As indicated by its full name — Light Emitting Diode — an LED is a diode that emits light. A diode is a device that allows current to flow in only one direction. Almost any two conductive materials will form a diode when placed in contact with each other — with a single p-n semiconductor junction between them. To create an LED, the n-type material is negatively charged, while the p-type material is positively charged. The atoms in the n-type material have extra electrons, while the atoms in the p-type material have electron holes — electrons missing from their outer rings.

Applying electrical current to the diode pushes the atoms in both materials toward the junction area. When the get close to each other, the n-type atoms donate their extra electrons to the p-type atoms, which accept them. When extra electrons in the n-type material fall into the holes in the p-type material, they release energy in the form of photons — the basic units of electromagnetic radiation.

All diodes release photons, but not all diodes emit light — just light-emitting diodes. The material in an LED is selected so that the wavelength of the released photons falls within the visible portion of the light spectrum. Different materials produce photons at different wavelengths, which appear as light of different colors.


LED Anatomy

The two basic types of LEDs are indicator-type LEDs and illuminator-type LEDs. Indicator-type LEDs are usually inexpensive, low-power LEDs suitable for use only as indicator lights in panel displays and electronic devices, or instrument illumination in cars and computers. Illuminator-type LEDs are durable, high-power devices capable of providing illumination. All illuminator-type LEDs share the same basic structure. They consist of a semiconductor chip (or die), a substrate that supports the die, contacts to apply power, bond wire to connect the contacts to the die, a heat sink, lens, and outer casing.


How LEDs Produce Different Colors

LEDs produce different colors by using various materials which produce photons at different wavelengths. Those individual wavelengths appear as light of different colors.

LEDs use materials that can handle the necessary levels of electricity, heat, and humidity. High-brightness red and amber LEDs use the aluminum indium gallium phosphide (AlInGaP) material system. Blue, green and cyan LEDs use the indium gallium nitride (InGaN) system.

Together, AlInGaP and InGaN cover almost the entire light spectrum, with a gap at green-yellow and yellow. One method of achieving a larger spectrum of colors is to mix different colors of LEDs in the same device.

Combining red, green, and blue LEDs in a single LED device, such as a lighting fixture or multi-chip LED, and controlling their relative intensities can produce millions of colors. Additionally combining red, green, and blue in equal amounts produces white light.


LED Fixture Anatomy

To be used for illumination, LEDs must be integrated into fixtures that incorporate optics, LED drivers, power supplies, and thermal management. Well-designed LED lighting fixtures integrate all of these critical components into the fixture itself.


What are the advantages of LEDs?

LEDs offer a variety of advantages to lighting professionals and ultimate beneficiaries of LED lighting systems — from creative individuals to innovative businesses to visionary cities and countries:

  • High-levels of brightness and intensity — LEDs generate high lumen output, ensuring brightness of white and color light.
  • Exceptional range — Color, dynamic color, and tunable white-light LED luminaires can produce millions of colors or color temperature ranges — extremely accurately — without gels or filters.
  • Energy-efficiency — LED lighting can be 5x more energy-efficient than incandescent and halogen sources — cutting costs while lowering environmental impact.
  • Low-voltage and current requirements — LED lighting systems offer simple, flexible installation and use.
  • Low radiated heat — Since LEDs don't emit infrared radiation, they can be installed in heat-sensitive areas, near people and materials, and in small spaces where collected heat might be dangerous.
  • High reliability — LEDs can operate in colder temperatures and withstand impact and vibrations, making them suitable for extreme environments or areas that are difficult to access. LEDs have no moving parts of filaments that can break or fail.
  • No UV rays or infrared radiation — Because LEDs do not emit harmful UV rays that can degrade materials or fade paints and dyes, they're ideal for use in retail stores, museums, and art galleries.
  • Long source life — LEDs offer a significantly longer useful life than conventional light sources, which reduces the cost an inconvenience of maintenance and replacement.
  • Easy control — LEDs can be digitally (and automatically) controlled for maximum efficiency and flexibility.



What Exactly Is a Lumen?

