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If the reflectance from the reference surface is 100% and the reflectance from the measurement surface is also approximately 100%, clear interference stripes appear. However, if the reflectance from the measurement surface is only 1%, the contrast is not as defined.

And all the above means that we now have a simple, gut-level intuitive picture of laser coherence. What is it? Coherent laser light is just pinhole-light produced by an infinite mirror-tunnel, with amplification. Sort of like those disco-era mirror-infinity toys from Spencer Gifts. But the depths of their virtual tunnel wouldn't be dark. On each reflection, the light passes through the laser-medium and gets slightly brighter. And on each pass, the "virtual source" seems farther away inside the tunnel. Viewed from the end, each deeper segment of the "tunnel" appears slightly brighter and smaller ...and the far end of the tunnel looks like an infinitely bright, infinitely tiny star. If you stare into the depths of the Amplifying Disco Infinity Mirror, the "star" is small and bright enough to punch a hole right through your retina. And it doesn't even have to be very bright to do this! A hundred-watt incandescent light bulb doesn't slice up your retina, but a quarter-watt laser can burn a tattoo permanently into the back of your eye. "Coherent" can also mean "sharp when focused," since focused Coherent light must all converge to an infinitely small point. (Yeah yeah diffraction limit. We're talking simple idealized geometrical optics here.)

The interference light becomes lighter and darker at intervals equal to half the wavelength of the light source (λ/2). These patterns of light and dark are called interference stripes. The height of a target can be determined by counting the number of interference stripes. The physicist Christiaan Huygens and others have proved that interference stripes of light form a graph (waveform) with a fixed period, as in the figure below. Optical interferometers use this physical phenomenon to ensure high-resolution measurement even at low magnifications.

Well, after a few years in the physics business I did figure it out. Jeeze, I just shoulda known... That explanation is WRONG. The explanation of Coherent Light found in most K12 introductory textbooks is pure garbage. It's worse than just wrong. It gave me a mental barrier. It led me directly into misconceptions, and I couldn't go forward until I'd un-learned them again.

For example, when using a 408 nm light source, the interference stripe spacing (wavelength) is 0.204 μm. This value represents the height difference of the measured surface. Because the height difference from peak to peak in the waveform graph is 0.204 μm, a resolution of 0.1 nm is possible by dividing the waveform graph into 2000 segments between the peaks. Optical interferometers measure changes in height by measuring the changes between light and dark in regular interference stripes.

With our gut-level intuitive understanding of Laser Coherence, we can now construct a basic list of coherent light sources Sources in increasing coherence Bright cloudy sky (least spatially coherent) Fluorescent tube lamp Frosted incandescent bulb Sun during clear weather Clear incandescent bulb Clear incandescent bulb w/noncoil filament (aquarium bulb) LED Electric welding arc 50ft away Laser (coherence-leng in MMs, up to a few Meters) Starlight (coherence leng 1000s KM) Note that the list also is a list of DEcreasing visible source-width, with the cloudy sky at the top and the distant stars at the bottom.

On the other hand, Partial spatial coherence is a whole 'nother kettle of fish, and is nearly impenetrable without recourse to algebra and trig. Even further: the mathematical "coherence" concept in general; the mixture of spatial and temporal coherence ...is not mentioned anywhere here.

If fig. 1 above is wrong, then what's right? If we could actually see individual light waves, what would coherent light look like? Fortunately the explanation is quite simple. Take a look at figure 2A below. That's what perfectly coherent light would look like if we could see the waves. Coherent light is simple: it's light which comes from a very small light source. Light from a single source is always coherent, since incoherence requires two sources. Spatially coherent light has another name: "sphere waves" or "plane waves." Or even simpler: "pinhole light" or "pointsource light."

But somehow this explanation just wouldn't stick to my brain. It didn't fit with everything else I knew. And worse still, I couldn't use the explanation as a tool. On one hand, the typical explanation of monochromatic laser light was very useful in many situations. Pure color means single frequency, and that implies narrow peaks in the spectrum graphs, and tiny spots on the radio dial. "Monochromatic light" connects with audio, where the pure tones such as flute-notes are monochromatic, while an impure broad-spectrum tone sounds like pink noise (or perhaps violins.) And in holography, whenever the frequency of light is moved high or low, I could imagine how this would slide all those tiny diffraction patterns around on my film. That would blur the patterns and make holography impossible, so clearly a hologram camera needs a very monochromatic light source. As a concept, "Monochromatic" works!

Determining whether the shape of the target is on an upward slope or downward slope is not possible when using interference stripes generated from a single-wavelength light source. However, this problem can be solved using phase shift interferometry.

Measurements may also not be possible when there is a significant difference between the light reflected from the reference mirror and light reflected from the measurement area. White light interferometers handle mirrored surfaces well, but can struggle to measure very rough samples with steep angles.

The peak of the created composite waveform is the focal point position of the lens, and height can be determined through synchronization with the travel distance of the lens.

