Color rendering index: Difference between revisions

Revision as of 13:04, 18 November 2007

Revision as of 13:09, 18 November 2007

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

which gives the color rendering index with respect to each sample. The factor 4.6 was so chosen that the <math>R_a</math> of a standard warm-white Thaliumthalium lamp would be about 50. It also appeared that <math>R_i</math> could be negative (<math>\Delta E_i</math> ≥ 22), and this was indeed calculated for some lamp test colors The general color 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>

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In 1965, in order to be able to objectively compare the color rendering properties of light sources, the CIE introduced a standardised measuring method. This method calculates the color change of 14 test colors under the light source being tested relative to the colors measured under a reference illuminant. The first 8 test colors are relatively non-saturated colors and are evenly distributed over the complete range of hues. These 8 test colors are employed to calculate the general color rendering index <math>R_a</math>. The last 6 colors (numbered 9 to 14) are employed to supply extra information about the color rendering properties of the light sources.

Although an objective measure, the CRI has comebeen under a fair bit of criticismcriticised in recent years as it does not always correlate well with the subjective color-rendering quality for real scenes, particularly for modern (e.g. fluorescent)light lightsourcessources 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.).