Color rendering index: Difference between revisions

Revision as of 15:20, 18 December 2005

Revision as of 17:22, 20 December 2005

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CRI is a quantitatively measurable index, not a subjective one. A reference source, such as blackbody 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. For colour-temperatures of 5000K and above, the reference source is a simulated daylight (e.g. [[D65]]) rather than a true blackbody. Both sources are used to illuminate several standard samples. The perceived colours under the reference and test illumination (measured in the [[CIE 1931 colour 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 takingmeasuring the colourcolor differencesindices <math>\Deltaof E_i</math>, between the test coloureight andsample the eight samples,colors on the 1964 W*U*V* uniform colour space (which is now obsolete). The colourindices renderingof indexthe samples are first measured while being illuminated by the reference source, yielding indices <math>R_i[W_{i0},U_{i0},V_{i0}]</math> iswhere calculatedthe forindex each''i'' ofspecifies the eightparticular sample color. The indices of the samples are then measured under the test source yielding indices <math>[W_i,U_i,V_i]</math>. The distances <math>\Delta E_i</math> between the measured colors is then calculated:

:<math>\Delta E_i=\sqrt{(U_i-U_{i0})^2+(V_i-V_{i0})^2+(W_i-W_{i0})^2}\,</math>

The colour rendering index <math>R_i</math> is calculated for each of the eight samples:

:<math>R_i=100-4.6\Delta E_i\,</math>

which gives the colour rendering index with respect to each sample. The general colour rendering index <math>R_a</math> is then the average of these eight separate indices.

:<math>R_a=\frac{1}{8}\sum_{i=1}^8 R_i</math>

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 specta, or white LEDs. It is understood that the CIE is looking at developing newer colour-rendering performance metrics.