6

I have a Xrite Colorchecker Classic:

enter image description here

  • Where to find the color specifications for each square (either in CMJN or any other format)?

    Let's say I want to develop my own open-source color correction software (that can use a color checker), then these reference numbers would be necessary.

  • Also, which square of the bottom row is equivalent to a 18% gray card?

    A gray card is a flat object of a neutral gray color that derives from a flat reflectance spectrum. A typical example is the Kodak R-27 set, which contains two 8x10" cards and one 4x5" card which have 18% reflectance across the visible spectrum, and a white reverse side which has 90% reflectance.

3

The official specifications are available on X-Rite website at this URL: https://xritephoto.com/ph_product_overview.aspx?ID=938&Action=Support&SupportID=5884. The specifications have changed in November 2014, hence the two sets.

They are given in CIE Lab and the white point is CIE Illuminant D Series D50 with the following CIE XYZ values: [96.42, 100.00, 82.49] as given by ICC.

Example: in the case of November 2014 Colorchecker Classic, here it is:

ColorChecker24_After_Nov2014.txt
################################

LGOROWLENGTH 12                     
ORIGINATOR "ColorChecker24 - November2014 edition and newer"
MANUFACTURER "X-Rite - http://www.xrite.com"                

4/28/2015  # Time: 14:33            
"i1Pro 2 ; Serial number 1001785"           
"MeasurementCondition=M0    Filter=no"      

NUMBER_OF_FIELDS 4                      
BEGIN_DATA_FORMAT                       
SAMPLE_NAME Lab_L   Lab_a   Lab_b                       
END_DATA_FORMAT                     
NUMBER_OF_SETS 24                       
BEGIN_DATA                      
A1  37,54   14,37   14,92
A2  62,73   35,83   56,5
A3  28,37   15,42   -49,8
A4  95,19   -1,03   2,93
B1  64,66   19,27   17,5
B2  39,43   10,75   -45,17
B3  54,38   -39,72  32,27
B4  81,29   -0,57   0,44
C1  49,32   -3,82   -22,54
C2  50,57   48,64   16,67
C3  42,43   51,05   28,62
C4  66,89   -0,75   -0,06
D1  43,46   -12,74  22,72
D2  30,1    22,54   -20,87
D3  81,8    2,67    80,41
D4  50,76   -0,13   0,14
E1  54,94   9,61    -24,79
E2  71,77   -24,13  58,19
E3  50,63   51,28   -14,12
E4  35,63   -0,46   -0,48
F1  70,48   -32,26  -0,37
F2  71,51   18,24   67,37
F3  49,57   -29,71  -28,32
F4  20,64   0,07    -0,46
END_DATA
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  • Thank you for your answer! – Basj Nov 3 '19 at 18:41
  • Something I don't understand (it's the same in your answer and mine) @KelSolaar: the black square (bottom right square) of the Colorchecker should be Lab(20.64,0.07,-0.46), i.e. ... ~ RGB(50, 50, 50), i.e. this color: htmlcsscolor.com/hex/323232! This is definitely not what we want for the darkest black in a photo, it's too light! What do you think about this mystery? – Basj Nov 3 '19 at 23:19
  • 1
    The bottom right square is not really meant not represent the darkest black in a photo, merely something with low reflectance, charcoal, Vantablack or MIT's Blackest Black. However, it is part of the bottom row which is a perceptually uniform gradient and is super helpful to assess non-uniformity of the lighting. – Kel Solaar Nov 4 '19 at 6:34
7

After further research, here is a partial answer taking as source Blog-Couleur: Quelles sont les valeurs RVB de la colorchecker ? and BabelColor PDF and the Xrite website.

  • The last row seems to be white 95%, neutral 80%, neutral 65%, neutral 50%, neutral 35%, black 20%.

    On trouve entre le blanc et le noir, une série de 4 gris : neutral 0.35, neutral 0.50, neutral 0.65 et neutral 0.80. Le neutral 0.50 de luminosité 50 % est le fameux gris moyen de luminance 18 %.

    Thus, the fourth square of the last row seems to be the 50% neutral gray (18% reflectance) of a gray card. In sRGB, it should be close to RGB(119,119,119) according to some sources (color #777777 in hex) or RGB(122,122,121) or RGB(121,121,122) according to other sources.

    More about this here.

  • More generally, the website BabelColor / Danny Pascale gives a lot of data (RGB, Lab) related to various versions of Colorchecker:

    enter image description here

  • Here (PDF) is the data given by Xrite themselves:

    enter image description here

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  • "The code value of 118 in an 8-bit system corresponds to 18% of the maximum final output for sRGB." - ISO 12232, 3rd edition, 2019-02 "Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index", page 11. – Iliah Borg Nov 5 '19 at 19:22

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