Color-rendering index

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The Color Rendering Index (CRI) (sometimes called Color Rendition Index), is a measure of the ability of a light source to reproduce the colors 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 colors 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 black body radiation, it is nearly one hundred. The CRI is measured by comparing the color 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 63. Newer "triphosphor" fluorescent lamps often claim a CRI of 80 to 90.

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, almost blackbody radiators), and the test source with the same color temperature is compared against this. Both sources are used to illuminate eight standard samples. The perceived colors 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 color indices of eight sample colors on the 1964 W*U*V* uniform color space (which is now obsolete). The indices of the samples are first measured while being illuminated by the reference source, yielding indices $[W i 0, U i 0, V i 0]$ where the index i specifies the particular sample color. The indices of the samples are then measured under the test source yielding indices $[W i, U i, V i]$ . The distances $Δ E i$ between the measured colors is then calculated:

$\Delta E_i=\sqrt{(U_i-U_{i0})^2+(V_i-V_{i0})^2+(W_i-W_{i0})^2}\,$

The color rendering index $R i$ is calculated for each of the eight samples:

$R_i=100-4.6\Delta E_i\,$

which gives the color rendering index with respect to each sample. The factor 4.6 was so chosen that the Ra of a standard warm-white Thalium lamp would be about 50. It also appeared that Ri could be negative (∆Ei ≥ 22), and this was indeed calculated for some lamp test colours The general color rendering index $R a$ is then the average of these eight separate indices.

$R_a=\frac{1}{8}\sum_{i=1}^8 R_i$

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

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 color-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 color-rendering performance metrics.

In general it can be said that the importance of Ri decreases as its value relative to 100 increases. This is even more true for the Ra, which is the average of 8 individual Ri values, and which gives only a global impression of the colour rendering properties of a light source. Indeed, in practice it can occur that a light source with Ra = 85 is not always better than a light source whose Ra = 80. A second disadvantage of the Ra value is the fact that it gives no information as to the direction of the colour shift. A colour can be more saturated or less saturated without a change in the numerical value of ∆Ei, while in general a saturated colour is experienced as being more attractive. An attempt at rectifying this has been made by the introduction of the Colour Discrimination Index (CDI). Here the surface of the octagon is formed by the eight test colours in the u,v diagram as a measure of the colour rendering quality. A smaller surface means less saturated, pale colours. 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 colour 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 Colour Preference Index (CPI) in which even greater emphasis is placed on the flattering rendition of well-known objects (butter, grass, skin colour, etc.).