An LED is nothing like a classic incandescent light bulb. For starters, a light bulb heats up a filament to a very high temperature, which creates heat, and forces metal to emit light. This requires a lot of energy and a big power source. On the other hand, LEDs emit photons when electrical current passes through a semiconductor. The semiconductor material generates little to no heat during this process. This draws very little energy, and does not require much power at all. Additionally, a light bulb is non polarized. This mean that it will light up regardless of which way a DC electrical current is attached to its terminals. An LED on the other hand has a right and wrong way to connect electricity. If a DC current is wired in the wrong way, the LED will do nothing. Also, light bulbs are omnidirectional in light production, whereas LEDs are designed to emit light in a beam at a certain angle

You can place LEDs in series, but every time you do so, there is a voltage drop across the LED. This changes the amount of resistance required. For instance, if you calculate that one single LED requires a 220 ohm resistor, and then put 3 LED in series with this resistor, it is going to have too much resistance and be fairly dim.You need to decrease the resistance to maintain brightness because of the voltage loss throughout the circuit. With the formulas you already have learned for calculating voltage drops and the proper resistor, you should be able to figure this out. Or - you can do as I do - and use this online calculator.

A diode consists of a PN junction made of P-type silicon and N-type silicon separated by a depletion region. The depletion region acts like an insulator. Put simply, the P-region is connected to the anode, and the N-region is connected to the cathode. The depletion region sits between the two.

A lone prism could change the beam radius in a single axis, but this would also change the beam direction. Two prisms are required to maintain the beam’s original direction of propagation while manipulating its ellipticity. Anamorphic prism pairs maintain parallelism to the original direction, but they do displace the beam in the perpendicular direction. Using anamorphic prism pairs also requires precise angular alignment for proper functioning. It is not required, but it is useful for one prism to be oriented at Brewster’s angle, or the angle of incidence where there is no reflection of p-polarized light. The other surface of the prism would be at normal incidence to the beam and should be anti-reflection (AR) coated to maximize throughput. The precise alignment required leads many optical system integrators to buy them as a pre-aligned pair.

Because laser diodes diverge in an asymmetrical pattern, a spherical optic cannot be used to produce a circular collimated beam from a diode. The lens acts on both axes at the same time, which maintains the original asymmetry in the beam. Each axis can be treated separately using an orthogonal pair of cylinder lenses.

If you want to put different colors in parallel, each one needs it's own current limiting resistor. This is because each color of LED has it's own forward voltage, and forward voltages change depending on the type of LED and who manufactured it. It's important to always look up the forward voltage and calculate the correct resistance. For example, if you have 3 blue LEDs and 1 red LED wired in parallel, the three blue LEDs can share a single resistor, and the red one requires it's own different resistor. This is to ensure you don't accidentally destroy the red LED by giving it too much current, or under-power it by giving it too little.

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Cylinder lenses have more degrees of freedom than mounted anamorphic prism pairs, which makes them more challenging to align. Cylinder lenses can tilt, which makes anamorphic prisms more forgiving when attempting to align axes independently. Careful attention must also be paid to the focal length of cylinder lenses so that they are placed the correct distance away from the laser diode to produce a collimated, circularized output beam. Mounted anamorphic prism pairs are more user friendly. They are pre-aligned to a fixed expansion power, so eliminating the need to position and assemble them yourself like you would have to do with cylinder lenses. The prisms only have a single axis which must be independently aligned because the user is merely sliding the prism into the beam path. This removes an alignment step, saving the user both time and potential frustration. The physical location of the anamorphic prisms, relative to the incident laser beam’s location, is also less sensitive.

The small output apertures of laser diodes may lead to very large divergence angles, which can be challenging when trying to collimate the beam. Divergence has a direct effect on both the allowable length of the system and the required sizes of the lenses. The relationship between divergence and beam size is described in our Gaussian Beam Propagation application note. Because the relative positions of each cylinder lens are fairly fixed based on their focal length, it is possible to calculate the maximum beam width $ \left( \small{d} \right) $ at each lens using the focal length of the lens $ \left( \small{f} \right) $ and the divergence angle $ \left( \small{\theta} \right) $ of the axis that the lens is collimating. Each lens’ clear aperture must then be larger than the corresponding maximum beam width to avoid clipping the beam.

