Home Theater Fiber Optics - fiber optic lighting for homes

 2024-08-15 18:26:49

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Numerical apertureof optical fiber

Where ‘n’ is the refractive index of the medium between the cover glass and the objective front lens (e.g. air, water or oil).

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The particle size, weight of compression, temperature, and length of time are regulated to maintain the same finished product result from batch to batch.

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To simplify this further, as light travels through one medium to another, it changes speed (e.g. when passing from air to water, light slows down). When light passes across the boundary of two different medium at an angle other than 900, this results in a change of direction. Although the frequency of light doesn’t change, the resultant wavelength will be determined by nature of the medium.

All the Spectralon I have is rather easily scratched. Minor scratches and nicks can be sanded under running water. I usually just wet the sandpaper, put it on a table, flip the Labsphere disk face down on it and gently rub across the sandpaper. Of course, not all scratches and nicks will be completely removable depending on their depth. A deep gouge could affect the reflectivity and susequent use of a white balance tool because a deep gouge will photograph darker.

I actually have not tried it outside in the wind, so I was over thinking how it does in the wind, just a guess, and it is not toxic.

If that was clear as mud, kindly advise and I will try again. :D If it is irrelevant or you already know it, kindly ignore.

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So, for those that have these things, how easy are they to clean, and what should I not do - scrub too hard, leave to soak, etc, etc?

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Are these needed for everyday UV photography - in my opinion, not really. A generic PTFE tile would be good enough as a white tile, an Xrite black tile would do the job of a black tile in UV. Getting 'in between' grey tiles would be more difficult, but making your own could be done for minimal outlay, and they would be good enough for most photography. I suppose it depends on how accurate you need it and what you want to do with the data. Most of what I'm doing at the moment is for publication, so for me, these are necessary. Would I have been able to buy them new? No, far to expensive. Would I buy them if I get the chance for a good deal? Absolutely.

All, thanks for the info. I've not heard back from the vendor yet, so I am hoping the purchase went through ok. Yes, I've been on the Labsphere website now and checked what they say about it, so between that and info here I'm in a good place.

A couple of years ago my Spectralon rectangle (5"x5") got swiped by a ballpoint pen when I was out working, making notes and trying to swat away various little insect annoyances. I did manage to remove the ballpoint ink by careful sanding, but you can still see the indentation from the ballpoint. I think I was upset for about a year about that ballpoint incident !! But you know.....if you have tools, then you must use them and when using them, well, sometimes stuff happens. So I finally let the distress go. B)

Numerical apertureof lens

My experience with handling it and fabricating it, is that it is very slippery in sheet form, and sintered felt has a rubbery feel to it.

Andy, unfortunately, for some things there is a need. I'm currently writing a paper on some of my UV work and it would have been great to have a range of UV suitable grey tiles for one part of it. Spectralon is optically neutral, so reflects the same in UV and visible (and IR if that's needed) so is a great calibration material. Cost is a problem, but then it often is for calibration materials. They are eye wateringly expensive to buy new, but I try and keep an eye out for second hand equipment from lab clearances. If the sale goes through ok, fingers crossed, I should be getting them for about 15% of the new price.

I have a 4 set, but I have not used it because I don't want to get it dirty until I know how to clean it from the inside out.

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It is my impression that the darker grey standards are more easily contaminated than the white and light grey standards? The darker ones may be a bit more porous??? (Not sure.) The black standard does seem ever so slightly grittier. Labsphere does recommend NOT sanding the black one.

The powder is not going to jump out of the container, it will sit there in the container unless you scatter it all over. A lid keeps it sealed up.

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I know a house window washer, he has a huge RO unit on his truck, uses nothing other than pure OR water. He told me that it is the best thing to clean windows with because the RO water alone removes dirt better.

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Numerical apertureunit

However I have pure virgin sheets of it, and they do get dirty to some extent, and things in the air can attach them selves to all sorts of things.

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To be honest I have no idea whether Spectralon is PTFE. I've seen it described as a fluoropolymer, but whether that means it's chemically just PTFE I don't know. Presumably the one with 99% reflectance is pure fluoropolymer, but again having not chemically analysed it myself, that's speculation on my behalf. As for the ones with different reflectances, I'm guessing they add in different amounts of carbon black to the mix before making it - more carbon black, the blacker it is. I asked Labsphere once about making me a custom set with reflectances between 1 and 20%. The cost would have been enormous for anything which wasn't standard in their range, as they'd have to make a large batch even if I wanted a single disk.

