Color Rendering Index

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The CIE Color Rendering Index (CRI) (sometimes incorrectly 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.

“ 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 „

—CIE 17.4, International Lighting Vocabulary, (Schanda 2002)

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 & Sandor 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. (CIE 1995) notes that CRI is not a good indicator for use in visual assessment.

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[edit] History

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 in "representative" spectral bands, whereas their North American counterparts studied the colorimetric effect of the illuminants on reference objects.[1]

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 tristimulus values compared. 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 5000K, or a phase of CIE standard illuminant D (daylight) otherwise.

[edit] Test Color Method

The CRI is calculated 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 phase of daylight (e.g. D65) is used. Chromatic adaptation should be performed so that like quantities are compared. Specified in (Nickerson & Jerome 1965) and republished in (CIE 1995), the Test Color Method needs only colorimetric, rather than spectrophotometric, information.

CIE 1960 UCS. Planckian locus and coordinates of several illuminants shown in illustration below.
CIE 1960 UCS. Planckian locus and coordinates of several illuminants shown in illustration below.
  1. Using the 2° standard observer, find the chromaticity co-ordinates of the test source in the CIE 1960 color space.[2]
  2. 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.[3]
  3. If the test source has a CCT<5000K, use a black body for reference, otherwise use CIE standard illuminant D. Both sources should have the same CCT.
  4. Ensure that the chromaticity distance (DC) of the test source to the Planckian locus is under 5.4E-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.[4] DC=\Delta_{uv}=\sqrt{(u_r-u_t)^2+(v_r-v_t)^2}
  5. Illuminate the first eight standard samples, from the fifteen listed below, alternately using both sources.
  6. Using the 2° standard observer, find the chromaticity co-ordinates of the light reflected by each sample in the CIE 1964 color space.
  7. Chromatically adapt each sample by a von Kries transform.
  8. For each sample, calculate the Euclidean distance ΔEi between the pair of co-ordinates.
  9. Calculate the special (i.e., particular) CRI using the formula Ri = 100 − 4.6ΔEi[5][6]
  10. 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, \Delta \bar{E}_{UVW} and using that to calculate Ra:

R_a=100-4.6 \Delta \bar{E}_{UVW}

[edit] Chromatic adaptation

(u,v) chromaticity diagram with several CIE illuminants.
(u,v) chromaticity diagram with several CIE illuminants.

(CIE 1995) uses this von Kries chromatic transform equation to find the corresponding color (uc,i,vc,i) for sample i:

u_{c,i}=\frac{10.872+0.404 (c_r/c_t) c_{t,i} - 4 (d_r/d_t) d_{t,i}}{16.518+1.481 (c_r/c_t) c_{t,i} - (d_r/d_t) d_{t,i}}

v_{c,i}=\frac{5.520}{16.518+1.481 (c_r/c_t) c_{t,i} - (d_r/d_t) d_{t,i}}

c=\left(4.0-u-10.0v \right)/v

d=\left(1.708v-1.481u+0.404\right)/v

where subscripts r and t refer to reference and test light sources, respectively.


[edit] Test color samples

Name Appr. Munsell Appearance under daylight
TCS01 7,5 R 6/4 Light greyish red
TCS02 5 Y 6/4 Dark greyish yellow
TCS03 5 GY 6/8 Strong yellow green
TCS04 2,5 G 6/6 Moderate yellowish green
TCS05 10 BG 6/4 Light bluish green
TCS06 5 PB 6/8 Light blue
TCS07 2,5 P 6/8 Light violet
TCS08 10 P 6/8 Light reddish purple
TCS09 4,5 R 4/13 Strong red
TCS10 5 Y 8/10 Strong yellow
TCS11 4,5 G 5/8 Strong green
TCS12 3 PB 3/11 Strong blue
TCS13 5 YR 8/4 White skin
TCS14 5 GY 4/4 Moderate olive green (leaf)
TCS15 1 YR 6/4 Asian skin

As specified in (CIE 1995), the original test samples 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. These eight samples are employed to calculate the general color rendering index Ra. 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),[7] and their approximate Munsell notations are listed below.[8]

Owing to the obsolescence of the original samples, CIE TC 1-33 now recommends the use of a Macbeth (now X-Rite) color chart with 24 samples (Schanda & Sandor 2005), accordingly averaging over 24 samples.[9] Nevertheless, (Hung 2002) has determined that the patches in (CIE 1995) give better correlations for any color difference than the Macbeth chart, whose samples are not equally distributed in uniform color space.

[edit] Spectrophotometric method

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 5nm increments (CIE 2004), so it is suggested to use interpolation up to the resolution of the illuminant's spectrophotometry.