In the distant past, monochromatic coherent sources were also microwatt light sources, no getting around it. Creating coherent light meant throwing away almost all of the power. Sending many milliwatts of light through a wavelength-diameter pinhole was basically impossible. So, all the bizarre and wonderful capabilities of lasers were unreachable. But lasers easily solved the problem because, right at the start, they create some spherewave "pinhole light," as if their entire light output came from a single virtual pinhole; a pinhole which is less than 500nM across. Aha, those confocal/concentric resonator mirrors, the ones used in lasers? This means that the "virtual pinhole" in an actual laser is just a non-virtual, very real pinhole-image sitting in the space between the mirrors. (See wikipedia diagrams for optical cavities, http://en.wikipedia.org/wiki/Optical_cavity). And all of those Semiconductor Lasers with parallel mirrors: they just employ an "infinite mirror tunnel" in order to place their pointsource at virtual-infinity distance, where it behaves just like the light from a distant star. During its trip down the infinite tunnel, all the non-planewave light wanders out the sides of the tunnel. Only planewave light can persist in the tunnel and get amplified. So ...laser coherence is created by the mirror-tunnel. Not by transparency or stimulated emission or 'stacked sinewaves." Or in proper terms, coherence is created by the laser's Fabry-Perot resonator cavity, and not by any sideways packing of long narrow string-like "photons." And all the above means that we now have a simple, gut-level intuitive picture of laser coherence. What is it? Coherent laser light is just pinhole-light produced by an infinite mirror-tunnel, with amplification. Sort of like those disco-era mirror-infinity toys from Spencer Gifts. But the depths of their virtual tunnel wouldn't be dark. On each reflection, the light passes through the laser-medium and gets slightly brighter. And on each pass, the "virtual source" seems farther away inside the tunnel. Viewed from the end, each deeper segment of the "tunnel" appears slightly brighter and smaller ...and the far end of the tunnel looks like an infinitely bright, infinitely tiny star. If you stare into the depths of the Amplifying Disco Infinity Mirror, the "star" is small and bright enough to punch a hole right through your retina. And it doesn't even have to be very bright to do this! A hundred-watt incandescent light bulb doesn't slice up your retina, but a quarter-watt laser can burn a tattoo permanently into the back of your eye. "Coherent" can also mean "sharp when focused," since focused Coherent light must all converge to an infinitely small point. (Yeah yeah diffraction limit. We're talking simple idealized geometrical optics here.) OK, if spatially coherent light looks like an expanding bullseye, then what does INCOHERENT light look like? In the above diagram 2A, incoherence instead would look like multiple pinholes and bunches of overlapped bullseyes. Lots of interference patterns, and probably with the nodes dynamically swerving around. Either that or it would look like fig. 2b but with bunches of light rays from multiple pinholes, and the rays all cross each other throughout the light beam. In both cases if the incorherent light was focused by a lens, we wouldn't produce any infinitely tiny hot spot. Can't punch holes in razor blades. With our gut-level intuitive understanding of Laser Coherence, we can now construct a basic list of coherent light sources Sources in increasing coherence Bright cloudy sky (least spatially coherent) Fluorescent tube lamp Frosted incandescent bulb Sun during clear weather Clear incandescent bulb Clear incandescent bulb w/noncoil filament (aquarium bulb) LED Electric welding arc 50ft away Laser (coherence-leng in MMs, up to a few Meters) Starlight (coherence leng 1000s KM) Note that the list also is a list of DEcreasing visible source-width, with the cloudy sky at the top and the distant stars at the bottom. As a little kid, did you believe that the light from clear incandescent bulbs was more magical than the frosted ones? And the light of garage welders was even more magical still? If so, you were intuitively experiencing optical coherence. Your little brain was wanting to mess around with laser sources, rather than overcast daylight. A perfect ideal pointsource gives perfectly coherent light, while a wide diffuse source gives the least coherent light. Turn the idea backwards: if we start out with perfectly coherent laser light, but then we send it through a frosted screen, the light remains just as monochromatic, but it becomes incoherent. Hey, I noticed that we can actually buy an incoherent-izer, an opto device for our optical bench. They're just a rotating frosted screen with a little motor (since an unmoving frosted screen still leaves a small bit of micro-scale coherence or "laser speckle.") NO JPEG YET Fig. 4 A frosted screen makes light incoherent. REAL SOLAR DEATH-RAY And now I have the answer to a big question that plagued me in childhood. No doubt all the nasty little science-boys like me had come up with this one. Why can't I make a death ray light-source? I could just get my big plastic fresnel lens and focus sunlight, and then somehow collimate it into a half-mm beam. The 0.50mm burning spot would appear anywhere along the parallel beam miles long. Write CHAIRFACE on the freakin' moon! But if we think about this now, it turns out to be impossible. Adding extra lenses to our solar furnace just creates a projector, where our parallel solar deathray spreads out and becomes a wide image of the sun. The darned sun isn't a pointsource. No thin beam is possible unless we include a tenth-micron pinhole in the optical path, and that turns the power into microwatts. The solution to the problem is simple: JUST REPLACE THE SUN WITH A 10KM WHITE DWARF STAR HA HAAAA! Keep the sun's brightness the same, but shrink the sun until it appears in the sky like a tiny star, like an extremely intense pinpoint. Now just use any big lens to gather a square meter of sunlight, focus it down to 1mm, then collimate it with a 1mm water-cooled short-focus quartz lens stolen from an ultraviolet microscope. Yes, the whole device is still a projector, but if we project the image of a pointsource into the distance, the result is an intense collimated beam. Other than a bit of diffraction it should work great: a few hundred watts in a parallel CW beam 1mm wide. Slice-a offs you fingas! Winston Kock, one of the early laser people at Bell Labs, said that laser light is "sharper light" which can be used as a cutting tool. Exactly, exactly! Winston Kock actually gets it. But the actual central concept is that coherence or "pinhole light" is the whole reason for the "sharp light" which does the laser-cutting. Lasers aren't particularly bright. Hundred watt light bulbs? 5,000 watt spotlights for school play?? Or daytime sunlight? If our sun was 10KM wide, or reduced to 10^5 times smaller in visual angle, then its light would be spatially coherent like lasers, or like an electric welding arc, and glancing upwards during the day might slice grooves across our retinas. The lens of your eye will focus the white-dwarf sunlight to a pinpoint rather than to a dim and safe little 0.3deg solar disk on your retina. Only because sunlight is non-parallel, because our sun is an extended source, our 1.5 KWatt/m^2 sunlight doesn't act like dangerous laser light. Hmmm, hold on a sec. If sunlight is about 1500 watts per square meter, and your eye's pupil is about 1mm, then your pupil intercepts 1500W/.001^2 = 1.5mW. DOH! WRONG! OK, staring at white-dwarf sunlight would actually be just like staring into a cheap laser pointer. Those things don't become really dangerous to human eyes until up around 5mW. AHA, but using binoculars would be bad, very bad: 5000X smaller exit aperture, creating an eight watt parallel beam 1mm in diameter. Binoculars become like icepicks aimed at your eyeballs. Coherent light can be nasty.

coherence中文

Light interference occurs when two light waves collide, causing each to strengthen or weaken. This section describes the interference of two lights reaching point P at a certain distance from the surface of the target. If the difference in distance between the two light paths S1P and S2P is an integer multiple of the light wavelength λ, the two light waves will strengthen and become brighter at point P due to the wave peaks overlapping. If the optical path difference is an integer multiple of the wavelength λ + 1/2 the wavelength λ, the peaks and valleys of the waves will overlap, causing the waves to weaken and become darker.

Pinhole pinhole, ever hear of an optics device called a "Spatial Filter?" They're used to 'clean up' laser light and make it much more spatially coherent. A Spatial Filter is just a very small pinhole with a converging lens upstream: any "incoherent" parts of the beam will never make it through the tiny aperture. It restores an imperfect laser's point-sourcey-ness.

The composite waveform of the interference stripe intensity can be reproduced from the interference fringes captured at regular intervals using an arithmetic formula.

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Also, this article is aimed at the general public as well as grade-school teachers and students. So, no math whatsoever! Also, this article explains IDEAL coherence: light with perfect spatial coherence.

REAL SOLAR DEATH-RAY And now I have the answer to a big question that plagued me in childhood. No doubt all the nasty little science-boys like me had come up with this one. Why can't I make a death ray light-source? I could just get my big plastic fresnel lens and focus sunlight, and then somehow collimate it into a half-mm beam. The 0.50mm burning spot would appear anywhere along the parallel beam miles long. Write CHAIRFACE on the freakin' moon! But if we think about this now, it turns out to be impossible. Adding extra lenses to our solar furnace just creates a projector, where our parallel solar deathray spreads out and becomes a wide image of the sun. The darned sun isn't a pointsource. No thin beam is possible unless we include a tenth-micron pinhole in the optical path, and that turns the power into microwatts. The solution to the problem is simple: JUST REPLACE THE SUN WITH A 10KM WHITE DWARF STAR HA HAAAA! Keep the sun's brightness the same, but shrink the sun until it appears in the sky like a tiny star, like an extremely intense pinpoint. Now just use any big lens to gather a square meter of sunlight, focus it down to 1mm, then collimate it with a 1mm water-cooled short-focus quartz lens stolen from an ultraviolet microscope. Yes, the whole device is still a projector, but if we project the image of a pointsource into the distance, the result is an intense collimated beam. Other than a bit of diffraction it should work great: a few hundred watts in a parallel CW beam 1mm wide. Slice-a offs you fingas!

The white light interferometer is designed so that the optical path length from the CCD element to the reference mirror and that from the CCD element to the sample surface are the same. The asperity on the sample surface causes these path lengths to be unequal, which results in forming an interference pattern at the CCD element. The number of lines in the interference pattern is translated to peaks and troughs (heights) on the sample surface.