Light measurements can either be radiometric or photometric. Radiometric measurements measure all the wavelengths of a light source, both visible and invisible. Photometric measurements measure only the visible wavelengths of light. The total electromagnetic energy that a light source emits across all wavelengths is known as radiant flux, and is measured in watts. The total energy that a light source emits across the visible wavelengths of light is known as luminous flux, and is measured in lumens. Since visibility only has meaning in relation to a human viewer, photometric data takes into consideration the varying sensitivities of the human eye to different wavelengths (colors) of visible light. The sensitivity of a human eye with normal vision can be plotted as a bell-shaped curve. This curve is known as the spectral luminous efficiency function, and is often referred to as the eye-sensitivity curve. The eye-sensitivity curve shows that the human eye is most sensitive to light in the green part of the spectrum, around a wavelength of 550 nanometers (nm), and is progressively less sensitive to light toward both the red and blue ends of the spectrum.


To calculate lumens, different wavelengths of light are given more or less weight depending on where they fall on the eye-sensitivity curve. Two light sources with the same radiant flux falling on different parts of the curve will therefore have different lumen measurements. Imagine, for instance, two light sources of 1 watt of radiant flux each. One source emits a blue light at 480 nm, and one emits a green light at 555 nm. As the eye-sensitivity curve shows, the blue light appears significantly less bright than the green light, even though the total energy of the two lights is the same (see the figure at the top of the next page). To put it another way, the green light produces more lumens than the blue light, even though both lights produce the same amount of radiant energy. In practice, there are variations in every individual’s experience of the apparent intensity of a light source.


In 1924, the International Commission on Illumination (CIE), a recognized authority on light, illumination, color, and color spaces, standardized the responses of the human eye to visible light by defining a so-called standard observer. The standard observer has regular eye responses to visible light under specific conditions, which the standard defines. The eye-sensitivity curve used in lumens and other photometric measurements is the standard observer’s eye-sensitivity curve, not the eye-sensitivity curve of any actual observer. Lumens and related measurements are therefore approximations or idealizations, which are usually good enough for evaluations and comparisons of different light sources. 



How Many Watts Is That LED?

Because of incandescent light bulbs, most people are used to looking at wattage to determine the light output of a light source: a 100-watt lamp puts out more light than a 60-watt lamp. All general service incandescent lamps use the same filament material heated to the same temperature, so the only way to increase their light output is to increase the wattage. This is one of the main reasons why incandescent lamps are so energy wasteful.


LED light sources are much more efficient at converting watts to lumens. Different materials can be used within the LED sources themselves, each of which has its own light extraction efficacy. For these and other reasons, two different LED sources can consume the same number of watts but differ widely in lumen output. Because watts can’t be used as an index of light output, evaluating the “brightness” of LED sources for a given situation requires you to think differently about lighting. A standard 60-watt incandescent lamp emits a total of about 800 lumens, but the light is emitted equally in all directions.


When you’re reading at your office desk, your book does not receive all 800 lumens from your desktop lamp, nor do you need it to. The crucial measurement is delivered light. According to the IES, serious reading requires an average of 50 footcandles (fc) or 500 lux (lx) on the page. Many linear LED under-cabinet fixtures and other task lights can deliver this level of light while consuming far less than 60 watts. For example, an under-cabinet LED light from a leading supplier can deliver 50 fc in typical desktop situations while consuming only about 6 watts per foot.



The Importance of Delivered Light

Instead of lumen output, the best and most relevant measurement for evaluating LED lighting fixtures and for making accurate comparisons with conventional lighting fixtures is delivered light. The formal term for measurements of delivered light is illuminance. Roughly speaking, illuminance is the intensity of light falling on a surface area. If the area is measured in square feet, the unit of illuminance is footcandles (fc). If measured in square meters, the unit of illuminance is lux (lx).


Delivered light describes how much useful light a lighting fixture can deliver to a task area. Useful light is the portion of a lighting fixture’s light output that is effectively directed to a task area, discounting any wasted light. The task area can be any space or surface that requires illumination — an entrance hallway, a common office space with desktop computers, a kitchen countertop, or the face of a Mayan pyramid in Guatemala. Light can be wasted in a number of ways: It can be partially blocked or dispersed within the fixture housing, it can be emitted in a direction away from the task area, or it can be lost through filtering, lensing, fixture positioning, or any of a number of other factors relevant to a specific installation.  


Materials were taken from www.colorkinetics.com



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