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.