What ismonochromaticlight

Height resolution with an optical interferometer does not depend on the magnification of the objective lens because a composite waveform representing the interference stripe intensity can be accurately reproduced through calculation even if the depth of field of the objective lens is high. The interference stripes of light appear at regular intervals because the light wavelength is constant. This means that, if the wavelength being used is known in advance, it is possible to calculate what the composite waveform from the interference stripe intensity will look like. High resolutions can then be achieved by reproducing a composite waveform graph from the interference intensity captured at regular intervals and separating the reproduced composite waveform for processing.

The confocal range finding system, which uses laser reflection intensity for detection, can measure shapes that have high angular characteristics with low noise.

As shown in the figures to the left, with a height difference of (1/2 + n) × the wavelength λ of the light source, no changes are noticeable in the interference stripes, so determining the correct height difference is not possible.

Optical path difference and light interferenceWhen the optical path difference is an integer multiple of the wavelength λ + λ/2

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So coherent light is just "pointsource light?" Paraphrasing Feynman: Now I Understand Evvvvvrrreeethiiing! Finally it all makes perfect sense: starlight is ULTIMATELY coherent, that's why Stellar Interferometry works. Starlight has coherence-lengths in thousands of KM, starlight is far more coherent than any human-made laser light. And the most distant stars are just like ideal point sources. I remember AA Michelson discovering that Betlegeuse is far less coherent than other stars. Ha, far less like a microscopic pointsource! Then I suddenly remember Dennis Gabor, inventing holography before lasers existed. To create his pseudo-lasers he just took light from an ordinary mercury-arc lamp and passed it through a pinhole. Mercury's emission line made it nearly monochromatic, and the pinhole gave it the spatial coherence.

Instead I'm following the usual distinction made by the intro textbooks. In these books, perfectly coherent light is explained separately from perfectly monochromatic light (i.e., spatial coherence is not temporal coherence.) Here I'm ignoring single-frequency waves, and only explaining the ideal pinhole-light, white light from wavelength-size apertures. Also, I'm not treating light from extended apertures. I'm explaining the light from distant stars, not the light from nearby Betelgeuse.

ADDENDUM: General mathematical theory of EM partial coherence If you read the first paragraphs here, you'd know that this article only describes Spatial Coherence, not temporal coherence or monochromatic light. (Important! Don't miss it.) Also, this article is aimed at the general public as well as grade-school teachers and students. So, no math whatsoever! Also, this article explains IDEAL coherence: light with perfect spatial coherence. On the other hand, Partial spatial coherence is a whole 'nother kettle of fish, and is nearly impenetrable without recourse to algebra and trig. Even further: the mathematical "coherence" concept in general; the mixture of spatial and temporal coherence ...is not mentioned anywhere here. Instead I'm following the usual distinction made by the intro textbooks. In these books, perfectly coherent light is explained separately from perfectly monochromatic light (i.e., spatial coherence is not temporal coherence.) Here I'm ignoring single-frequency waves, and only explaining the ideal pinhole-light, white light from wavelength-size apertures. Also, I'm not treating light from extended apertures. I'm explaining the light from distant stars, not the light from nearby Betelgeuse. In other words, where kids and the public are concerned, the term "coherent light" has a distinct meaning. It does not mean generalized coherence. Instead, for the greater public, "coherent light" means "light of perfect spatial-coherence," such as white light from ideal pinholes. But at the university level things are very different, where the term "Coherence" means a general theory; a mathematical description of non-ideal partial coherence which combines monochromatic light with the light from non-pinhole extended sources. A general theory of coherence does not divide temporal coherence from spatial. Do "textbooks" get Coherence completely wrong? Yes: grade-school textbooks, K-12 textbooks. Also articles written for public consumption, they get it wrong too. But the college textbooks, they're fine. They go into the rigorous details of partial coherence, and mixtures of spatial and temporal coherence, and they don't teach us that photons are like little sine-waves which can pack together like cardboard egg-cartons. SEE ALSO: Optics 421: Space & temp coherence Van-Cittert/Zernike theorem Etendue Common Laser Misconceptions Laser light is not "in-phase" light Laser light is not parallel light In-phase emission does not lead to coherent light Electricity misconceptions Essay collection List of misconceptions Electricity F.A.Q. http://amasci.com/miscon/coherenc.html Created and maintained by Bill Beaty. Mail me at: . View My Stats

So ...laser coherence is created by the mirror-tunnel. Not by transparency or stimulated emission or 'stacked sinewaves." Or in proper terms, coherence is created by the laser's Fabry-Perot resonator cavity, and not by any sideways packing of long narrow string-like "photons."

A perfect ideal pointsource gives perfectly coherent light, while a wide diffuse source gives the least coherent light. Turn the idea backwards: if we start out with perfectly coherent laser light, but then we send it through a frosted screen, the light remains just as monochromatic, but it becomes incoherent. Hey, I noticed that we can actually buy an incoherent-izer, an opto device for our optical bench. They're just a rotating frosted screen with a little motor (since an unmoving frosted screen still leaves a small bit of micro-scale coherence or "laser speckle.") NO JPEG YET Fig. 4 A frosted screen makes light incoherent.

In other words, where kids and the public are concerned, the term "coherent light" has a distinct meaning. It does not mean generalized coherence. Instead, for the greater public, "coherent light" means "light of perfect spatial-coherence," such as white light from ideal pinholes. But at the university level things are very different, where the term "Coherence" means a general theory; a mathematical description of non-ideal partial coherence which combines monochromatic light with the light from non-pinhole extended sources. A general theory of coherence does not divide temporal coherence from spatial.

To get right down to it, light isn't a transverse wave. Or more specifically, light isn't a "transverse wave in the Aether," instead light is a wave in magnetic and electric fields where the field vectors point sideways. But the flux lines themselves don't wiggle sideways, and the flux doesn't contain any sine-wave shapes. Take a look at the video below left. The animated graph depicts the field strengths found along a single straight line: the values of the fields when a light wave is passing towards the right. The only sinewaves present are found in the pattern of intensity, in the sine-graph of field strength measurements. That sine wave is not a flux shape in space. In that vid, the only space involved is a straight-line axis with no wiggles.

