Color rendering index: Difference between revisions

Revision as of 19:06, 13 December 2007

Revision as of 13:58, 17 December 2007

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Although an objective measure, the CRI has been criticised in recent years as it does not always correlate well with subjective color-rendering quality for real scenes, particularly for modern light sources with spikey emission spectra (e.g. fluorescent lights or white LEDs). It is understood that the CIE is looking at developing newer color-rendering performance metrics.

In general it can be said that the importance of <math>R_i</math> decreases as its value relative to 100 increases. This is even more true for the <math>R_a</math>, which is the average of 8 individual <math>R_i</math> values, and which gives only a global impression of the color rendering properties of a light source. Indeed, in practice it can occur that a light source with <math>R_a = 85</math> is not always better than a light source with <math>R_a = 80</math>. A second disadvantage of the <math>R_a</math> value is the fact that it gives no information as to the direction of the color shift. A color can be more saturated or less saturated without a change in the numerical value of ∆Ei, while in general a saturated color is experienced as being more attractive. An attempt at rectifying this has been made by the introduction of the [[Color Discrimination Index]] (CDI). Here the surface of the octagon is formed by the eight test colors in the u,v diagram as a measure of the color rendering quality. A smaller surface means less saturated, pale colors. A larger surface means greater saturation, stronger contrasts, more lively, and so on. The objection to this method is that the principle of true-to-nature color rendering is abandoned. It also appears that equal surfaces do not always correspond to equal visual assessments. The CIE is rather hesitant about this method. The same goes for the so-called Color Preference Index (CPI) in which even greater emphasis is placed on the flattering rendition of well-known objects (butter, grass, skin color, etc.).

==See also==