Color rendering index

The Colour rendering index, or CRI, is a measure of the ability of a light source to reproduce the colours 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 colours 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 blackbody radiation, it is nearly a hundred. The CRI is measured by comparing the colour 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 62. Newer "triphosphor" fluorescents often claim a CRI of 80 to 85.

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, blackbody radiators), and the test source with the same colour temperature is compared against this. Both sources are used to illuminate eight standard samples. The perceived colours 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.

The standard formula consists of measuring the colour indices of eight sample colours on the 1964 W*U*V* uniform colour space (which is now obsolete). The indices of the samples are first measured while being illuminated by the reference source, yielding indices ${\displaystyle [W_{i0},U_{i0},V_{i0}]}$ where the index i specifies the particular sample colour. The indices of the samples are then measured under the test source yielding indices ${\displaystyle [W_{i},U_{i},V_{i}]}$. The distances ${\displaystyle \Delta E_{i}}$ between the measured colours is then calculated:

${\displaystyle \Delta E_{i}={\sqrt {(U_{i}-U_{i0})^{2}+(V_{i}-V_{i0})^{2}+(W_{i}-W_{i0})^{2}}}\,}$

The colour rendering index ${\displaystyle R_{i}}$ is calculated for each of the eight samples:

${\displaystyle R_{i}=100-4.6\Delta E_{i}\,}$

which gives the colour rendering index with respect to each sample. The general colour rendering index ${\displaystyle R_{a}}$ is then the average of these eight separate indices.

${\displaystyle R_{a}={\frac {1}{8}}\sum _{i=1}^{8}R_{i}}$

Although an objective measure, the CRI has come under a fair bit of criticism in recent years as it does not always correlate well with the subjective colour-rendering quality for real scenes, particularly for modern (e.g. fluorescent) lightsources with spikey emission spectra, or white LEDs. It is understood that the CIE is looking at developing newer colour-rendering performance metrics.

See also

• Color temperature and Correlated Colour Temperature (CCT)

External link

• Formula to calculate CRI