So, for those that have these things, how easy are they to clean, and what should I not do - scrub too hard, leave to soak, etc, etc?

Numerical apertureof objective lens

Because Spectralon is a thick porous mass, it seems to me that it may be possible that dirt in the air, smoke, whatever, might infiltrate the porosity of the substrate, and populate the matrix.

I use a much finer sandpaper grit than is recommended by Labsphere. I'll go look it up and get back to you on that. Also be sure that your sandpaper is not dyed in any way. I had some kind of reddish sandpaper once which seemed to release reddish dye when used wet. Not good for Spectralon. :D

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I should mention that the actual reflectivity does vary by some small amount for Spectralon standards. Mine were calibrated, so I know what that variation is. Variation has to be within a narrow range for a standard to be considered a standard. (we all know this....)

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You are over thinking it, because there is way worse dust that is actually bad for you, and we would not be using this in any way that would scatter it around and get it into our environment at all.

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There is an inverse relationship between the angular aperture and the working distance of an objective. I have covered working distance in my article entitled ‘Looking Down and Through: Microscope Optics 3: Oil Immersion Objectives’, but to briefly recap, the working distance is the actual distance (in millimetres or microns) between the objective front lens and the surface of the cover slip when the object is in sharp focus. Objectives with short working distances will consequently have a greater ability to gather more oblique light rays from a specimen compared to longer working distance objectives. Angular aperture is usually determined by the optics within the objective and each objective lens will have an optimal focal length and working distance- it can’t simply be increased by moving the objective closer to the slide!

In my opinion, the main functional difference between sheet PTFE and sintered PTFE is the lack of reflection. It is essentially the difference between shiny paint and flat paint.

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Metaphorically, think of it this way: if you are standing in front of a door with a key hole which leads into another room, then when you are at a distance, you will only be able to see a little of the light and objects within the room. If you press your eye against the key hole, you will then see more of the detail and light in the room as you have, in theory, increased the angular aperture of your eye.

I mentioned this only as a curiosity. When using a reflective standard to judge reflectivity of a photographic subject under a given illumination, it is NOT necessary to physically adjust your photo brightness (HSB model) to "match" the reflectivity of the standard in use. I assumed everyone would understand this, but it is important to clarify this for readers who are not familiar with reflectivity standards.

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Because of the "gamma" curve applied to raw file data, it is also equally sometimes impossible to reproduce in a photograph an "equally spaced" range of tones corresponding to the reflectivity of the 2/25/50/75/99 standard.

Spectralon is PTFE, but powder and other forms of PTFE might actually be a better material, only in the sense that they are more easily and inexpensively replaced with new fresh clean structural depth.

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Jonathan, if you scroll down in Post #34 you can see the deterioration of my Spectralon 25% reflective standard. Nothing lasts forever! I've had the standards since 2012. They have been used under botanical field conditions which can vary from desert dust to wet fields to windy pollen laden gardens. I do not know what specifically happened to the grey standard. I have had good use from my standards, so if I need to replace one every 5 years, that is not a problem. Using the standards has taught me a lot.

Labsphere has put them thru some pretty rugged tests. Example: left some outdoors for a year or so. After a clean-up, the Spectralon reflectivity was the same. (This does assume no contaminants like dye, oil, etc.)

The NA of an objective is an important aspect as it relates to the final image formation seen when looking down through the eyepieces (which will be covered in full in a forthcoming blog article). Briefly, resolution relates to the amount of detail which can be seen in the final formation of an image. An objective with a high magnification would be unable to resolve detail in your sample without a similarly high NA.

Numerical apertureof microscope

Here's an example: I photograph my 50% reflectivity standard in a scene. I measure the brightness (HSB) of the 50% reflective standard in the photograph and find it to be, say, 65%. I then look for some subject with 65% brightness in the scene and observe that that subject also has 50% reflectivity under the conditions of which I made the photo.

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Dust, dirt and lint can be rinsed off Spectralon standards. They are very hydrophobic. I pat them gently dry with a microfiber cloth and let them stand for awhile before putting the lid back on. The purists amongst us would use distilled water for rinsing Spectralon. I admit to using my local tap water once or twice, so don't tell Labsphere, please.

