Understanding how LEDs work and what materials are used for ideal operation in various spectral ranges will aid in selecting the right color and ideal wavelengths for a variety of applications. This article will discuss the newest LED technology and products that are available for your application, the various colors of LEDs and their respective wavelengths, and the theory of LED operation. Our goal is to help you make a knowledgeable decision when choosing an LED device to meet your specifications.
Light emitting diodes (LEDs) are semiconductors that convert electrical energy into light energy. The color of the emitted light depends on the semiconductor material and composition. LEDs are generally classified into three wavelengths: Ultraviolet, visible, and infrared.
The wavelength range of commercially available LEDs with single-pixel-output power of at least 5mW is 360nm to 950nm. Each wavelength range, which will be discussed further in this article, is made from a specific semiconductor material family, regardless of the manufacturer.
Colors of LEDs
In applications where LEDs are being viewed directly, or being used as illuminators, exact color is far more important than exact output in lumens or candela (see Figures 1 and 2).
The human eye is relatively insensitive to light intensity changes and the brain compensates quite well for what intensity changes occur. For example, when looking at an LED video screen on a building, the average person will not notice an intensity drop off of 20 percent as portions of the screen are viewed at 10 to 20 degrees off axis, as compared to the portion directly on axis. This is a gradual change moving toward the edge of your vision and it is not perceived. By the same token, if LEDs in one location are 10nm different in wavelength than one another, the viewer will easily see this color difference and find it distracting.
The majority of white LEDs being used today are made from a blue LED pumping a yellow phosphor. Viewed directly, the LED will appear to be white as the blue and yellow wavelengths are mixed together in the package. This product is ideal for general outdoor lighting and indoor hallway lighting. However, for illumination where color rendering is important (measured as a color-rendering index or CRI), this type of LED falls very short.
Color rendering is measured on a scale where 100 is a perfect match to sunlight across the visible spectrum. When the CRI falls below 80, viewing objects by eye will not result in seeing the true color. As a comparison, incandescent lights typically have a CRI above 80, while standard cool fluorescent lights have a CRI in the range of 60-65. This is why it is difficult to determine the true color of clothing in a store illuminated with fluorescent lights.
Color rendering is very important when reading topographical maps with an LED flashlight or when an electrician needs to discern wire colors with an LED flashlight. In museum illumination, a high color rendering index is vital to the perception of color in paintings and other works of art.
Poor color rendering is evident when a subject is illuminated by white LEDs; it is because the green and red components are weak. Sunlight has output at all visible wavelengths with relatively gradual and smooth transitions when graphed as power vs. wavelength; all colors can be determined equally well in sunlight. Fluorescent lamps and phosphor-pumped, white LEDs lack the smooth output-versus-wavelength curve or transition that is found with natural sunlight; hence the colors viewed by the eye will not be true.
An alternative white LED technology to phosphor-pumped LEDs is RGB or RGBA LEDs. These combine red, green, and blue, or red, green, blue, and amber chips to create white light. These LEDs produce a light with much higher color rendering index and therefore produce colors that are more true in illumination applications.
The LED chips have been available for many years and the concept demonstrated by many different LED companies. The problem has been color stability. Red and amber LED chips have a high wavelength and intensity shift over ambient temperature compared to green and blue LED chips. Without proper compensation over temperature shifts, the white light will become warmer (more red) at low temperature and colder (more blue) at high temperature.
Within the last few years LED controllers specifically designed for multicolor LED arrays have come onto the market at a reasonable price. With the introduction of these controllers, the “light engine” market for multi-color LEDs has increased significantly. These controllers also allow for creating any color of interest from violet to red.
LEDs for monochromatic applications have huge advantages over filtered lamps; the wavelength spectra are better defined than what can be achieved with a white light source and filter. For general lighting applications, the energy savings can easily be 100 times the cost of using a filtered incandescent lamp. This creates huge dividends in applications such as architectural lighting and traffic signals. Low-power portable highway LED signage can easily be powered by a small solar panel instead of a large generator, offering a distinct advantage.
LEDs are more reliable than lasers, generally cost less, and can be driven with lower cost circuitry. The European Union has now joined with the U.S. in classifying LEDs as a separate entity. Fortunately, LEDs do not carry the same eye safety concerns or warnings that lasers and laser diodes often have. Also, LEDs cannot be made into extremely small, highly collimated and optically dense spots. In applications where extremely high power density within a small area is required, a laser is almost always required.
Ultraviolet LEDs (UV LEDs): 320nm – 360nm
UV LEDs are rapidly becoming commercialized, specifically used for industrial curing applications and medical/biomedical uses. Until recently, the lower wavelength limitation for high-efficiency die was at 390nm. It has been moved to 360nm and further developments over the next few years will likely see the commercialization of high efficiency die in the 320nm region.
The material primarily used for UV LEDs is gallium nitride/aluminum gallium nitride (GaN/AlGaN). At this time the technology does not yield high-power LEDs as compared to the blue wavelengths, and the market is unsettled as several companies are moving to improve their processes.