LEDs have different levels of brightness that are typically measured in MCD (millacandella). One thousand millacandella is equivalent to the brightness of one candle. So, an LED like the one pictured above with an intensity of 6,000mcd is equal to the brightness of 6 candles. It is not uncommon to also see extremely bright high-power LEDs to be measured in Lumens - another unit of light measurement - or Watts.

x represents position and v represents momentum. The uncertainty principle adds some limitations to the design of beam shapers. For example, for a design with very well-defined position, the spatial frequencies become less defined. Applying the uncertainty principle to diffraction theory, i.e. the Fourier transform relation in the Fresnel integral, a characteristic parameter $ \beta $ is obtained:

An LED can be turned off using a switch placed in series between it and the power supply. This does not have to be just a basic toggle switch. You can explore specialty switches such as tilt or, magnet switches. You can learn all about switches in the Switch lesson of the Electronics class.

However, should you want to calculate the proper resistor for maximizing brightness, you can calculate this by using this equation. Even more simply, you can search online for "LED resistor calculator."

Can a single chipLEDproduce white light

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However, the extra degrees of freedom offered by cylinder lenses result in more flexibility, which might be useful in research settings and prototyping. Cylinder lenses can also provide higher throughput than anamorphic prism pairs, especially when the lenses feature AR coatings. Light does not need to travel through as much material in cylinder lenses compared to anamorphic prism pairs, and p-polarized light will be lost if anamorphic prisms are used at Brewster’s angle. For more information, please visit our Anamorphic Prism Pairs application note.

SMDLEDpolarity

So far, we have discussed shaping light using field mapping or beam integration where diffraction effects play major roles in the design and performance of the optics. Diffraction is the deviation of light from propagating in a straight line that is not caused by reflection or refraction. These diffraction effects cause laser beams to diverge as they propagate. On the other hand, a beam whose profile is described by a Bessel function, which is defined as the exact and invariant solution of the Helmholtz equation, does not experience diffraction; i.e. it does not spread out as it propagates.4 These beams are also self-healing, which means that they can reform at any point after obstruction. However, ideal Bessel beams are impossible to generate because they would require an infinite amount of energy. Instead, approximate Bessel beams knowns as quasi-Bessel beams can be generated by interference of plane waves formed by a conical surface such as an axicon.

In order to overcome the depletion region, a little bit of voltage must be sacrificed. This is called the voltage drop. In a standard silicon diode like the one pictured this is typically 0.7V. However, and LED can have a much higher voltage drop. A typical voltage drop in an LED is 1.8 to 3.3 volts, and this varies by color. In other words, if you have a 5V signal and it passes through an LED with a 1.8V drop, the voltage that comes out the other end will be 3.2V. It can fluctuate above or below this value depending on the type of LED.This drop in voltage is what is referred to as an LED's "Forward Voltage."

When a positive voltage is applied to the P-region and the N-region is connected to ground, the depletion region all but disappears and allows electricity to flow. In this state the diode is considered forward biased.

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If you have three diodes in series, you will lose 0.7V through each diode and the voltage at the far end of this chain will be 2.9V. If you consider LEDs offer a voltage loss more than double this amount, connecting three in series would add up to a significant loss, and is the reason LEDs should be connected in series sparingly.

Like standard spherical lenses, cylinder lenses use curved surfaces to converge or diverge light, but they only have optical power in one dimension. Cylinder lenses do not affect light in the perpendicular dimension. This cannot be achieved using standard spherical lenses because light will focus or diverge uniformly in a rotationally symmetric manner. This property of cylinder lenses makes them useful for forming laser light sheets and circularizing elliptical beams.

In order to understand LEDs, you need to first understand what a diode is.A diode is an electronic component that allows electricity to flow through in one direction, and all but stops it from flowing the opposite way.A diode's primary role is to route electricity within a circuit. This is extremely useful for preventing an electrical signal from taking unwanted or unexpected routes or flowing in the wrong directions.