In summary, without a correspondingly high NA, a high magnification objective will have low resolution. Most microscope companies offer objectives which have high NA values for use with immersion medium. If you are in the lucky position of buying a custom microscope, or buying new objectives for your existing instrument, you should always consider buying objectives which offer the highest NA value which you can afford.

It isn't toxic, they use PTFE powder for lubricating all sorts of things, including pianos and other musical instruments.

I also have Spectralon standards for investigations in documentary botanical photography. Bjørn and I needed something to use to determine whether we could obtain similar standardized results across different UV platforms (camera + lens + filter + illumination + converter). Yes, we can obtain very similar results across different UV platforms but certainly not identical results. But close enough for all practical purposes of displaying an sRGB JPG online. :D

Does anyone have a link to the Labsphere instructions for cleaning the Spectralon? Maybe someone already posted that? I didn't find it.

Numerical Aperturecalculator

Ok, this could be a great thing or a ridiculous purchase. I just put in an offer and won a set of 8 Spectralon diffuse reflectance standards from Labsphere on a well known online auction site. 2 of the standards are described as having "visible marks, roughly 1". These are likely removable, as standards are also fully washable per the manufacturer's website (see instructions)". I see this as a bit of a risk, hence I put in a low offer which to be honest I was surprised was accepted.

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The light from the microscope source passes through the specimen/slide and continues through the air (or an immersion medium) as a cone of light between the cover glass and the objective front lens. The ‘angular aperture’ refers to the maximum angle of the edges of this image-forming cone of light which can be collected by the objective front lens when the specimen is in focus. In addition to an increasing NA, image brightness and image detail (resolution) are also related to the angular aperture.

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Numerical aperture is proportional to refractive index. For example, air has a refractive index of 1.00, water has a refractive index of 1.33, whereas many of the immersion oils have refractive indexes around 1.52.

I've just been in correspondence with Labsphere about making me a 5x5" target with 5 strips. It would be awesome to have.

I've looked a PTFE alone for reflectance, between UV and IR. It's certainly good enough for most work, but the one I had wasn't as optically neutral as Spectralon across the range of wavelengths. Would it work as a white tile for UV work, yes absolutely. However the complexity comes when trying to get sample which have controlled reflectances which aren't 99%. For my work I needed ones between 0 and 20%. I needed as many as possible in that range to try and develop a calibration curve. In the end I made them from carbon black, magnesium oxide and plaster of paris. It cost me £10 to by the raw materials and took a couple of months to make them once I had settled on a consistent method, and I made 6 targets in that range, enabling me to test my theory. They weren't as optically neutral as I would have liked as reflectance changed a bit with wavelength, but they did the job I needed of them. At the end of the day, if I need to validate what I make against something standard, I need to get my own standards. Even just having a device to measure reflectance, I still need standards to verify what it is telling me. In theory I'd need to get my standards re-verified every year (or however long is recommended) if I'm using them to support other peoples research.

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Numerical apertureformula

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NOTE: They recommend using 220-240 grit waterproof emery sandpaper. I was MUCH happier using a 400 grit sandpaper. The 240 seemed to dig too easily.

As a side note, PTFE is funny stuff. Yes it is repellent, but once it gets dirty it is not always easy to clean. I worked with low surface energy materials in my PhD and post doc days. PTFE exhibits hysteresis in its repellency. Once wetted, its not as easy to dewet. Some of the materials we were making in the lab had much lower surface energies than PTFE. We were making stuff with water contact angles around 170 degrees, and it would even repel oils. Still a fluorinated polymer, but a different surface structure.

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I enjoy attempting to match brightness to reflectivity in some of my photos because I'm curious just how light & dark the tones "really are" in a UV photo.

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The main concern, like any white balance product, is to keep the material clean and free of other materials that would change the purity and cause an alteration to the original pure white balance.

That is what I would use, RO water, It has a tendency to be 'hungry water', because it is so clean it attracts things to it. Unless they advise against using that.

The Spectralon standards are also useful for determining exposure range of various platforms. Sometimes it is impossible to get the 2% black and the 99% white reflective Spectralon standards equally well exposed in one shot. (I hope that is no surprise to anyone.) Because of the "gamma" curve applied to raw file data, it is also equally sometimes impossible to reproduce in a photograph an "equally spaced" range of tones corresponding to the reflectivity of the 2/25/50/75/99 standards. So you need to decide when using Spectralon standards how you want use them and how you are going to interpret the results. [EDIT: Please see clarification of this rambling of mine in Post #13 below.]

