The Color Rendering Index (CRI) is the relative ability of a light source to replicate colors generated by a reference light source of the same color temperature. It is measured as a percentage.
What this means is that a CRI of 100%, taken at a color temperature of noon day sunlight (6000k) will exactly reproduce the colors found on a sunny day at noon.
A CRI of 50% taken at noon day sun will partially reproduce the colors when compared to that of a normal noon day.
Some marketers use differing color temperatures as a reference to attain 100% CRI, when the actual CMI would be much less than 100% if noon day sun light was used as a reference.
If you notice how a landscape changes color as the sun sets. This is a change in the color temperature. 100% CRI at sunset would result in a yellow or orange appearance. That same measure against noon day sun might only produce a CRI of 60% or less.
If this has not confused you try this wikipedia definition:
"The Color Rendering Index (CRI) (sometimes called Color Rendition Index), is a measure of the ability of a light source to reproduce the colors of various objects being lit by the source. It is a method devised by the International Commission on Illumination (CIE). The best possible rendition of colors is specified by a CRI of one hundred, while the very poorest rendition is specified by a CRI of zero. For a source like a low-pressure sodium vapor lamp, which is monochromatic, the CRI is nearly zero, but for a source like an incandescent light bulb, which emits essentially black body radiation, it is nearly one hundred. The CRI is measured by comparing the color rendering of the test source to that of a "perfect" source which is generally a black body radiator, except for sources with color temperatures above 5000K, in which case a simulated daylight (e.g. D65) is used. For example, a standard "cool white" fluorescent lamp will have a CRI near 63. Newer "triphosphor" fluorescent lamps often claim a CRI of 80 to 90.
CRI is a quantitatively measurable index, not a subjective one. A reference source, such as black body radiation, is defined as having a CRI of 100 (this is why incandescent lamps have that rating, as they are, in effect, almost blackbody radiators), and the test source with the same color temperature is compared against this. Both sources are used to illuminate eight standard samples. The perceived colors under the reference and test illumination (measured in the CIE 1931 color space) are compared using a standard formula, and averaged over the number of samples taken (usually eight) to get the final CRI. Because eight samples are usually used, manufacturers use the prefix "octo-" on their high-CRI lamps."
Examples of color temperature and CRI
Candle: 1700k 100 CRI
High Pressure Sodium: 2100k 25 CRI
Incandescent: 2700k 100 CRI
Tungsten Halogen: 3200k 95 CRI
Cool White: 4200k 62 CRI
Clear Metal Halide: 5500k 60 CRI
Natural Sunlight: 5000-6000k 100 CRI
Daylight Bulb: 6400k 80 CRI
Many products marketed as 'natural sunlight' light sources are far from noon day sun.
Color Temperature
The color temperature is a measurement that is used to define the color of a light source. This is also used to indicate the 'whiteness' or 'warmness' of a light source.
Smarterlight LEDs are available in many levels of Kelvin color temperature. The descriptions used in this site for color temperature conform to the following:
Warm light: |2500-3500 Kelvin: typical incandescent light bulb
Cool light: |3500-4500 Kelvin: typical retail space 'white' florescent lighting
Pure light: |4500-5500 Kelvin: used for high color definition
Noon sunlight: |5500-6000 Kelvin: normal daylight at mid day on a mid-summer day
The following table offers a broader example of the types of color temperature:
1900 Kelvin |Candle light or sunlight at sunrise or sunset
27K - 2700 Kelvin |Often used as accent lighting to blend in with fluorescent 2700K applications.
3K - 3200 Kelvin |Used as a primary light source for retail applications.
3700 Kelvin |Coated lamps. Used where a "softer" metal halide light source is desired.
4000 Kelvin: |Used in general lighting; factories: parking lots, warehouses
5K - 5500 Kelvin |Daylight lamps: horticulture, aquariums, high color definition.
5600 Kelvin |Nominal sunlight (mid day during mid summer)
6000 Kelvin |Starts to get a blue tint like some automotive headlights
For the scientists: The color temperature of light is typically measured in degrees Kelvin or just Kelvin. This scale designates the light sources spectral distribution. The Kelvin scale is a temperature that is on the Celsius scale where 0 degrees Kelvin = -273 degrees Celsius. The Kelvin is the light emitted from a black body energy source at that given temperature.
For a Glossary of Lighting Terms you can visit http://www.sylvaniaautocatalog.com/new_sylvania/glossary.htm