Fig. 3 SOMEONE GETS IT RIGHT! The above unattributed diagram found in online archives. In the distant past, monochromatic coherent sources were also microwatt light sources, no getting around it. Creating coherent light meant throwing away almost all of the power. Sending many milliwatts of light through a wavelength-diameter pinhole was basically impossible. So, all the bizarre and wonderful capabilities of lasers were unreachable. But lasers easily solved the problem because, right at the start, they create some spherewave "pinhole light," as if their entire light output came from a single virtual pinhole; a pinhole which is less than 500nM across. Aha, those confocal/concentric resonator mirrors, the ones used in lasers? This means that the "virtual pinhole" in an actual laser is just a non-virtual, very real pinhole-image sitting in the space between the mirrors. (See wikipedia diagrams for optical cavities, http://en.wikipedia.org/wiki/Optical_cavity). And all of those Semiconductor Lasers with parallel mirrors: they just employ an "infinite mirror tunnel" in order to place their pointsource at virtual-infinity distance, where it behaves just like the light from a distant star. During its trip down the infinite tunnel, all the non-planewave light wanders out the sides of the tunnel. Only planewave light can persist in the tunnel and get amplified. So ...laser coherence is created by the mirror-tunnel. Not by transparency or stimulated emission or 'stacked sinewaves." Or in proper terms, coherence is created by the laser's Fabry-Perot resonator cavity, and not by any sideways packing of long narrow string-like "photons." And all the above means that we now have a simple, gut-level intuitive picture of laser coherence. What is it? Coherent laser light is just pinhole-light produced by an infinite mirror-tunnel, with amplification. Sort of like those disco-era mirror-infinity toys from Spencer Gifts. But the depths of their virtual tunnel wouldn't be dark. On each reflection, the light passes through the laser-medium and gets slightly brighter. And on each pass, the "virtual source" seems farther away inside the tunnel. Viewed from the end, each deeper segment of the "tunnel" appears slightly brighter and smaller ...and the far end of the tunnel looks like an infinitely bright, infinitely tiny star. If you stare into the depths of the Amplifying Disco Infinity Mirror, the "star" is small and bright enough to punch a hole right through your retina. And it doesn't even have to be very bright to do this! A hundred-watt incandescent light bulb doesn't slice up your retina, but a quarter-watt laser can burn a tattoo permanently into the back of your eye. "Coherent" can also mean "sharp when focused," since focused Coherent light must all converge to an infinitely small point. (Yeah yeah diffraction limit. We're talking simple idealized geometrical optics here.) OK, if spatially coherent light looks like an expanding bullseye, then what does INCOHERENT light look like? In the above diagram 2A, incoherence instead would look like multiple pinholes and bunches of overlapped bullseyes. Lots of interference patterns, and probably with the nodes dynamically swerving around. Either that or it would look like fig. 2b but with bunches of light rays from multiple pinholes, and the rays all cross each other throughout the light beam. In both cases if the incorherent light was focused by a lens, we wouldn't produce any infinitely tiny hot spot. Can't punch holes in razor blades. With our gut-level intuitive understanding of Laser Coherence, we can now construct a basic list of coherent light sources Sources in increasing coherence Bright cloudy sky (least spatially coherent) Fluorescent tube lamp Frosted incandescent bulb Sun during clear weather Clear incandescent bulb Clear incandescent bulb w/noncoil filament (aquarium bulb) LED Electric welding arc 50ft away Laser (coherence-leng in MMs, up to a few Meters) Starlight (coherence leng 1000s KM) Note that the list also is a list of DEcreasing visible source-width, with the cloudy sky at the top and the distant stars at the bottom. As a little kid, did you believe that the light from clear incandescent bulbs was more magical than the frosted ones? And the light of garage welders was even more magical still? If so, you were intuitively experiencing optical coherence. Your little brain was wanting to mess around with laser sources, rather than overcast daylight. A perfect ideal pointsource gives perfectly coherent light, while a wide diffuse source gives the least coherent light. Turn the idea backwards: if we start out with perfectly coherent laser light, but then we send it through a frosted screen, the light remains just as monochromatic, but it becomes incoherent. Hey, I noticed that we can actually buy an incoherent-izer, an opto device for our optical bench. They're just a rotating frosted screen with a little motor (since an unmoving frosted screen still leaves a small bit of micro-scale coherence or "laser speckle.") NO JPEG YET Fig. 4 A frosted screen makes light incoherent. REAL SOLAR DEATH-RAY And now I have the answer to a big question that plagued me in childhood. No doubt all the nasty little science-boys like me had come up with this one. Why can't I make a death ray light-source? I could just get my big plastic fresnel lens and focus sunlight, and then somehow collimate it into a half-mm beam. The 0.50mm burning spot would appear anywhere along the parallel beam miles long. Write CHAIRFACE on the freakin' moon! But if we think about this now, it turns out to be impossible. Adding extra lenses to our solar furnace just creates a projector, where our parallel solar deathray spreads out and becomes a wide image of the sun. The darned sun isn't a pointsource. No thin beam is possible unless we include a tenth-micron pinhole in the optical path, and that turns the power into microwatts. The solution to the problem is simple: JUST REPLACE THE SUN WITH A 10KM WHITE DWARF STAR HA HAAAA! Keep the sun's brightness the same, but shrink the sun until it appears in the sky like a tiny star, like an extremely intense pinpoint. Now just use any big lens to gather a square meter of sunlight, focus it down to 1mm, then collimate it with a 1mm water-cooled short-focus quartz lens stolen from an ultraviolet microscope. Yes, the whole device is still a projector, but if we project the image of a pointsource into the distance, the result is an intense collimated beam. Other than a bit of diffraction it should work great: a few hundred watts in a parallel CW beam 1mm wide. Slice-a offs you fingas! Winston Kock, one of the early laser people at Bell Labs, said that laser light is "sharper light" which can be used as a cutting tool. Exactly, exactly! Winston Kock actually gets it. But the actual central concept is that coherence or "pinhole light" is the whole reason for the "sharp light" which does the laser-cutting. Lasers aren't particularly bright. Hundred watt light bulbs? 5,000 watt spotlights for school play?? Or daytime sunlight? If our sun was 10KM wide, or reduced to 10^5 times smaller in visual angle, then its light would be spatially coherent like lasers, or like an electric welding arc, and glancing upwards during the day might slice grooves across our retinas. The lens of your eye will focus the white-dwarf sunlight to a pinpoint rather than to a dim and safe little 0.3deg solar disk on your retina. Only because sunlight is non-parallel, because our sun is an extended source, our 1.5 KWatt/m^2 sunlight doesn't act like dangerous laser light. Hmmm, hold on a sec. If sunlight is about 1500 watts per square meter, and your eye's pupil is about 1mm, then your pupil intercepts 1500W/.001^2 = 1.5mW. DOH! WRONG! OK, staring at white-dwarf sunlight would actually be just like staring into a cheap laser pointer. Those things don't become really dangerous to human eyes until up around 5mW. AHA, but using binoculars would be bad, very bad: 5000X smaller exit aperture, creating an eight watt parallel beam 1mm in diameter. Binoculars become like icepicks aimed at your eyeballs. Coherent light can be nasty.

As a kid I was always confused by explanations of coherent light. I'd been told that coherence had something to do with the sinusoidal shape of photons. Light is supposedly made up of little wiggling string-shapes; transverse waves. Textbooks show each photon as a kind of little "snake" moving side to side. And, supposedly, whenever all the "snakes" pack together side by side with their wiggles aligned, that's Coherence. Atoms in a laser are all emitting their light in phase-lock, and supposedly the end result is a special kind of "Inphase Light," where the little sine-waves stack up together, much like egg cartons.

To ensure accurate measurement with a white light interferometer, the target must be leveled, otherwise known as zeroing. With conventional systems, ensuring the target is level required users to check the interference stripes visually and readjust several times. Keyence's 3D Surface Profiler has a built-in zeroing support function to detect any tilting of the target and automatically calculates the correction angle. Being able to determine the correction angle in advance allows for easy, reliable, and fast adjustment.

When using a white light source, the interference stripes on the measurement surface at the focal point of the objective lens become stronger, and disappear when moving away from the focal point. Using a composite waveform created by superimposing interference stripes of different wavelengths makes it possible to detect the interference intensity peaks.

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If you read the first paragraphs here, you'd know that this article only describes Spatial Coherence, not temporal coherence or monochromatic light. (Important! Don't miss it.)

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When using interferometers to measure objects with steep angles, due to the concentration of interference patters in those areas, accurate information cannot be gathered.

Coherent lightsource examples

If we could see light and radio waves, could we find any little sinewave-snakes anywhere? Nope. Take a look at the second video above right. It shows what EM waves would actually look like, if we could see them. It's an animation of flux lines surrounding a very tiny light source. The EM waves expand like layers of an onion. The flux lines break loose from the source, close upon themselves to form loops, then fly off into space. Of course if we graphed the field strengths on a voltage axis, they would form sine waves, as shown in the first video. But the flux itself isn't like a snake. It points entirely sideways all the time, like closed rings with no sine wiggles. And of course there isn't any "Aether medium" which could wiggle like transverse sine waves. No little snakes flying through space. Like atoms being little solar systems, the wiggles were "lies to children." Or, they were simply wrong.

