Understanding the Color Rendering Index: Why the CRI is Important to Photography

Digital Photography, Multimedia
Page content

Color Rendering Index

The color rendering index is used to describe the ability of lighting equipment to reproduce the visual effect of natural light sources. This visual effect shows how colors are displayed by objects when they are hit by light sources. The color rendering index is usually used by lighting equipment manufacturers to let consumers know how high or how low the visual effect reproduction level of lighting equipment are. The values of this index goes from 0 to 100. The higher the value of the CRI, the higher the quality of its color rendering ability would be.

Applications

The color rendering index, or the CRI, of a piece of lighting equipment is important to people using those lighting equipment in color-critical applications like cinematography and photography. In these color-critical applications, it is important that the lighting equipment gives them the most accurate representation of colors in their work compared to produced colors in the real world where things are illuminated by natural light sources. Photographers particularly use CRI not just for the visual quality of the light, but also in choosing the right light source for a particular shot.

Determining the Color Rendering Index Value of Lighting Equipment

To determine the CRI value of a piece of lighting equipment, 8 standard low saturated pastel colors are viewed under lighting with a blackbody source and under the light of the piece of lighting equipment being rated. The two lighting sources must have the same color temperature for this test to be valid. By averaging the estimated differences of the colors under the two light sources, the CRI value is determined. In a more recent form of the test method, instead of 8 test colors, the method uses 10 colors. These colors include the first 8 plus 2 skin tone colors.

Color Temperature

The color rendering index does not determine the color temperature of a light source. The two terms represent two different properties of a light source. Color temperature describes the light source’s color appearance. The CRI describes how well colors are rendered in objects by the light source. However, as previously discussed, these two properties are related in terms of determining the CRI rating of a piece of lighting equipment.

CRI Values for Common Light Sources

Here are the values of commonly used lighting equipment in homes, businesses, industrial buildings and corporate offices:

  • Clear Mercury – 17
  • White Deluxe Mercury – 45
  • Warm White Fluorescent Tube – 45
  • Cool White Fluorescent Tube – 65
  • Deluxe Warm White Fluorescent – 73
  • Daylight Fluorescent – 79
  • Metal Halide 4200K – 85
  • Deluxe Cool White Fluorescent – 86
  • Metal Halide 5400K – 93
  • Low Pressure Sodium – 0-18
  • High Pressure Sodium – 25
  • 100-Watt Incandescent – 100

From this chart, you can see that the best piece of lighting equipment for applications that need high CRI values are incandescent or metal halide lighting equipment. Originally, fluorescent lighting equipment were considered to have low to medium CRI values. However, recent developments in fluorescent manufacturing are slowly increasing the CRI values of fluorescent lighting equipment.