[edit] Typical values

Light source CCT (K) CRI
Low Pressure Sodium (LPS/SOX) 1800 ~5
Clear Mercury-vapor 6410 17
High Pressure Sodium (HPS/SON) 2100 24
Coated Mercury-vapor 3600 49
Halophosphate Warm White Fluorescent 2940 51
Halophosphate Cool White fluorescent 4230 64
Tri-phosphor Warm White Fluorescent 2940 73
Halophosphate Cool Daylight Fluorescent 6430 76
"White" SON 2700 82
Quartz Metal Halide 4200 85
Tri-phosphor Cool White fluorescent 4080 89
Ceramic Metal Halide 5400 96
Incandescent/Halogen Light Bulb 3200 100

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 black body radiators. The best possible faithfulness to a reference is specified by a CRI of one hundred, while the very poorest is specified by a CRI of zero. A high CRI by itself does not imply a good rendition of color, because the reference itself may have an imbalanced SPD if it has an extreme color temperature (see Criticism section).

[edit] Criticism and resolution

(Ohno 2006) and others have criticised CRI for not always correlating well with subjective color rendering quality in practice, particularly for light sources with spiky emission spectra such as fluorescent lamps or white LEDs. (Davis & Ohno 2006) identify several issues, which they address in their Color Quality Scale (CQS):

  • The color space in which the color distance is calculated (CIEUVW) is obsolete and nonuniform. Use CIELAB or CIELUV instead.
  • The chromatic adaptation transform used (Von Kries) is inadequate. Use CMCCAT2000 or CIECAT02 instead.
  • Calculating the arithmetic mean of the errors diminishes the contribution of any single large deviation. Two light sources with similar CRI may perform significantly differently if one has a particularly low special CRI in a spectral band that is important for the application. Use the root mean square deviation instead.
  • The metric is not perceptual; all errors are equally weighted, whereas humans favor certain errors over others. 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.
  • A negative CRI is difficult to interpret. Normalize the scale from 0 to 100 using the formula R_{out}=10 \ln \left[\exp(R_{in}/10)+1\right]
  • The CRI can not be calculated for light sources that do not have a CCT (non-white light).
  • Eight samples are not enough since manufacturers can optimize the emission spectra of their lamps to reproduce them faithfully, but otherwise perform poorly. Use more samples (they suggest fifteen for CQS).
  • The samples are not saturated enough to pose difficulty for reproduction.
  • CRI merely measures the faithfulness of any illuminant to an ideal source with the same CCT, but the ideal source itself may not render colors well if it has an extreme color temperature, due to a lack of energy at either short or long wavelengths (i.e., it may be excessively blue or red). Weight the result by the ratio of the gamut area of the polygon formed by the fifteen samples in CIELAB for 6500K to the gamut area for the test source. 6500K is chosen for reference since it has a relatively even distribution of energy over the visible spectrum and hence high gamut area. This normalizes the multiplication factor.

(CIE 2007) "reviews the applicability of the CIE colour rendering index to white LED light sources based on the results of visual experiments." Chaired by Davis, CIE TC 1-69(C) is currently investigating "new methods for assessing the colour rendition properties of white-light sources used for illumination, including solid-state light sources, with the goal of recommending new assessment procedures ... by March, 2010."[10]

[edit] See also

[edit] Footnotes

  1. ^ American approach is expounded in (Nickerson 1960), and the European approach in (Barnes 1957), and (Crawford 1959). See (Schanda & Sandor 2003) for a historical overview.
  2. ^ Note that when CRI was designed in 1965, the most perceptually uniform chromaticity space was the CIE 1960 UCS, the CIE 1976 UCS not yet having been invented.
  3. ^ For calculation of the CCT in CIELUV see (Schanda & Danyi 1977).
  4. ^ (CIE 1995), Section 5.3: Tolerance for reference illuminant
  5. ^ The coefficient was chosen as 4.6 so that the CRI of the CIE standard illuminant F4, a "warm white" calcium halophosphate fluorescent lamp would be 51 (Schanda & Sandor 2003)(Schanda 2002). Today's fluorescent "full-spectrum lights" boast CRIs approaching 100; e.g, Philips TL950 or EP patent 1184893 . (Thornton 1972) compares older products.
  6. ^ It appeared that Ri could be negative (ΔEi ≥ 22), and this was indeed calculated for some lamp test colors.
  7. ^ TCS spectra in CSV form, Korea Research Institute of Standards and Science.
  8. ^ Munsell Renotation Data, Munsell Color Science Laboratory, Rochester Institute of Technology
  9. ^ X-Rite ColorChecker Chart
  10. ^ CIE Activity Report. Division 1: Vision and Color, pg.21, January 2008.

[edit] References

[edit] External links