Light and color basics

Light-emitting diodes (LEDs) differ from other light sources, such as incandescent and fluorescent lamps, in the way they generate white light. We are accustomed to lamps that emit white light. But what does that really mean? What appears to our eyes as "white" is actually a mix of different wavelengths in the visible portion of the electromagnetic spectrum. Electromagnetic radiation in wavelengths from about 380 to 770 nanometers is visible to the human eye.

Source: US Department of Energy

Incandescent, fluorescent, and high-intensity discharge (HID) lamps radiate across the visible spectrum, but with varying intensity in the different wavelengths. The spectral power distribution (SPD) for a given light source shows the relative radiant power emitted by the light source at each wavelength. Incandescent sources have a continuous SPD, but relative power is low in the blue and green regions. The typically "warm" color appearance of incandescent lamps is due to the relatively high emissions in the orange and red regions of the spectrum.

SPDs for fluorescent and HID sources are provided for comparison. These sources have "spikes" of relatively higher intensity at certain wavelengths, but still appear white to our eyes. Unlike incandescent, fluorescent and HID sources, LEDs are near-monochromatic light sources. An individual LED chip emits light in a specific wavelength. This is why LEDs are comparatively so efficient for colored light applications.

In traffic lights, for example, LEDs have largely replaced the old incandescent + colored filter systems. Using colored filters or lenses is actually a very inefficient way to achieve colored light. For example, a red filter on an incandescent lamp can block 90 percent of the visible light from the lamp. Red LEDs provide the same amount of light for about one-tenth the power (12 watts compared to 120+ watts) and last many times longer. However, to be used as a general light source, "white" light is needed. LEDs are not inherently white light sources.