Understanding CRI

By Jim Moody

 

A color rendering index (CRI) is a quantitative measure of a light source to reveal the colors of various objects faithfully in comparison with an ideal natural light source. Light sources with a high CRI are desirable in color critical applications such as film and video reproduction. It is defined by the International Commission on Illumination (CIE) as:

Color rendering: Effect of an illuminant on the color appearance of the object by conscious or subconscious comparison with their color appearance under a reference illuminant. (CIE 17.4-1987 International lighting vocabulary).

Color temperature has been described most simply as a method of describing the color characteristics of light, usually either warm (yellowish) or cool (bluish) and measuring it in degrees of Kelvin (®K).

The scale was devised by a British physicist William Thomas. Later Queen Victoria knighted him as Lord Kelvin in 1866.

The scale is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. All actual temperatures are above zero.

The CRI of a light source does not indicate the apparent color of the light source; that information is given by the Correlated Color Temperature (CCT). The CRI is determined by the light source’s spectrum. For example, an incandescent lamp has a continuous spectrum, a fluorescent lamp has a discrete line spectrum. The incandescent lamp has the higher CRI.

 

Light source CCT (K) CRI
Low-pressure sodium (LPS/SOX) 1800 -44
Clear mercury-vapor 6410 17
High-pressure sodium (HPS/SON) 2100 24
Coated mercury-vapor 3600 49
Halophosphate warm-white fluorescent 2940 51
Halophosphate cool-white fluorescent 4230 64
Tri-phosphor warm-white fluorescent 2940 73
Halophosphate cool-daylight fluorescent 6430 76
“White” SON 2700 82
Standard LED Lamp 2700-5000 83
Quartz metal halide 4200 85
Tri-phosphor cool-white fluorescent 4080 89
High CRI LED Lamp (Blue LED) 2700-5000 95
Ceramic discharge metal-halide lamp 5400 96
Ultra High CRI LED Lamp (Violet LED) 2700-5000 99
Incandescent/halogen bulb 3200 100

 

The Value often quoted as “CRI” on commercially available lighting products is properly called the CIE Ra value. “CRI” being a general term and CIE Ra being the International standard color rendering index.

Numerically, the highest possible CIE Ra value is 100 and would only be given to a source identical to standardized daylight or a “Black Body”. Note: Incandescent lamps are effectively black bodies. For some light sources low-pressure sodium lighting has negative CRI; fluorescent lights range from about 50, for the basic type, up to about 98 for the best multi-phosphor type. Typical LED, having about 80+ CRI, while some of the manufacturers claim that their LEDs have achieved up to 98 CRI.

CIE Ra’s ability to predict color appearance has been criticized in favor measures based on color appearance models such as CIECAM02 and for daylight simulators the CIE Metameriem index. CRI is not a good indicator for use in visual assessment, especially for sources below 50000 Kelvin.

A newer version of CRI, R96, has been developed, but it has not replaced the better known Ra (General color rendering Index)
Lighting film and video have encountered issues attempting to use LED lighting on set. The color spectra of LED lighting primary colors do not match the expected color wavelength for band passes of film emulsions and digital sensors. As a result, color rendering can be completely unpredictable in optical prints, transferrers to digital media from film and video camera recordings. This phenomenon with respect to motion picture film has been documented in an LED lighting evaluation series of tests produced by the Academy of Motion Picture Arts and Sciences scientific staff. (Solid State Lighting Report).

To that end, various other metrics such as the TLCI (Television Lighting Consistency Index) have been developed to replace the human observer with a camera observer. Similar to the CRI, the metric measures quality of a light source as it would appear on camera on a scale from 0 to 100. Some manufacturers say their products have TLCI values of up to 99.

Most film and video photographers and gaffers say they only occasionally use a meter, trusting their practiced eye instead. Once a test pattern is illuminated with the illuminator they are using for the production, the video controller will adjust the camera(s) to balance the rendering. In film a test is also done that can be developed at the lab and corrections can be accomplished. This does not negate the value and importance of the CRI scales. When first working in these medias it is wise to check often, especially with the other team members who can assist in the evaluation.

Note: Most statics and definitions taken from Wikipedia
Jim Moody is the author of Concert Lighting and The Business of Theatrical Lighting Design