Another type of laser beam shaping is circularizing a beam, which involves converting an oval or differently shaped profile to a circular one. Laser diodes with no collimating optics will have different divergence angles in the x- and y-axes because of the rectangular shape of the diode’s active region, resulting in oblong beam shapes (Figure 7). Circular profiles are often desired to form symmetric, compact final focused spots.

Multicolor or "RGB" LEDs are individual red, green, and blue LEDs built into a single LED package. Typically, each color has its own anode, and they all share a common cathode. By varying the voltage on each anode, the amount of red, green and blue light being emitted can be varied. By mixing these lights together, you can create nearly any color in the visible light spectrum. These LEDs are miniaturized and attached to the multicolor LED strips we will encounter in the next lesson.

High-end laser diodes often have anamorphic prism pairs built into their laser head to circularize beams. Many lower cost diodes, however, do not. The cost of purchasing a separate anamorphic prism pair and a less-expensive diode lacking an integrated anamorphic prism pair can be less than that of a more expensive diode.

One last thing, if there is a million or more ohms, we then measure in mega-ohms. For example, this resistor is worth 1,000,000Ω. This is shortened to 1MΩ.

Determining how much resistance a resistor offers is a little trickier and can be established by deciphering the colored stripes from left to right towards the tolerance marking. You will typically see four stripes, but you may also encounter resistors with five.Resistors with four stripes are the most common. These will likely be the type you are working with most.When reading a resistor with four stripes, the first two stripes are combined together to form a number between 1 and 99. The third marking is the multiplier. The last marking determines the tolerance, which is basically the accuracy of the resistor and not typically important to know about when working with LEDs. If you would like to learn more, check out the Resistors lesson in the Electronics Class.

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LEDs come in different shapes and sizes. The 5mm domed is the most common, but you are liable to also find them in 3mm domed, 10mm domed, rectangle, oval, and square (to name a few).

A laser beam shape is typically defined by its irradiance distribution and phase. The latter is essential in determining the uniformity of a beam profile over its propagation distance. Therefore, beam shapers are designed to redistribute the irradiance and phase of an optical beam to attain a desired beam profile that is maintained along the desired propagation distance. Common irradiance distributions include Gaussian, in which irradiance decreases with increasing radial distance, and flat top beams, also known as top hat beams, in which irradiance is constant over a given area (Figure 1). A detailed description of Gaussian beam propagation can be found in our Gaussian Beam Propagation application note and information on quantifying the quality of a laser’s irradiance distribution can be found in our Laser Resonator Modes application note.

Finally, go to the "Blink" example code and change all the number 13s to the number 7. By doing this, you are simply changing the digital pin that is being pulsed on and off from 13 to 7 in order to match the circuit you built on your breadboard.

Aside from blinking an LED, we can also make one fade. To do this we need to use a PWM pin. The PWM pins are special digital pins on the Arduino that allow for an analog-like output that simulates an output voltage between 0 and 5V. They are all labeled with a ~ in front of the pin number.PWM stands for pulse width modulation. Put simply, PWM is toggling a pin on and off so fast that it gives the appearance of dimming the LED. A dim LED glowing at 1/4 brightness means that the signal being sent to it is toggled off much more than it is toggled on. For instance, it is turned off 75% of the time and turned on 25% of the time. A brighter LED at 3/4 brightness is receiving a PWM signal that is the opposite. So it would be on 75% of the time and off 25%. The thing is, it is happening so fast, you don't see that it is being turned on and off, but only experience the LED as being slightly dim.Anyhow, if you want to fade the LED, from the examples menu select:03.Analog --> FadingOnce done, swap the wire connected from digital pin 7 to digital pin 9, and upload the code to your Arduino.The LED should now fade in and out.

Now, let's say the first two numbers were to change, and the multiplier were to decrease. In this example, the first two colors when combined create the number 68. When multiplied by 10, we get the number 680. Since 680Ω is less than 1,000, we just call this resistor 680Ω.

