Two Ways to Look at Light
There are two systems of measurement commonly used to describe the color properties of a
light source: "color temperature," which expresses the color appearance of the light itself,
and "color rendering index" (CRI), which suggests how an object illuminated by that light will
appear in relation to its appearance under other common light sources. Both can be valuable
in evaluating and specifying light sources.
Color Temperature - the Appearance of Light
The color temperature of a light source is a numerical measurement of its color appearance. It is based on
the principle that any object will emit light if it is heated to a high enough temperature, and that the color
of that light will shift in a predictable manner as the temperature is increased. The system is based on the
color changes of a theoretical "blackbody radiator" as it is heated from a cold black to a white hot state.
With increased temperature, the blackbody would shift gradually from red to orange to yellow to white and,
finally, to blue white. A light source’s color temperature is the temperature, expressed in "degrees" kelvin
(K), at which the color of the blackbody would most closely match the color of the light source. The color
is described as corelated color temperature (CCT). Example: a fluorescent lamp with a color temperature
rating of 4100K has a color appearance similiar to that of a blackbody heated to 4,100 kelvin (3327°
Celsius).
Warm vs. Cool - the Psychology of Light
Some people find it confusing that low color temperature light sources are called "warm" while those with
higher temperatures are referred to as "cool." In fact, these descriptions have nothing to do with the
temperature of the blackbody radiator, but rather refer to the way color groups are perceived - the
psychological impact of lighting. Colors and light sources from the blue end of the spectrum are referred
to as cool, and those toward the red/orange/yellow side of the spectrum are described as warm.
How Light Affects the Color of Objects
Color Rendering Index (CRI) is a system derived from visual experiments. It assesses the impact of
different light sources on the perceived color of objects and surfaces.
As a general statement: the higher the CRI of a light source,
the better and more natural colors appear under the light source.
In measuring CRI, scientists compare how eight specific colors appear under a light source to how the same
colors appear under a reference light source. There are two reference sources: incandescent lighting for
warm color lamps, and daylight for cool color lamps. Any CRI rating of 80 or above is
normally considered high and indicates that the source has good color properties. In this system, both
incandescent lamps and daylight are considered to have "perfect" CRI’s of 100 even though we know
materials appear differently when viewed under these two light sources. CRI is therefore meaningful in
comparing lamps that are close in color temperature.
Color temperature and CRI provide some helpful information, but they are not perfect.
Used in conjunction, they can provide excellent benchmarks for the comparison of light sources.
The following chart will help you understand the correlation between color temperature, Kelvin temperature and CRI.
| Lamp Description |
Designation | Atmosphere | Color Rendering Index (CRI) |
Kelvin Color Temperature |
|---|---|---|---|---|
| Cool White | CW | Cool | 62 | 4100K |
| Cool White Deluxe | CWX | Cool | 89 | 4200K |
| Daylight | D | Daylight | 79 | 6500K |
| Lite White | LW | Cool | 51 | 4100K |
| White | W | Neutral | 58 | 3500K |
| Warm White | WW | Warm | 53 | 3000K |
| Warm White Deluxe | WWX | Warm | 79 | 3000K |
| Natural | N | Warm | 81 | 3400K |
| Colortone 50 | C50 | Daylight | 92 | 5000K |
| Colortone 75 | C75 | Daylight | 95 | 7500K |
| Spec 30 | Spec 30 | Warm | 70 | 3000K |
| Spec 35 | Spec 35 | Neutral | 73 | 3500K |
| Spec 41 | Spec 41 | Cool | 70 | 4100K |
| Ultralume 30 | 30 U | Warm | 85 | 3000K |
| Ultralume 35 | U 35 | Neutral | 85 | 3500K |
| Ultralume 41 | U 35 | Cool | 85 | 4100K |