The scientific measurement and quantification of LED color is called Colorimetry. Its units are typically given as chromaticity coordinates or in wavelength. Color perception is very complicated because it not only depends on the various physical properties of light such as the emission and optical properties of the light source, the wavelengths transmitted, etc. but also on things such as surrounding objects, the devices mechanical properties, the viewer’s eye response as well as their psychological state. The CIE (Commission International de l'Eclairage) has established standards for the measurement of visible light as it relates to the "standard human eye response." This so called standard observer curve was first established in 1931 (see figure 1a). From this curve, the tristimulus values for accurately defining a color are obtained. The X, Y, Z tristimulus system is based on the assumption that every color is a combination of three primary colors; red, green and blue. The 1931 CIE Chromaticity Diagram (see figure 1b) is derived from the tristimulus values by the following:

x = X/(X+Y+Z) or x = Red/(Red + Green + Blue)

y = Y/(X+Y+Z) or y = Green/(Red + Green + Blue)

Since (x + y + z) =1, the third axis, z = 1 – (x + y)

FIG 1a – 1931 CIE Tristimulus Color Matching Curves

FIG 1b – CIE 1931 Chromaticity Diagram

The chromaticity coordinates are normally specified by the x and y axis only. In general, most specifications provided by LED manufacturers do not list the chromaticity coordinates, but rather the peak and dominant wavelength (unless the LED is white). The dominant wavelength, specified in nanometers, is obtained from the color coordinates discussed above. It is essentially the color that is actually perceived by the human eye.

The peak wavelength is the wavelength at the maximum spectral intensity. The peak value is easy to obtain and is therefore the most common value specified by LED manufacturers, however, it has little practical significance for applications that are viewed with the human eye since two LEDs may have the same peak wavelength but can be perceived as different colors.

Correlated Color Temperature (CCT)

The apparent color of an object changes as the temperature increases. All objects emit light when sufficiently hot. The brightness and color of the light emitted is a function of temperature. If the object is hot enough to glow, like a tungsten lamp, it is said to be "incandescent". Incandescent sources that emit radiation with 100% efficiency are called "Black Body Radiators" or Planckian Sources. An ideal black body glows with a color that is solely dependent on the object's temperature (in ºK - Kelvin) and therefore, its temperature can be used as a color standard. This concept of color temperature can be applied only to sources that are black body radiators.

On the CIE chart (figure 2), the chromaticities of black bodies at different temperatures fall on the Planckian locus. Other sources that have chromaticities lying near this black body can be described as having a "Correlated Color Temperature", CCT. The CCT of a source is the temperature of the blackbody radiator which has the chromaticity most similar to that of the light source. For example, if a source has a CCT of 3200 º K, that means it has the same color that a black body would have if heated to 3200 º K. Sources with higher color temperatures have maximum intensities at shorter wavelengths. (6500 ºK is the standard for white light used for most white LEDs)

FIG 2 - CIE Chart w/color temperature

Currently, the most accurate method for measuring color is by using a Spectroradiometer. This device performs a complete spectral power distribution of the source being measured from which all photometric, radiometric and colorimetric parameters can be mathematically calculated. The wavelength accuracy of the equipment should be better than .5nm with .1nm preferred. There are several factors that can affect the value obtained. One of these is temperature. As the ambient temperature rises, so to does the LED wavelength. This increase will typically be from .1nm/ºC - .2nm/ºC depending on the type of LED used.