The color rendering index (CRI),What is CRI? the definition of CRI

Color rendering index

The color rendering index (CRI) (sometimes called color rendition index), is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such as photography and cinematography. It is defined by the International Commission on Illumination as follows:

Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant

Note that the CRI by itself does not indicate what the color temperature of the reference light source is; therefore, it is customary to also cite the correlated color temperature (CCT).

According to (Schanda & Sándor 2005), CRI is being deprecated in favor of measures based on color appearance models, such as CIECAM02 and, for daylight simulators, the CIE Metamerism Index. (Guo & Houser 2004) and (CIE 1995) note that CRI is not a good indicator for use in visual assessment, especially for sources below 5000 kelvin (K).

A newer version of the CRI has been developed (R96a), but it has not replaced the better-known Ra (general color rendering index).

History

Most researchers use daylight as the benchmark for which to compare color rendering of electric lights; one notable researcher, P.J. Bouma, in 1947 described daylight as the ideal source of illumination for good color rendering[3] because “it (daylight) displays (1) a great variety of colours, (2) makes it easy to distinguish slight shades of colour, and (3) the colours of objects around us obviously look natural.”

Around the middle of the 20th century, color scientists took an interest in assessing the ability of artificial lights to accurately reproduce colors. European researchers attempted to describe illuminants by measuring the spectral power distribution (SPD) in "representative" spectral bands, whereas their North American counterparts studied the colorimetric effect of the illuminants on reference objects.

The CIE assembled a committee to study the matter and accepted the proposal to use the latter approach, which has the virtue of not needing spectrophotometry, with a set of Munsell samples. Eight samples of varying hue would be alternately lit with two illuminants, and the color appearance compared. Since no color appearance model existed at the time, it was decided to base the evaluation on color differences in a suitable color space, CIEUVW. In 1931 the CIE adopted the first formal system of colorimetry, which is based the tri-chromatic nature of the human visual system[5][6]. CRI is based upon this system of colorimetry.

To deal with the problem of having to compare light sources of different correlated color temperatures (CCT), the CIE settled on using a reference black body with the same color temperature for lamps with a CCT of under 5000 K, or a phase of CIE standard illuminant D (daylight) otherwise. This presented a continuous range of color temperatures to choose a reference from. Any chromaticity difference between the source and reference illuminants were to be abridged with a von Kries-type chromatic adaptation transform.

Test Method

The CRI is calculated by comparing the color rendering of the test source to that of a "perfect" source which is a black body radiator for sources with correlated color temperatures under 5000 K, and a phase of daylight otherwise (e.g. D65). Chromatic adaptation should be performed so that like quantities are compared. Specified in (Nickerson & Jerome 1965) and republished in (CIE 1995), the Test Method (also called Test Sample Method or Test Color Method) needs only colorimetric, rather than spectrophotometric, information.

Using the 2° standard observer, find the chromaticity co-ordinates of the test source in the CIE 1960 color space. Determine the correlated color temperature (CCT) of the test source by finding the closest point to the Planckian locus on the (u,v) chromaticity diagram. If the test source has a CCT<5000 K, use a black body for reference, otherwise use CIE standard illuminant D. Both sources should have the same CCT. Ensure that the chromaticity distance (DC) of the test source to the Planckian locus is under 5.4×10−3 in the CIE 1960 UCS. This ensures the meaningfulness of the result, as the CRI is only defined for light sources that are approximately white.[9] Illuminate the first eight standard samples, from the fifteen listed below, alternately using both sources. Using the 2° standard observer, find the chromaticity co-ordinates of the light reflected by each sample in the CIE 1964 color space. Chromatically adapt each sample by a von Kries transform. For each sample, calculate the Euclidean distance ΔEi between the pair of co-ordinates. Calculate the special (i.e., particular) CRI using the formula Ri = 100 − 4.6ΔEi[10][11] Find the general CRI (Ra) by calculating the arithmetic mean of the special CRIs. Note that the last three steps are equivalent to finding the mean color difference, and using that to calculate Ra: [edit] Chromatic adaptation (CIE 1995) uses this von Kries chromatic transform equation to find the corresponding color (uc,i,vc,i) for sample i: where subscripts r and t refer to reference and test light sources, respectively.