And photons? ...the photons are either dimensionless particles, or they're broad wavefunctions which are 'quantized.' They're either like infinitely small bullets flying in straight lines, or they're like enormous expanding EM pond-ripples from a thrown pebble. Photons aren't shaped like twisty snakes, they're nothing like a transverse wave on a string.

As a little kid, did you believe that the light from clear incandescent bulbs was more magical than the frosted ones? And the light of garage welders was even more magical still? If so, you were intuitively experiencing optical coherence. Your little brain was wanting to mess around with laser sources, rather than overcast daylight.

Place of installation is limited due to the equipment’s high sensitivity to vibrations. Shock-absorbing tables are necessary for installation.

Coherent lightsource meaning

Keyence's 3D Surface Profiler with a built-in white light interferometer uses a white LED to determine the interference stripe intensity at each height interval by moving the objective lens. Height information from the focal point position is obtained by using a linear scale to measure the lens position at the point where the interference stripes become stronger. This method is called vertical scanning interferometry (VSI).

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Focusing on the observation screen may be difficult with a white light interferometer because interference signals are weaker for low-reflectivity targets. The VK-X3000 camera is able to focus on low-reflectivity targets with high sensitivity thanks to the included laser auto-focus function.

Quantum coherence

And finally I know why lasers are so wonderful: lasers are pinhole light sources which are ...actually bright! It's always been easy to make some coherent light, just use a normal light source and an optically small pinhole (a halfwave diameter.) A frosted light bulb can become a coherent light source. But a pinhole aperture this small will block nearly all the light from any conventional source. To experiment with this, get a slide projector and make a slide with a pinhole: an Al foil layer perforated by a needle. Add a narrowband green filter, and that's your Gabor-approved 1940s laser source. Make some holograms? Heh, a bit long exposure-time though.

A. B. Fig. 2 A coherent light source emits waves and/or particles. A perfectly coherent source is just a point-source. A single small light source sends out electromagnetic waves in all directions as shown above. Of course these diagrams are only two-dimensional, while the real situation is 3D. We can visualize a coherent wavefront to be spherical. The waves are like layers of a spherical onion, but where the onion is expanding at the speed of light, with new layers constantly added in the center. OR... we could imagine that the tiny light source is sending out a stream of particles flying off in all directions. The paths of these particles are the "rays" of light. Since they all fly outwards from a single point, none of the rays cross each other. And if this light is passed through a converging lens, it's focused to a perfect point. Coherent light is just some: Rays which never cross each other; parallel or radial Perfect wavetrains in 3D; nested sphere-waves or plane-waves So coherent light is just "pointsource light?" Paraphrasing Feynman: Now I Understand Evvvvvrrreeethiiing! Finally it all makes perfect sense: starlight is ULTIMATELY coherent, that's why Stellar Interferometry works. Starlight has coherence-lengths in thousands of KM, starlight is far more coherent than any human-made laser light. And the most distant stars are just like ideal point sources. I remember AA Michelson discovering that Betlegeuse is far less coherent than other stars. Ha, far less like a microscopic pointsource! Then I suddenly remember Dennis Gabor, inventing holography before lasers existed. To create his pseudo-lasers he just took light from an ordinary mercury-arc lamp and passed it through a pinhole. Mercury's emission line made it nearly monochromatic, and the pinhole gave it the spatial coherence. Pinhole pinhole, ever hear of an optics device called a "Spatial Filter?" They're used to 'clean up' laser light and make it much more spatially coherent. A Spatial Filter is just a very small pinhole with a converging lens upstream: any "incoherent" parts of the beam will never make it through the tiny aperture. It restores an imperfect laser's point-sourcey-ness. And finally I know why lasers are so wonderful: lasers are pinhole light sources which are ...actually bright! It's always been easy to make some coherent light, just use a normal light source and an optically small pinhole (a halfwave diameter.) A frosted light bulb can become a coherent light source. But a pinhole aperture this small will block nearly all the light from any conventional source. To experiment with this, get a slide projector and make a slide with a pinhole: an Al foil layer perforated by a needle. Add a narrowband green filter, and that's your Gabor-approved 1940s laser source. Make some holograms? Heh, a bit long exposure-time though. Fig. 3 SOMEONE GETS IT RIGHT! The above unattributed diagram found in online archives. In the distant past, monochromatic coherent sources were also microwatt light sources, no getting around it. Creating coherent light meant throwing away almost all of the power. Sending many milliwatts of light through a wavelength-diameter pinhole was basically impossible. So, all the bizarre and wonderful capabilities of lasers were unreachable. But lasers easily solved the problem because, right at the start, they create some spherewave "pinhole light," as if their entire light output came from a single virtual pinhole; a pinhole which is less than 500nM across. Aha, those confocal/concentric resonator mirrors, the ones used in lasers? This means that the "virtual pinhole" in an actual laser is just a non-virtual, very real pinhole-image sitting in the space between the mirrors. (See wikipedia diagrams for optical cavities, http://en.wikipedia.org/wiki/Optical_cavity). And all of those Semiconductor Lasers with parallel mirrors: they just employ an "infinite mirror tunnel" in order to place their pointsource at virtual-infinity distance, where it behaves just like the light from a distant star. During its trip down the infinite tunnel, all the non-planewave light wanders out the sides of the tunnel. Only planewave light can persist in the tunnel and get amplified. So ...laser coherence is created by the mirror-tunnel. Not by transparency or stimulated emission or 'stacked sinewaves." Or in proper terms, coherence is created by the laser's Fabry-Perot resonator cavity, and not by any sideways packing of long narrow string-like "photons." And all the above means that we now have a simple, gut-level intuitive picture of laser coherence. What is it? Coherent laser light is just pinhole-light produced by an infinite mirror-tunnel, with amplification. Sort of like those disco-era mirror-infinity toys from Spencer Gifts. But the depths of their virtual tunnel wouldn't be dark. On each reflection, the light passes through the laser-medium and gets slightly brighter. And on each pass, the "virtual source" seems farther away inside the tunnel. Viewed from the end, each deeper segment of the "tunnel" appears slightly brighter and smaller ...and the far end of the tunnel looks like an infinitely bright, infinitely tiny star. If you stare into the depths of the Amplifying Disco Infinity Mirror, the "star" is small and bright enough to punch a hole right through your retina. And it doesn't even have to be very bright to do this! A hundred-watt incandescent light bulb doesn't slice up your retina, but a quarter-watt laser can burn a tattoo permanently into the back of your eye. "Coherent" can also mean "sharp when focused," since focused Coherent light must all converge to an infinitely small point. (Yeah yeah diffraction limit. We're talking simple idealized geometrical optics here.) OK, if spatially coherent light looks like an expanding bullseye, then what does INCOHERENT light look like? In the above diagram 2A, incoherence instead would look like multiple pinholes and bunches of overlapped bullseyes. Lots of interference patterns, and probably with the nodes dynamically swerving around. Either that or it would look like fig. 2b but with bunches of light rays from multiple pinholes, and the rays all cross each other throughout the light beam. In both cases if the incorherent light was focused by a lens, we wouldn't produce any infinitely tiny hot spot. Can't punch holes in razor blades. With our gut-level intuitive understanding of Laser Coherence, we can now construct a basic list of coherent light sources Sources in increasing coherence Bright cloudy sky (least spatially coherent) Fluorescent tube lamp Frosted incandescent bulb Sun during clear weather Clear incandescent bulb Clear incandescent bulb w/noncoil filament (aquarium bulb) LED Electric welding arc 50ft away Laser (coherence-leng in MMs, up to a few Meters) Starlight (coherence leng 1000s KM) Note that the list also is a list of DEcreasing visible source-width, with the cloudy sky at the top and the distant stars at the bottom. As a little kid, did you believe that the light from clear incandescent bulbs was more magical than the frosted ones? And the light of garage welders was even more magical still? If so, you were intuitively experiencing optical coherence. Your little brain was wanting to mess around with laser sources, rather than overcast daylight. A perfect ideal pointsource gives perfectly coherent light, while a wide diffuse source gives the least coherent light. Turn the idea backwards: if we start out with perfectly coherent laser light, but then we send it through a frosted screen, the light remains just as monochromatic, but it becomes incoherent. Hey, I noticed that we can actually buy an incoherent-izer, an opto device for our optical bench. They're just a rotating frosted screen with a little motor (since an unmoving frosted screen still leaves a small bit of micro-scale coherence or "laser speckle.") NO JPEG YET Fig. 4 A frosted screen makes light incoherent. REAL SOLAR DEATH-RAY And now I have the answer to a big question that plagued me in childhood. No doubt all the nasty little science-boys like me had come up with this one. Why can't I make a death ray light-source? I could just get my big plastic fresnel lens and focus sunlight, and then somehow collimate it into a half-mm beam. The 0.50mm burning spot would appear anywhere along the parallel beam miles long. Write CHAIRFACE on the freakin' moon! But if we think about this now, it turns out to be impossible. Adding extra lenses to our solar furnace just creates a projector, where our parallel solar deathray spreads out and becomes a wide image of the sun. The darned sun isn't a pointsource. No thin beam is possible unless we include a tenth-micron pinhole in the optical path, and that turns the power into microwatts. The solution to the problem is simple: JUST REPLACE THE SUN WITH A 10KM WHITE DWARF STAR HA HAAAA! Keep the sun's brightness the same, but shrink the sun until it appears in the sky like a tiny star, like an extremely intense pinpoint. Now just use any big lens to gather a square meter of sunlight, focus it down to 1mm, then collimate it with a 1mm water-cooled short-focus quartz lens stolen from an ultraviolet microscope. Yes, the whole device is still a projector, but if we project the image of a pointsource into the distance, the result is an intense collimated beam. Other than a bit of diffraction it should work great: a few hundred watts in a parallel CW beam 1mm wide. Slice-a offs you fingas! Winston Kock, one of the early laser people at Bell Labs, said that laser light is "sharper light" which can be used as a cutting tool. Exactly, exactly! Winston Kock actually gets it. But the actual central concept is that coherence or "pinhole light" is the whole reason for the "sharp light" which does the laser-cutting. Lasers aren't particularly bright. Hundred watt light bulbs? 5,000 watt spotlights for school play?? Or daytime sunlight? If our sun was 10KM wide, or reduced to 10^5 times smaller in visual angle, then its light would be spatially coherent like lasers, or like an electric welding arc, and glancing upwards during the day might slice grooves across our retinas. The lens of your eye will focus the white-dwarf sunlight to a pinpoint rather than to a dim and safe little 0.3deg solar disk on your retina. Only because sunlight is non-parallel, because our sun is an extended source, our 1.5 KWatt/m^2 sunlight doesn't act like dangerous laser light. Hmmm, hold on a sec. If sunlight is about 1500 watts per square meter, and your eye's pupil is about 1mm, then your pupil intercepts 1500W/.001^2 = 1.5mW. DOH! WRONG! OK, staring at white-dwarf sunlight would actually be just like staring into a cheap laser pointer. Those things don't become really dangerous to human eyes until up around 5mW. AHA, but using binoculars would be bad, very bad: 5000X smaller exit aperture, creating an eight watt parallel beam 1mm in diameter. Binoculars become like icepicks aimed at your eyeballs. Coherent light can be nasty.

