Color rendering index: Difference between revisions

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The '''colorColor renderingRendering indexIndex (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 temperature]]s 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 6263. Newer "triphosphor" fluorescentsfluorescent lamps often claim a CRI of 80 to 8590.

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 <math>[W_{i0},U_{i0},V_{i0}]</math> where the index ''i'' specifies the particular 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:

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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 general color rendering index <math>R_a</math> is then the average of these eight separate indices.

The factor 4.6 was so chosen that the Ra of a standard warm-white TL

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 <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>

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.).

==See also==