LEDanode cathode diagram

The reference system of cylinder lenses is defined by two orthogonal dimensions: the power direction and the non-power direction. The “power direction” runs along the curved length of the lens and is the only axis of the lens with optical power (Figure 8). The “non-power direction” of the cylinder lens runs along the length of the lens without any optical power. The cylinder lens’ length along the non-power direction can vary without affecting the lens’ optical power. Cylinder lenses can have a variety of form factors including rectangular, square, circular, and elliptical shapes.

Like resistors, diodes also need to be interpreted based on their packaging.There are typically three ways to tell a standard 5mm LED's anode from its cathode.1) The leg connected to the anode is typically longer than the one connected to the cathode.2) The body of the LED typically has a flat spot on the cathode side. 3) If you look inside the LED, the little metal bit connected to the anode lead is much smaller than the cathode.

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The ratio of both lenses’ focal lengths should match the ratio of the x and y beam divergences in order to achieve a symmetrical output beam. Similar to standard collimation, the laser diode is placed at the shared focal point of both lenses and the distance between the lenses is kept equal to the difference of their focal lengths (Figure 9).

Axicons form a quasi-Bessel beam with nearly zero diffraction over a given region, known as their depth-of-field (DOF). After this region, the beam continues propagating in a ring-like pattern (Figure 5). Traditional, refractive axicons are considered either conical lenses or prisms. Light transmits through them and then refracts at the conical surface. Reflective axicons with a reflective conical surface are also employed in certain situations such as ultrafast laser systems. The broad wavelength bandwidth inherent to ultrafast lasers would experience significant chromatic dispersion transmitting through a refractive axicon, while this dispersion is avoided in reflective axicons (Figure 6). Quasi-Bessel beams can also be generated using holographic methods with high diffraction efficiencies but suffer from a diffraction-modulated axial profile.

While diodes charge a toll to cross the depletion region in the form of voltage, they offer no real resistance. If you put only diodes in a circuit without a load to use up the electricity, it will virtually look like a short circuit and draw as much current as the power supply is able to provide. Since that is likely higher than the diode's maximum current rating, it will release the diode's magic smoke.

If the LEDs are all the same, they can easily be put in parallel to your heart's content - well, within reason. You need to keep in mind how much current they are drawing in relation to how much current your power supply can provide.

LEDpolarity marking on PCB

Bessel beams experience little to no diffraction within their propagation distance and provide an excellent DOF, which makes them ideal for applications such as laser material processing and corneal surgery. Clean cuts with sharp edges can be generated in the DOF because of the uniform beam diameter.

LEDs also draw different amounts of power. In fact, some high power LEDs draw so much power that they are mounted on metal heatsinks to dissipate heat. While LEDs such as these tend to be very bright, they sometimes require special constant current circuitry to drive them.

As you can see from this extreme closeup of an LED pictured above, it is actually quite different from a light bulb. LED is an abbreviation for light emitting diode. It is a special type of electronic component that emits photons when electricity flows through it in the right direction. Over the last two decades, LED technology has changed and improved dramatically. LEDs now come in so many countless types and configurations, it would be impossible to survey them all in this class. On account of their versatility and pervasiveness, you could say that LEDs currently shine brighter than all other lights. Let's see why.

LEDs have different viewing angles, or beam widths. What this means is that the visible brightness of the LED seems to decrease when you are looking at the LED from and a spot outside of its ideal viewing angle. This angle also determines the size of the spotlight created by the LED. The viewing angle on an LED can vary widely.

LEDs can come grouped together into display modules. With these LED dot, bar, and 7-segment numerical displays, each individual light-up segment is a discrete LED. For instance, the 8X8 matrix on the left actually has 64 separate LEDs inside of it.

To power an LED, you simply connect it to a battery pack in series with an appropriate current limiting resistor. As a general rule of thumb, a 470 ohm resistor is typically more than enough resistance for any 5mm LED that is being powered with 9V or less.

All diodes are polarized. This means they have an anode (positive side) and cathode (negative side). You can tell the difference because the cathode has a little line painted around it. What this means is that electricity can only flow through in one direction. A positive voltage should be connected to the anode and the cathode should be connected to ground.

LEDpositive negative symbol

When the P-region is connected to ground and the N-region is connected to a positive voltage, the depletion region actually grows in size. This ensures little to no electricity is able to flow through the diode between power and ground. In this configuration the diode is considered reverse biased.