To measure short distances, use a measuring stick called a ruler, 4 to 5 m long. You can make your own by following the steps below.

Because Keyence's 3D Surface Profiler is equipped with both white light interferometry and laser confocal scanning, nearly any sample can be measured, including those with steep angles.

Winston Kock, one of the early laser people at Bell Labs, said that laser light is "sharper light" which can be used as a cutting tool. Exactly, exactly! Winston Kock actually gets it. But the actual central concept is that coherence or "pinhole light" is the whole reason for the "sharp light" which does the laser-cutting. Lasers aren't particularly bright. Hundred watt light bulbs? 5,000 watt spotlights for school play?? Or daytime sunlight? If our sun was 10KM wide, or reduced to 10^5 times smaller in visual angle, then its light would be spatially coherent like lasers, or like an electric welding arc, and glancing upwards during the day might slice grooves across our retinas. The lens of your eye will focus the white-dwarf sunlight to a pinpoint rather than to a dim and safe little 0.3deg solar disk on your retina. Only because sunlight is non-parallel, because our sun is an extended source, our 1.5 KWatt/m^2 sunlight doesn't act like dangerous laser light. Hmmm, hold on a sec. If sunlight is about 1500 watts per square meter, and your eye's pupil is about 1mm, then your pupil intercepts 1500W/.001^2 = 1.5mW. DOH! WRONG! OK, staring at white-dwarf sunlight would actually be just like staring into a cheap laser pointer. Those things don't become really dangerous to human eyes until up around 5mW. AHA, but using binoculars would be bad, very bad: 5000X smaller exit aperture, creating an eight watt parallel beam 1mm in diameter. Binoculars become like icepicks aimed at your eyeballs. Coherent light can be nasty.

The laser's in-phase emission arises in other topics: it's the basis for transparency of materials. For example, whenever atoms in a glass window absorb light waves, they re-emit those waves in phase, so the original wave is preserved and the material acts transparent. In-phase emission prevents the light from scattering when it interacts with the atoms in the glass. So yes, the atoms in the laser-rod or laser gas-tube emit light in phase... making the laser material transparent, and this preserves whatever coherence that the incoming light might already have had. The "in phase" textbook laser diagram below is actually, heh, explaining transparency. Incoherent light could also get amplified and bounce as shown below. So, the authors never bothered to tell us how the light became coherent in the first place.

Before measurement, slope correction of the sample with a goniometer stage is required.Because interference patterns become crowded when the sample is at an angle, proper measurement cannot be performed.

Whenever atoms in a laser are emitting EM waves in phase with incoming EM waves, the emitted waves add to the incoming light, making it brighter. Two plus two equals four. But amplification doesn't create any "in phase light." If two plus two is four, the resulting 4 is purely a number, and it isn't concealing any 2 + 2. Instead it could be one plus three, or nine minus five. I mean, when two in-phase waves add together to create an amplified wave, the original waves are gone. The larger wave doesn't forever travel along as two smaller "inphase waves" in the way all those intro laser explanations depict. Instead, all those diagrams should show that smaller waves add together to create single, larger waves. Amplification. Not some sort of "coherence-izing effect."

OK, if spatially coherent light looks like an expanding bullseye, then what does INCOHERENT light look like? In the above diagram 2A, incoherence instead would look like multiple pinholes and bunches of overlapped bullseyes. Lots of interference patterns, and probably with the nodes dynamically swerving around. Either that or it would look like fig. 2b but with bunches of light rays from multiple pinholes, and the rays all cross each other throughout the light beam. In both cases if the incorherent light was focused by a lens, we wouldn't produce any infinitely tiny hot spot. Can't punch holes in razor blades.