Near UV to Green LEDs: 395nm – 530nm
The material for this wavelength range of products is indium gallium nitride (InGaN). It is technically possible to make a wavelength anywhere between 395nm and 530nm. However, most large suppliers concentrate on creating blue (450nm - 475nm) LEDs for making white light with phosphors and green LEDs that fall into the 520nm - 530nm range for traffic signal green lighting.
Rapid advancements and improvements in efficiency are noted in the blue wavelength range, especially as the race to create brighter and brighter white illumination sources continues.
Yellow-Green to Red LEDs: 565nm – 645nm
Aluminum indium gallium phosphide (AlInGaP) is the semiconductor material used for this wavelength range. It is predominately used for traffic signal yellow (590nm) and red (625nm) lighting. The lime green (or yellowish-green 565nm) and orange (605nm) are also available from this technology, but they are somewhat limited. The technology is rapidly advancing on the red wavelength in particular because of the growing commercial interest in making red-green-blue white lights.
It is interesting to note that neither the InGaN or AlInGaP technologies is available as a pure green (555nm) emitter. Older, less efficient technologies do exist in this pure green region, but they are not considered efficient or bright. This is due largely to a lack of interest and/or demand from the marketplace, and therefore a lack of funding to develop alternative material technologies for this wavelength region.
Deep Red to Near Infrared (IRLEDs): 660nm – 900nm
There are many variations on device structure in this region, but all use a form of Aluminum Gallium Arsenide (AlGaAs) or Gallium Arsenide (GaAs) materials. There is still a push to increase the efficiency of these devices, but the increases are only incremental improvements. Applications include infrared (IR) remote controls, night vision illumination, industrial photo-controls, and various medical applications (at 660nm - 680nm).
Theory of LED Operation
LEDs are semiconductor diodes that emit light when an electrical current is applied in the forward direction of the device. An electrical voltage that is large enough for the electrons to move across the depletion region and combine with a hole on the other side to create an electron-hole pair must be applied. As this occurs, the electron releases its energy in the form of light and the result is an emitted photon.
The bandgap of the semiconductor determines the wavelength of emitted light. Shorter wavelengths equal greater energy and therefore higher bandgap materials emit shorter wavelengths. Higher bandgap materials also require higher voltages for conduction. Short wavelength UVblue LEDs have a forward voltage of 3.5 volts while near-IR LEDs have a forward voltage of 1.5 - 2.0 volts.
Things to Consider: Wavelength Availability and Efficiency
High-efficiency LEDs can be produced in any wavelength range, with one exception — the 535nm to 560nm range. The overriding factor as to whether or not a specific wavelength is commercially available has to do with market potential, demand, and industry-standard wavelengths. This is particularly pronounced in the 420nm - 460nm, 480nm - 520nm, and the 680nm - 800nm regions. Because there are no high-volume applications for these wavelength ranges, there are no high-volume manufacturers providing LED products for these ranges. It is possible, though, to find medium and/or small suppliers offering products to fill these particular wavelengths on a custom basis.
Each material technology has a spot within the wavelength range where it is most efficient. This point is very close to the middle of each range. As the doping level of the semiconductor increases or decreases from the optimal amount, efficiency suffers. That is why a blue LED has much greater output than green or near UV, amber has more than yellow-green, and near IR is better than 660nm. When given a choice, it is much better to design for the center of the range than at the edges. It is also easier to procure products if you are not operating at the edges of the material technology.
Supplying Current and Voltage to LEDs
While LEDs are semiconductors and need a minimum voltage to operate, they are still diodes and need to be operated in a current mode. There are two main ways to operate LEDs in DC mode. The easiest and most common is using a current limiting resistor (See Figure 5). The disadvantage to this method is the high heat and power dissipation in the resistor. In order for the current to be stable over temperature changes and from device-to-device, the supply voltage should be much greater than the forward voltage of the LED.
In applications where the operating temperature range is narrow (less than 30°C) or the output of the LED is not critical, a simple circuit utilizing a current limiting resistor may be used.
A better way to drive the LED is with a constant current source (see Figure 6). This circuit will provide the same current from device-to-device and over temperature shifts. It also has lower power dissipation than using a simple current limiting resistor.
Commercial, off-the-shelf LED drivers are available from a number of different sources. Typically these operate using pulse width modulation (PWM) principles for brightness control.
Pulsing LEDs in high current and/or high voltage mode for arrays in series-parallel configuration creates a unique set of problems. For the novice designer it is not practical to design a current-controlled pulse drive with the capability to deliver 5 amps and 20 volts. There are a few manufacturers of specialty equipment for pulsing LEDs (see Figure 7), such as Gardasoft Vision.
LEDs are now used in a large number of diverse markets and applications. Their high reliability, high efficiency, and lower overall system cost compared to lasers and lamps make these devices very affordable and attractive to both consumer and industrial segments. Each individual LED technology and/or color has been developed to address specific uses and requirements. For more information about choosing the right LED for your lighting task, please visit Opto Diode.