In low-performance systems where cost is a driving factor, Gaussian beams may be physically truncated by an aperture to form a pseudo-flat top profile. This is inefficient and wastes the energy in the outer regions of the Gaussian profile, but it minimizes system complexity and cost.

For instance, given this LED with a 3V voltage drop (forward voltage), 20mA operating current, and a 9V source, we can calculate that the proper resistance is 300 ohms. However, that is the absolute minimum resistor, and since resistors tend to have a tolerance range, it is best to increase the value a little to be on the safe side. It is safe to say then that a 330 ohm resistor should do the job. However, you don't want to increase it too much because the more resistance there is, the dimmer the LED becomes.

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Resistors with 5 stripes are a little less common, but just as easy to read. Let us briefly consider how to read them. Like the 4 band resistor, you first find the tolerance marking on the far edge, and then read left to right towards the marking. However, where they differ is in that the first three stripes get read as a single number, and the fourth stripe is the multiplier. So, in this case, we can determine the first number is 100 and it gets multiplied by 1,000, giving us a resistance of 100K. The fifth stripe is the tolerance marking.

If you already have an understanding of Arduino, controlling LEDs is easy.To blink the on-board LED connected to pin 13, simply open and upload the following example code:01.Basics --> BlinkIf you know how to use Arduino, chances are you have already done this a long time ago.

LEDDirect

How to identify positive and negative terminal ofLED

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Beam shaping modifies the properties of light at their most fundamental level and its efficiency is determined by Heisenberg uncertainty principle on time-bandwidth:1

If that was confusing, let us look at another example. This resistor has the same initial number of 10, but a multiplier of 1,000. When multiplied together, these numbers yield a resistance of 10,000Ω or 10KΩ.

Higher-performance applications requiring more efficiency often employ either refractive and diffractive laser beam shapers. These assemblies typically utilize field-mapping phase elements, such as aspheric or freeform lenses and diffractive elements, to redistribute the irradiance and phase profile of laser light. Figure 3 shows an example layout of a refractive field mapper that transforms a Gaussian beam profile to a flat top profile through wavefront distortion and the energy conservation condition.2 The amplitude and phase of the incident beam are changed after passing through both elements in a Galilean or Keplerian lens assembly. The resulting beam shaping is highly efficient (>96% throughput) and wavelength-independent within the range of the design. Refractive beam shapers allow for uniform irradiance distribution and flat phase fronts.

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Anamorphic prism pairs are other types of optics used for circularizing elliptical beams. Anamorphic prism pairs consist of two prisms used together to reshape a laser beam. They are typically used to change elliptical beam profiles into circular distributions, but they can also produce other elliptical beam profiles in a variety of sizes. The optical principle at play behind the reshaping is the same as that of cylinder lenses: refraction. Light is bent in one direction, or one axis, while the other axis remains constant (Figure 10). This compensates for the beam’s different divergence angles.

Adjusting the brightness of an LED can be adjusted quite simply using a 1K potentiometer in series with its current limiting resistor. This adjustable knob will sweep the resistance between 0 and 1K ohms. The addition of extra resistance will cause the LED to dim as the resistance increases.We can also replace the potentiometer with any variable resistor, such as a photocell to make it light controlled.To learn more about potentiometers and photocells, once again, I recommend checking out the Resistor Lesson of the Electronics Class.

A laser beam integrator, or homogenizer, is composed of multiple Ienslets that divide the beam into an array of smaller beams, or beamlets, followed by a lens or other focusing element that superimposes the beamlets at the target plane. They can be used with both coherent laser light and other incoherent light sources. Typically, the final output beam profile is the sum of the diffraction patterns determined by the lenslet array. Most laser beam integrators are used to generate a homogenized flat top profile from incident Gaussian beams. Beam homogenizers usually suffer from random irradiance fluctuations, which leads to beam profile that is not perfectly flat. Non-diffraction-based beam integrators, such as imaging integrators or waveguides, are also suitable for spatially incoherent incident light. The choice between diffraction or imaging beam integrators depends on the Fresnel number. As a rule of thumb, for Fresnel numbers <10, an imaging integrator will be needed to obtain a highly uniform flat top profile.3

LEDs also come in many different colors. Often the plastic is tinted to indicate what color they are. However, clear LEDs are deceptive in that you might assume they glow white, but can actually glow a host of different colors.