The theoretical maximum angular aperture of light entering the front lens of an objective is 1800. This would give a θ value of 900 (half of the angle of the light cone). As a result, the theoretical maximum NA of an objective would be one (which is equal to the sine of 90). The refractive index of air is also one, therefore the maximum (theoretical) NA of an objective with an air gap between the front lens and the specimen would only equal one.

The powder is compressed at at the same temperature for a given time, which adheres the particles together, but doesn't completely fuse them,

Of course PTFE (Teflon) is notorious for being a "non stick surface" (I never cook on it myself, I don't have any cooking equipment using it), but it is supposedly food doesn't get stuck to it, and it is easy to clean.

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It is sintered PTFE. Starting with fine virgin PTFE powder, it is put under a specific compression, in a heated oven with a specific temperature.

One of my dark grey standards has begun to look spotty and may have to be retired. I don't recall what is added to make the grey standards or how that colourant might affect them. I should pick up the phone and ask Labsphere about it. They bear no blame for the spotty grey one because my using these standards "in the field" like I have done for the last 5 years is very hard on them. Pollen is a very sticky contaminating substance on both sensors and standards! Generally I'm pleased that my Spectralon items are holding up so well.

Yeah, then I guess you should probably take a spectrum after cleaning, so you can see how close it is to the manufacturer spectrum?

To help to understand NA, it is useful to also have some understanding of refraction. In microscopy and optics, refraction refers to the change in direction of light waves which results from a change in the medium though which light passes (for example, glass, air, oil or water).

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The NA of an objective is the simply the ability of the lenses to collect light at a fixed distance from the sample which you are viewing. When light passes through and leaves a specimen, it enters the front lens of an objective as an inverted cone. However, a percentage of this image-forming light is refracted and reflected. Objectives which have a high NA allow for increasingly oblique light waves to be collected by the front lens which will in turn form a final image which is not only relatively brighter, but contains more information and detail and is highly resolved.

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Snell’s Law describes the relationship between the angles of incidence and refraction of light as it travels through the boundary of two different medium (e.g. from air to glass). The Law states that the ratio of the sines of the angles of incident and refracted light are equivalent to the reciprocal of the ratio of the refractive indices through which the light passes.

Yeah, then I guess you should probably take a spectrum after cleaning, so you can see how close it is to the manufacturer spectrum?

Numerical aperture nacalculator

Therefore, the refractive index of air is a limiting factor in achieving the highest possible NA of an objective. As a result, objectives with NA values greater than one are the immersion objectives where the air gap is replaced by a medium such as water or oil. An angular aperture of 1800 is physically unachievable- the widest angle of light which can be collected by an objective is around 1440. Consequently, the maximum achievable NA of a non-immersion objective is approximately 0.95 (which is equal to the sine of 72).

Powdered PTFE works great for white UV white balance, except that it is not too convenient, because you have to be careful not to spill it, and it can blow away also,

Refraction is described in a formula known as ‘Snell’s Law’. Refraction was first described in the year 984 by a Persian physicist and mathematician called Ibn Sahl. In 984, he presented a manuscript in which he described how mirrors and curved lenses focused and bent light. Snell’s Law is actually named after a Dutch mathematician and astronomer called Willebrord Snellius (1580-1626). Although he was credited for mathematically describing refraction, it is more accurate to say that he ‘rediscovered’ diffraction after the work of Ibn Sahl.

Etched onto the barrel of each objective on a microscope, you will find a variety of information. In addition to the magnification and the optical correction (see my article published entitled ‘Looking Down and Looking Through: The Optics of a Microscope 2: The Objectives’ for more information on aberrations and corrections), you will find a number without units. This is the Numerical Aperture (or ‘NA’) of the objective.

The stuff may get dirty on the inside, below the surface, and the whole point of the structure is to give depth to the whiteness.

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Another potential problem with Spectralon is that it can become contaminated with hand oils or other organic substances or chemicals which may not be removeable. So don't touch the Spectralon directly with fingers/hands. Try not to spash coffee or beer on it. And so forth.

Where ‘θ’ is half of the angle of the cone of light which is collected by the front lens lens (i.e., the angular aperture).