THIS IS WRONG. LASERS DON'T WORK LIKE THAT. Fig. 1 The bad diagram. Did you learn this one in school? If so, you may need to un-learn it before you can understand coherence. Coherent light does not behave anything like this. If fig. 1 above is wrong, then what's right? If we could actually see individual light waves, what would coherent light look like? Fortunately the explanation is quite simple. Take a look at figure 2A below. That's what perfectly coherent light would look like if we could see the waves. Coherent light is simple: it's light which comes from a very small light source. Light from a single source is always coherent, since incoherence requires two sources. Spatially coherent light has another name: "sphere waves" or "plane waves." Or even simpler: "pinhole light" or "pointsource light." A. B. Fig. 2 A coherent light source emits waves and/or particles. A perfectly coherent source is just a point-source. A single small light source sends out electromagnetic waves in all directions as shown above. Of course these diagrams are only two-dimensional, while the real situation is 3D. We can visualize a coherent wavefront to be spherical. The waves are like layers of a spherical onion, but where the onion is expanding at the speed of light, with new layers constantly added in the center. OR... we could imagine that the tiny light source is sending out a stream of particles flying off in all directions. The paths of these particles are the "rays" of light. Since they all fly outwards from a single point, none of the rays cross each other. And if this light is passed through a converging lens, it's focused to a perfect point. Coherent light is just some: Rays which never cross each other; parallel or radial Perfect wavetrains in 3D; nested sphere-waves or plane-waves So coherent light is just "pointsource light?" Paraphrasing Feynman: Now I Understand Evvvvvrrreeethiiing! Finally it all makes perfect sense: starlight is ULTIMATELY coherent, that's why Stellar Interferometry works. Starlight has coherence-lengths in thousands of KM, starlight is far more coherent than any human-made laser light. And the most distant stars are just like ideal point sources. I remember AA Michelson discovering that Betlegeuse is far less coherent than other stars. Ha, far less like a microscopic pointsource! Then I suddenly remember Dennis Gabor, inventing holography before lasers existed. To create his pseudo-lasers he just took light from an ordinary mercury-arc lamp and passed it through a pinhole. Mercury's emission line made it nearly monochromatic, and the pinhole gave it the spatial coherence. Pinhole pinhole, ever hear of an optics device called a "Spatial Filter?" They're used to 'clean up' laser light and make it much more spatially coherent. A Spatial Filter is just a very small pinhole with a converging lens upstream: any "incoherent" parts of the beam will never make it through the tiny aperture. It restores an imperfect laser's point-sourcey-ness. And finally I know why lasers are so wonderful: lasers are pinhole light sources which are ...actually bright! It's always been easy to make some coherent light, just use a normal light source and an optically small pinhole (a halfwave diameter.) A frosted light bulb can become a coherent light source. But a pinhole aperture this small will block nearly all the light from any conventional source. To experiment with this, get a slide projector and make a slide with a pinhole: an Al foil layer perforated by a needle. Add a narrowband green filter, and that's your Gabor-approved 1940s laser source. Make some holograms? Heh, a bit long exposure-time though. Fig. 3 SOMEONE GETS IT RIGHT! The above unattributed diagram found in online archives. In the distant past, monochromatic coherent sources were also microwatt light sources, no getting around it. Creating coherent light meant throwing away almost all of the power. Sending many milliwatts of light through a wavelength-diameter pinhole was basically impossible. So, all the bizarre and wonderful capabilities of lasers were unreachable. But lasers easily solved the problem because, right at the start, they create some spherewave "pinhole light," as if their entire light output came from a single virtual pinhole; a pinhole which is less than 500nM across. Aha, those confocal/concentric resonator mirrors, the ones used in lasers? This means that the "virtual pinhole" in an actual laser is just a non-virtual, very real pinhole-image sitting in the space between the mirrors. (See wikipedia diagrams for optical cavities, http://en.wikipedia.org/wiki/Optical_cavity). And all of those Semiconductor Lasers with parallel mirrors: they just employ an "infinite mirror tunnel" in order to place their pointsource at virtual-infinity distance, where it behaves just like the light from a distant star. During its trip down the infinite tunnel, all the non-planewave light wanders out the sides of the tunnel. Only planewave light can persist in the tunnel and get amplified. So ...laser coherence is created by the mirror-tunnel. Not by transparency or stimulated emission or 'stacked sinewaves." Or in proper terms, coherence is created by the laser's Fabry-Perot resonator cavity, and not by any sideways packing of long narrow string-like "photons." And all the above means that we now have a simple, gut-level intuitive picture of laser coherence. What is it? Coherent laser light is just pinhole-light produced by an infinite mirror-tunnel, with amplification. Sort of like those disco-era mirror-infinity toys from Spencer Gifts. But the depths of their virtual tunnel wouldn't be dark. On each reflection, the light passes through the laser-medium and gets slightly brighter. And on each pass, the "virtual source" seems farther away inside the tunnel. Viewed from the end, each deeper segment of the "tunnel" appears slightly brighter and smaller ...and the far end of the tunnel looks like an infinitely bright, infinitely tiny star. If you stare into the depths of the Amplifying Disco Infinity Mirror, the "star" is small and bright enough to punch a hole right through your retina. And it doesn't even have to be very bright to do this! A hundred-watt incandescent light bulb doesn't slice up your retina, but a quarter-watt laser can burn a tattoo permanently into the back of your eye. "Coherent" can also mean "sharp when focused," since focused Coherent light must all converge to an infinitely small point. (Yeah yeah diffraction limit. We're talking simple idealized geometrical optics here.) OK, if spatially coherent light looks like an expanding bullseye, then what does INCOHERENT light look like? In the above diagram 2A, incoherence instead would look like multiple pinholes and bunches of overlapped bullseyes. Lots of interference patterns, and probably with the nodes dynamically swerving around. Either that or it would look like fig. 2b but with bunches of light rays from multiple pinholes, and the rays all cross each other throughout the light beam. In both cases if the incorherent light was focused by a lens, we wouldn't produce any infinitely tiny hot spot. Can't punch holes in razor blades. With our gut-level intuitive understanding of Laser Coherence, we can now construct a basic list of coherent light sources Sources in increasing coherence Bright cloudy sky (least spatially coherent) Fluorescent tube lamp Frosted incandescent bulb Sun during clear weather Clear incandescent bulb Clear incandescent bulb w/noncoil filament (aquarium bulb) LED Electric welding arc 50ft away Laser (coherence-leng in MMs, up to a few Meters) Starlight (coherence leng 1000s KM) Note that the list also is a list of DEcreasing visible source-width, with the cloudy sky at the top and the distant stars at the bottom. As a little kid, did you believe that the light from clear incandescent bulbs was more magical than the frosted ones? And the light of garage welders was even more magical still? If so, you were intuitively experiencing optical coherence. Your little brain was wanting to mess around with laser sources, rather than overcast daylight. A perfect ideal pointsource gives perfectly coherent light, while a wide diffuse source gives the least coherent light. Turn the idea backwards: if we start out with perfectly coherent laser light, but then we send it through a frosted screen, the light remains just as monochromatic, but it becomes incoherent. Hey, I noticed that we can actually buy an incoherent-izer, an opto device for our optical bench. They're just a rotating frosted screen with a little motor (since an unmoving frosted screen still leaves a small bit of micro-scale coherence or "laser speckle.") NO JPEG YET Fig. 4 A frosted screen makes light incoherent. REAL SOLAR DEATH-RAY And now I have the answer to a big question that plagued me in childhood. No doubt all the nasty little science-boys like me had come up with this one. Why can't I make a death ray light-source? I could just get my big plastic fresnel lens and focus sunlight, and then somehow collimate it into a half-mm beam. The 0.50mm burning spot would appear anywhere along the parallel beam miles long. Write CHAIRFACE on the freakin' moon! But if we think about this now, it turns out to be impossible. Adding extra lenses to our solar furnace just creates a projector, where our parallel solar deathray spreads out and becomes a wide image of the sun. The darned sun isn't a pointsource. No thin beam is possible unless we include a tenth-micron pinhole in the optical path, and that turns the power into microwatts. The solution to the problem is simple: JUST REPLACE THE SUN WITH A 10KM WHITE DWARF STAR HA HAAAA! Keep the sun's brightness the same, but shrink the sun until it appears in the sky like a tiny star, like an extremely intense pinpoint. Now just use any big lens to gather a square meter of sunlight, focus it down to 1mm, then collimate it with a 1mm water-cooled short-focus quartz lens stolen from an ultraviolet microscope. Yes, the whole device is still a projector, but if we project the image of a pointsource into the distance, the result is an intense collimated beam. Other than a bit of diffraction it should work great: a few hundred watts in a parallel CW beam 1mm wide. Slice-a offs you fingas! Winston Kock, one of the early laser people at Bell Labs, said that laser light is "sharper light" which can be used as a cutting tool. Exactly, exactly! Winston Kock actually gets it. But the actual central concept is that coherence or "pinhole light" is the whole reason for the "sharp light" which does the laser-cutting. Lasers aren't particularly bright. Hundred watt light bulbs? 5,000 watt spotlights for school play?? Or daytime sunlight? If our sun was 10KM wide, or reduced to 10^5 times smaller in visual angle, then its light would be spatially coherent like lasers, or like an electric welding arc, and glancing upwards during the day might slice grooves across our retinas. The lens of your eye will focus the white-dwarf sunlight to a pinpoint rather than to a dim and safe little 0.3deg solar disk on your retina. Only because sunlight is non-parallel, because our sun is an extended source, our 1.5 KWatt/m^2 sunlight doesn't act like dangerous laser light. Hmmm, hold on a sec. If sunlight is about 1500 watts per square meter, and your eye's pupil is about 1mm, then your pupil intercepts 1500W/.001^2 = 1.5mW. DOH! WRONG! OK, staring at white-dwarf sunlight would actually be just like staring into a cheap laser pointer. Those things don't become really dangerous to human eyes until up around 5mW. AHA, but using binoculars would be bad, very bad: 5000X smaller exit aperture, creating an eight watt parallel beam 1mm in diameter. Binoculars become like icepicks aimed at your eyeballs. Coherent light can be nasty.