If you would like to blink an external LED, insert an LED into a breadboard. Connect a 150 ohm resistor in series with the LED's cathode. Using a black solid core wire, connect the opposite end of the resistor (the side not connected to the LED) to ground on the Arduino.Using a red solid core wire, connect the LED's anode to digital pin 7 on the Arduino.

Polarized LEDlight

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However, focusing a flat top beam through a lens will not result in a flat top profile at the final focused spot, as the lens will affect the beam profile. When a flat top focused spot is desired, field mappers are instead used to convert Gaussian beams to collimated Airy disk profiles, which form flat top spots after being focused by a diffraction-free lens (Figure 4).

Some applications benefit from beam profiles different from that of the laser source, which is typically Gaussian. For example, flat top profiles are advantageous in applications such as certain materials processing systems because they often result in more accurate and predictable cuts and edges than Gaussian beams (Figure 2). However, introducing a beam-shaping optics increases system complexity and cost.

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Before we dive too deep into LEDs, it is important to understand a bit more about how the anode and the cathode actually work. While this is going to get a little bit technical, it will be important for understanding LEDs later on.

where $ \small{r_o} $ is the input beam half-width, $ \small{r_i} $ is the output beam half-width, $ \small{C} $ is a constant, $ \small{\lambda} $ is the wavelength, and $ \small{z} $ is the distance to the output plane. The value of $ \small{\beta} $ is very important when designing or considering a beam shaping application since larger values correspond with better beam shaping performance. For example, for $ \small{\beta} < 4 $, the beam shaper will not produce acceptable results for essentially any laser application, while a $ \small{ 4 < \beta <16} $ would provide low performance. Hence, for optimal performance, experimental conditions that lead to $ \small{\beta} > 16 $ should be employed. This formula implies that it will be more straightforward to design beam shapers for larger beams, shorter wavelengths, and shorter focus distances.

Diffractive beam shapers utilize diffraction, rather than refraction, to shape the laser beam into a specific irradiance distribution. Diffractive elements employ an etching process to create a specific micro- or nanostructure in a substrate. Typically, the design wavelengths and function of the element are dependent on the height and zone spacing, respectively. Hence, using a diffractive optical element at the design wavelength is essential in order to avoid performance errors. Compared to refractive beam shapers, diffractive elements are also more dependent on alignment, divergence, and the beam position in the plane of the nominal working distance. On the other hand, diffractive optical elements are very advantageous in space-limited laser setups since they are usually made of a single element, rather than multiple refractive lenses.

Since an LED is basically a diode and offers no resistance in a circuit, it typically requires a component called a resistor (that offers a fixed amount of resistance) in series with it.

For instance, in the following example, the first two lines represent 1 and 0, which is combined together as the number 10. This is then multiplied by 10,000 (which is the multiplier). The result is 100,000Ω. However, when a resistor is 1,000 or more ohms, we measure it in kilo-ohms. A kilo-ohm is basically equal to 1,000 ohms. So, 100,000Ω is shortened to 100kΩ. Basically, it is 1,000 ohms times 100. All we are essentially doing is removing three zeros from the number, and replacing them with with k.

Resistors have their value printed on them in color codes. To begin with, you can probably just get away using an online resistor calculator to determine their value. If you chose to go this route, you may skip this section.However, if you are curious on how to decipher them yourself, read on. I'm about to show you. Telling its current rating can typically be established by the size of the resistor. This is something you will figure out intuitively in time, and not remarkably important for the kind of low-current circuits you will be working with when getting started.

If you look very carefully inside of an LED, you will be able to see its anode and cathode. The thin wire bond attached to the anode bridges across to the center of a small reflective bowl attached to the cathode. In the center of the reflective bowl sits the semiconductor die. When current flows from the anode to the cathode, the semiconductor material emits photons, reflects off the bowl, and is further amplified by the resin material of the LED.