RAL, sRGB, L*a*b*… What is color ?

What is color ?

The color observed (CO) of an object is the combination of the color itself (C), the light source (L) and the sensitivity of the observer (S).

CO = L * C * S

The light can change modifying the color observed.

The CIE (Commission internationale de l'Eclairage) was to standardize the light source … and the observer to reach the absolute object color.

Standardized light source are Daylight D65, C, Incandescent light A, Fluorescent light F2, F11 for instance. The most used is the D65. Those are represented as spectral power distribution.

Standardized observer has been established by numeruous observers and systematic visual tests integration small and large field of view : CIE 1931 with 2° FOV ​and CIE 1964 with 10° FOV respectively.

From the previous visual tests and the three-component theory of color vision, the concept for the XYZ tristimulus values has been built so that each visible color has non-negative XYZ coordinates. It resulted the CIE 1931 standard Observer having the color matching functions shown in the figure here below.

The XYZ tristimulus value are then directly calculated using the Standard Observer color-matching functions. For a given intensity spectrum  emitted or being the result of the light reflected by the object, those values are :

The CIE standard observer color matching functions

So with a standardized lighting and observer, we can measure the absolute color C !

From this XYZ color space, a chromaticity diagram as been defined as the first lightness independent representation to easily visualize colors : this is the CIE xyY color space with x= X / (X+Y+Z), y = Y / (X+Y+Z), z = Z / (X+Y+Z)

The CIE 1931 color space chromaticity diagram. The outer curved boundary is the spectral (or monochromatic) locus, with wavelengths shown in nanometers.[1]

What is a color space ? What is L*a*b*

The CIE 1931 XYZ color space is the basis for almost all other color spaces.

The CIE RGB color space is a linear transformation of the CIE XYZ. It is defined by three primary colors that are monochromatic (blue at 435.8nm, green at 546.1nm, red at 700nm).

Gamut of the CIE RGB primaries and location of primaries on the CIE 1931 xy chromaticity diagram

Gamut of the CIE RGB primaries and location of primaries on the CIE 1931 xy chromaticity diagram [1]

Other derivatives of the CIE XYZ are the CIE LUV, CIE LAB.

The CIE L*a*b*, with L for lightness and a and b for the color components is a non linear transformation of the CIE XYZ with the intention to approximate the human vision.

It is the device-independent model used as a reference that mimics the non linear response of the eye and one of the main usefullness of this model is to mesure perceptual color differences as the Euclidean distance between two L*a*b* coordinates.

delta E = sqrt(L^2+a^2+b^2)

What is a gamut ?

The CIE XYZ color space represents all the colors the the standard observer can see but all those colors can not be reproduced with three primary colors.

The gamut is subset of the CIE XYZ colors. In color reproduction it is often a triangle where the extremities are defined by three primary colors.

In additive color reproduction system, common RGB gamut color space are sRGB, Adobe RGB.

In substractive reproduction, the most used gamut is the CMYK.[2]

CIE 1931 xy chromaticity diagram showing the gamut of the sRGB color space and location of the primaries. The D65 white point is shown in the center. The Planckian locus is shown with color temperatures labeled in Kelvin. The outer curved boundary is the spectral (or monochromatic) locus, with wavelengths shown in nanometers (blue). Note that the colors in this displayed file are being specified using sRGB. Areas outside the triangle cannot be accurately colored because they are out of the gamut of sRGB, therefore they have been interpreted.


What is the RGB of a RAL Colour ?

RAL, as Pantone, HKS… are colour matching systems providing to the users a set of color references that can be used for quality control.

Those colors are absolute and list of L*a*b* coordinates for those color references are available.[3]

Those colors are often organised in Hue, Lightness and Chroma. 


But the reproduction of a L*a*b* color implies the choice of a gamut.

When looking on internet for the RGB values of a given RAL color we found various results : for instance RAL 1000 can be

(214, 199, 148), (http://www.ralcolor.com/)

(200, 186, 127), (http://www.easyrgb.com/index.php?X=TINT#Result)

(204, 204, 153), (http://www.pats.ch/formulaire/unites/unites11.aspx)

(201, 194, 140) (http://www.e-paint.co.uk/Lab_values.asp?cRange=RAL%20Classic&cRef=RAL%201000&cDescription=Green%20beige)


RAL ID   L* a* b* R G B     Name
RAL 1000   77,13 -0,59 25,90 205 186 1336 #CBDA88 #CCC58F Green beige
RAL 2000   60,35 34,64 54,65 218 110 0 #DA6E00 #DD7907 Yellow orange



[1] http://en.wikipedia.org/wiki/CIE_1931_color_space

[2] http://en.wikipedia.org/wiki/Gamut

[3] http://en.wikipedia.org/wiki/List_of_RAL_colors