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Light-Emitting Diodes (LEDs)


LEDs are a type of diode that are capable of transforming electric current into light. They are used in a wide variety of applications, as they provide for an efficient and inexpensive way of generating light.

The Many Application of LEDs

Diodes (and transistors!) have been a fundamental part of the evolution of the digital electronics world.  In fact, LEDs are a type of diode that are capable of transforming electric current into light!  These devices come in all shapes and sizes, and are used ubiquitously around us.  Likely, the next artificial light you see will be emmitted by an LED.

LED costume for professional stage performers, created by the artist Beo Beyond.

The low energy consumption, low maintenance and small size of LEDs make them very usful as status indicators and displays on a variety of equipment and installations.  In addition, advances in high-power and high-efficiency have made them useful for general lighting purposes, such as in traffic stop lights, car headlights, and in a large number of different applications.

LED Characteristics: Color

LEDs are made from a variety of semiconductor compounds mixed at different ratios to produce a distinct wavelength of color.  Different compounds emit light in specific regions of the visible light spectrum and therefore produce different intensity levels.  

Note that the color of an LED is not given by the plastic casing around it, but rather by the compounds that comprise it as shown by the color of the clear-capsule LEDs above!

Color Generation and RGB LEDs

Whereas the concept of color is a fairly intuitive one, the theory behind the perception of color is actually complex and not fully understood.  After many years of study, it's been demonstrated that the human eye contains specialized retinal cells that carry pigments with different spectral sensitivities (note that similar cells are present in many different species of animals!).  Known as cone cells, they are sensitive to three different spectra of light:

Cone type Spectral Sensitivity Range Peak wavelength
S 400–500nm 420–440nm
M 450–630nm 534–555nm
L 500–700nm 564–580nm

Roughly speaking, once light within these wavelengths stimulates the different cone cells, the brain combines the information giving rise to what we, as humans, experience as color.  The range of color is vast because light stimulates the receptors with varying degrees of intensity.  In addition, the perception of color is highly subjective as color is not a property of the electromagnetic radiation, but rather a feature of visual perception by an observer. 

In the context of LEDs, in particular the multi-colored kinds, they typically comprise 3 different LEDs (Red, Green, and Blue) packed tighly under a single casing.  As each LED's intensity can be controlled independently, we can generate different combinations of light, which our eyes will perceive as the different colors in the spectrum.

This principle of operation underlies pixel color in our LCD screens, where red, green and blue color dots placed next to each other give rise to the different colors we perceive.  The graphic below shows a simulation of how combining (in this case by way of linear addition) the three primary colors—Red, Green, and Blue—generates additional the colors we perceive as cyan, magenta, yellow, and white.

Color Mixing Diagram

In the case of our RGB LEDs, if we set the brightness of all three individual LEDs to be the same, then we will perceive the emitted light to be white.  Similar to the simulation above, if we turn off the red LED, so that just the green and blue LEDs have the same brightness, then the light will appear cyan.  With LEDs, the trickiest color to generate is actually black as it's not so much a color as it is the absense of light.  In this case, the best we can do is to turn off all three LEDs.

LED Characteristics: Polarity

Similar to regular diodes, LEDs only allow current flow in one direction.  Thus, reversing the polarity of the power supply to an LED will not damage it (fortunately!), but no light will be emitted either.

LED Anatomy

For LEDs that have terminals (or legs)—like those used in this activity—the positive terminal (anode) is denoted by a longer lead.  In addition, the negative terminal (cathode) is also denoted by a flat side on the capsule around the LED itself!

LED Characteristics: Forward Bias (Voltage and Current)

In order to emit light, LEDs need to operate under what's called a "forward bias" condition.  For doing so, a power supply should provide enough power (current and voltage) to meet that condition.  The specific voltage and current values for this condition vary for each LED model, and they're specificied in the corresponding datasheet (a technical user guide for the component).

Forward Voltage vs Forward Current Chart

Typically, we would connect a resistor in series with the LED to protect if from excessive current flow.  The value of the resistor will depend on the forward voltage and current of the LED.  For this activity we'll use a 1KOhm resistor and a red LED.  Note that different colors and types of LEDs will need different current and voltage levels to emit light at the same intensity and brightness.

LED Characteristics: Luminous Flux and Intensity

Characterizing the brightness of LEDs is typically done using two measurements:  luminous flux (in lumens), which is a measure of the total light output from a source; and, luminous intensity (in candelas), which is a measure of how bright the beam of light is in a particular direction.

LED Cone Angles

A great example of this, illustrated above, can be seen by comparing two LEDs with identical luminous flux of 0.2 lumens (at a current of 30mA).  Because the LED on the left has a lens that narrows the cone of light emitted to 15°, in contrast to the one on the right that uses a lens that concentrates the light in a cone of 30°, the luminous intensity is 3.7 candelas for the former, and 0.9 candelas for the latter.

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