CIE 1960 UCS. Planckian locus and co-ordinates of several illuminants shown in illustration below. (u,v) chromaticity diagram with several CIE illuminants. Chromatic adaptation of TCSs lit by CIE FL4 (short, black vectors, to indicate before and after) to a black body of 2940 K (cyan circles).

Test color samples

As specified in (CIE 1995), the original test color samples (TCS) are taken from an early edition of the Munsell Atlas. The first eight samples, a subset of the eighteen proposed in (Nickerson 1960), are relatively low saturated colors and are evenly distributed over the complete range of hues.^[12] These eight samples are employed to calculate the general color rendering index R_a. The last seven samples provide supplementary information about the color rendering properties of the light source; the first four for high saturation, and the last three as representatives of well-known objects. The reflectance spectra of these samples may be found in (CIE 2004),^[13] and their approximate Munsell notations are listed aside.^[14]

R96_a method

In the CIE's 1991 Quadrennial Meeting, Technical Committee 1-33 (Color Rendering) was assembled to work on updating the color rendering method, as a result of which the R96_a method was developed. The committee was dissolved in 1999, releasing (CIE 1999), but no firm recommendations, partly due to disagreements between researchers and manufacturers.^[15]

The R96_a method has a few distinguishing features:^[16]

• A new set of test color samples
• Six reference illuminants: D65, D50, black bodies of 4200 K, 3450 K, 2950 K, and 2700 K.
• A new chromatic adaptation transform: CIECAT94.
• Color difference evaluation in CIELAB.
• Adaptation of all colors to D65 (since CIELAB is well-tested under D65).

It is conventional to use the original method; R96_a should be explicitly mentioned if used.

[edit] New test color samples

As discussed in (Schanda & Sándor 2005), (CIE 1999) recommends the use of a ColorChecker chart owing to the obsolescence of the original samples, of which only metameric matches remain.^[17] In addition to the eight ColorChart samples, two skin tone samples are defined (TCS09^* and TCS10^*). Accordingly, the updated general CRI is averaged over ten samples, not eight as before. Nevertheless, (Hung 2002) has determined that the patches in (CIE 1995) give better correlations for any color difference than the ColorChecker chart, whose samples are not equally distributed in a uniform color space.

Example

The CRI can also be theoretically derived from the SPD of the illuminant and samples since physical copies of the original color samples are difficult to find. In this method, care should be taken to use a sampling resolution fine enough to capture spikes in the SPD. The SPDs of the standard test colors are tabulated in 5 nm increments (CIE 2004), so it is suggested to use interpolation up to the resolution of the illuminant's spectrophotometry.

Starting with the SPD, let us verify that the CRI of reference illuminant F4 is 51. The first step is to determine the tristimulus values using the 1931 standard observer. Calculation of the inner product of the SPD with the standard observer's color matching functions (CMFs) yields (x,y',z)=(109.2,100.0,38.9) (after normalizing for Y=100). From this follow the xy chromaticity values:

The tight isotherms are from 2935K–2945K. FL4 marked with a cross.

The next step is to convert these chromaticities to the CIE 1960 UCS in order to be able to determine the CCT:

Relative SPD of FL4 and a black body of equal CCT. Not normalized.

Examining the CIE 1960 UCS reveals this point to be closest to 2938 K on the Planckian locus, which has a co-ordinate of (0.2528, 0.3484). The distance of the test point to the locus is under the limit (5.4×10⁻³), so we can continue the procedure, assured of a meaningful result:

We can verify the CCT by using McCamy's approximation algorithm to estimate the CCT from the xy chromaticities:

CCT_est. = − 449n³ + 3525n² − 6823.3n + 5520.33, where .

Substituting (x,y) = (0.4402,0.4031) yields n=0.4979 and CCT_est. = 2941 K, which is close enough. (Robertson's method can be used for greater precision, but we will be content with 2940 K in order to replicate published results.) Since 2940 < 5000, we select a Planckian radiator of 2940 K as the reference illuminant.

The next step is to determine the values of the test color samples under each illuminant in the CIEUVW color space. This is done by integrating the product of the CMF with the SPDs of the illuminant and the sample, then converting from CIEXYZ to CIEUVW:

From this we can calculate the color difference between the chromatically adapted samples (labeled "CAT") and those illuminated by the reference. (The Euclidean metric is used to calculate the color difference in CIEUVW.) The special CRI is simply R_i = 100 − 4.6ΔE_UVW.

Finally, the general color rendering index is mean of the special CRI's: 51.