Incoherentlight

A single small light source sends out electromagnetic waves in all directions as shown above. Of course these diagrams are only two-dimensional, while the real situation is 3D. We can visualize a coherent wavefront to be spherical. The waves are like layers of a spherical onion, but where the onion is expanding at the speed of light, with new layers constantly added in the center. OR... we could imagine that the tiny light source is sending out a stream of particles flying off in all directions. The paths of these particles are the "rays" of light. Since they all fly outwards from a single point, none of the rays cross each other. And if this light is passed through a converging lens, it's focused to a perfect point. Coherent light is just some: Rays which never cross each other; parallel or radial Perfect wavetrains in 3D; nested sphere-waves or plane-waves

Coherentwave

So where is the equivalent power in the concept of "coherence?" How do I use those stacked-snakes to explain many other things? Where do the parallel wiggles clarify a radio antenna, a loudspeaker, or water waves? And if my laser isn't coherent enough to make holograms, can I draw a very simple picture of the problem's exact nature? A simple picture that any kid could understand? No. It just didn't connect.

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And even more important than all of the above... I realized that the in-phase emissions in lasers don't even create any "in-phase light" in the first place! [It's important enough to say twice: coherent light isn't created by in-phase stimulated emission. That's a big one.] In-phase emissions are important of course. But they only cause light amplification. They create amplified, brighter light. So what creates the coherence? I'll get to that, but first more about the error.

Because laser scanning microscopes use a photomultiplier (PMT) with a wide sensitivity range, targets that include areas of both high and low reflectivity can be measured accurately.

But lasers easily solved the problem because, right at the start, they create some spherewave "pinhole light," as if their entire light output came from a single virtual pinhole; a pinhole which is less than 500nM across. Aha, those confocal/concentric resonator mirrors, the ones used in lasers? This means that the "virtual pinhole" in an actual laser is just a non-virtual, very real pinhole-image sitting in the space between the mirrors. (See wikipedia diagrams for optical cavities, http://en.wikipedia.org/wiki/Optical_cavity). And all of those Semiconductor Lasers with parallel mirrors: they just employ an "infinite mirror tunnel" in order to place their pointsource at virtual-infinity distance, where it behaves just like the light from a distant star. During its trip down the infinite tunnel, all the non-planewave light wanders out the sides of the tunnel. Only planewave light can persist in the tunnel and get amplified.

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Light interference occurs when two waves of light converge, causing the amplitude of the now combined wave to increase or decrease. White light interferometers use this phenomenon to capture the 3D data of a sample. The figure on the right is a structural diagram of an interferometer. The light emitted from the source is separated into reference and measurement beams. While the reference beam is passed through the reference mirror through a beam splitter, the measurement beam is reflected and guided to the sample surface. The passed beam is reflected by the reference mirror to the CCD image sensor and forms an interference pattern. The other beam is reflected off the sample surface, passes the beam splitter, and forms an image through the CCD image sensor.

Do "textbooks" get Coherence completely wrong? Yes: grade-school textbooks, K-12 textbooks. Also articles written for public consumption, they get it wrong too. But the college textbooks, they're fine. They go into the rigorous details of partial coherence, and mixtures of spatial and temporal coherence, and they don't teach us that photons are like little sine-waves which can pack together like cardboard egg-cartons.

Coherent lightmeaning

Keyence's 3D Surface Profiler is also equipped with laser confocal scanning, which can achieve a lateral resolution of 0.13 μm 0.01 Mil.

Laser and ultrasonic technology for precision non-contact measurements. Outside diameter, ovality, wall thickness, concentricity, inside diameter, and more.

Prior to measuring, sample tilt correction must be performed using a goniometric stage. Tilted samples can cause closely-spaced interference patters, which hinder measurement accuracy. Some white light interferometry systems are equipped with a tilt mechanism that automatically corrects the sample tilt.

With light interference, if the surface does not reflect well, measurement is difficult to perform, limiting the type of targets that can be measured. Measurement also cannot be performed if there is an extreme difference between the reflected light from the reference surface and the reflected light from the measurement surface. Interferometers work well with mirrored-surfaces, but have difficulty measuring samples with extreme projections and depressions and samples whose surface is not very reflective.

As shown in the figure to the left, the interference pattern from a singlewavelength light source is the same for both upward and downward slopes, making it impossible to determine the direction. To solve this problem, the height is measured by capturing four interference stripe images as the objective lens and target are moved by λ/8 (1/8 wavelength) of the light source. This measurement method is called phase shift interferometry (PSI).

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From long to short wavelength, the EM spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma rays.

Since interferometers operate on white light, the lateral resolution of these systems will be the same as a conventional optical microscope - approximately 0.43 μm 0.02 Mil.