

Color Rendering Index (CRI)
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CRI is a unit of measure that defines how well colors are
rendered by different illumination conditions
in comparison to a standard (i.e. a thermal radiator or daylight).
When CRI is calculated, it can be rated on a scale from 0  100.
On this scale, a CRI of 100 would represent that all color samples illuminated by a light
source in question,
would appear to have the same color as those same samples illuminated by a reference
source. A negative CRI can be calculated, but a goal for most lighting
systems that produce white light is to have a positive CRI.
The technical CRI report was prepared
by the
CIE (Commission Internationale de l'Eclairage) Division 1 Vision and Colour.
Calculating
CRI back to top

Colorimetric data must be transformed from CIE 1931
values (X, Y, Z, x, y) to the (u, v) coordinate of the 1960 diagram by means of the
following:
u = 4X/(X + 15Y + 3Z)
(11a)
v = 6Y/(X + 15Y + 3Z)
(11b)

To account for the adaptive color shift due to the
different state of chromatic adaptation under the lamp to be tested, k, and under
the reference illuminant, r, use the following formula:
(12)
The values u'k,i and v'k,i are the chromaticity coordinates of a test color sample, i, after the
adaptive color shift, obtained by moving the light source to be tested to the reference
illuminant, i.e. u'k = ur v'k = vr and should not be mixed up with CIE 1976 u', v'
coordinates.

The functions c and d for use in equation (12) shall
be calculated for the light source to be tested uk, vk, and the test color samples i under the light source to be tested uk,i, vk,i according to the following:
c = (4  u  10v)/v
(13)
d = (1.708v + 0.404  1.481u)/v

Color imetric data must now be transformed into the 1964 Uniform Space
coordinates by the following:
The values u'k = ur, v'k = vr are the chromaticity coordinates of the light source to be tested after consideration of
the adaptive color shift. The values Yr,i and Yk,i must be normalized so that Yr = Yk = 100.

One must calculate the difference between the resultant color shift of
the test color sample under the test lamp k and illuminated by the reference r.
The following 1964 Colour Difference Formula should be used:
(15)

Calculate the Special Color Rendering Index, Ri, for
each test color sample by the following:
Ri = 100  4.6DEi
(16)

Calculate the general Color Rendering Index, CRI, by the following:
(17)
The calculation uses 8 test
color samples typically.
CIE test
color samples: back to top
No.

Approximate
Munsell Notation

Colour appearance under daylight

1

7,5 R 6/4

Light greyish red

2

5 Y 6/4

Dark greyish yellow

3

5 GY 6/8

Strong yellow green

4

2,5 G 6/6

Moderate yellowish green

5

10 BG 6/4

Light bluish green

6

5 PB 6/8

Light blue

7

2,5 P 6/8

Light violet

8

10 P 6/8

Light reddish purple

9

4,5 R 4/13

Strong red

10

5 Y 8/10

Strong yellow

11

4,5 G 5/8

Strong green

12

3 PB 3/11

Strong blue

13

5 YR 8/4

Light yellowish pink (human complexion)

14

5 GY 4/4

Moderate olive green (leaf green)

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