To determine the color rendering capabilities of different light sources, there are two important measurements to consider: Color Temperature and Color Rendering Index.
Color Temperature
, or Chromaticity
, is a measure in degrees Kelvin that indicates the appearance of a source.
If a steel rod were placed into a fire it would first turn red, then orange as it heats up,
until it finally turns bluish white. The temperatures of the rod and the color
at each temperature describe the color of a source. It may sound like a
contradiction, but low color temperature lamps have more red wavelengths, thus
creating a warm feeling. High color temperature lamps have more blue wavelengths
creating a cool feeling.
In general, the higher the light levels in a space, the cooler the source. For example, offices in the 50 to 100 footcandle range, typically use fluorescent lamps with a color temperature of 4100 degrees Kelvin. Lobby areas (10 to 20 footcandles) usually have incandescent lighting with a color temperature of 2700 degrees Kelvin.
Chromaticity doesn't tell us how well the light source renders colors, it just
tells us the apparent color of the source - whether it provides a cool or warm feeling. The Color Rendering Index
, or CRI
, of a source indicates how well it renders eight standard colors compared to a perfect reference lamp of the same color temperature. The
comparison is only valid for lamps of the same color temperature. The CRI Index ranges from 1
to 100. A lamp with a CRI of 80 will render colors better than a lamp with a
CRI of 50.
But the CRI alone doesn't indicate color accuracy. To compare the color rendering capability of two light sources, first choose two sources with an appropriate color temperature for the situation. Then compare the CRI of each source. The higher CRI source will probably render the colors in the space more accurately.
