VBAI - CIELAB color measurement with "measure colors setup"

[QMT-ado]

Hello,

We are currently working on a colorimetric application under VBAI 2018 SP1.

When we use the step "measure color", we noticed that the color measure "L* a* b*" was not in agreement with international standards:

Indeed, "a*" is normally between -128 and 128. On the other hand, VBAI measures only from -86,18 to 98,24.
Similarly for b*, VBAI measures from -107.86 to 94.48 instead of -128 to 128.

Do you know why VBAI presents these ranges of measurements, which are not at the international standards?
Would you have an explanation on the fact that these bounds are not from -128 to 128 ?

Thank you in advance, have a nice day
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Bonjour,

Nous travaillons actuellement sur une application colorimétrique sous VBAI 2018 SP1.

Lorsque nous utilisons le step "measure color", nous avons remarqué que la mesure de couleur "L* a* b*" n'etait pas aux normes internationales :

En effet, la grandeur a* est normalement comprise entre -128 et 128. En revanche, VBAI mesure uniquement de -86,18 à 98,24.
De même pour b*, VBAI mesure de -107,86 à 94,48 au lieu de -128 à 128.

Savez vous pourquoi VBAI présente ces plages de mesures qui ne sont pas aux normes internationales ?
Auriez vous une explication sur le fait que ces bornes ne soit pas de -128 à 128.

Merci d'avance, bonne journée

[rolfk]

Your understanding of the CIELab standard isn't quite correct. There is no hard limit on the a and b components of CIELab in the theoretical model. The practical maximum and minimum values depend on the used color samples, object surface and lighting. And as you can see they are not what you claim they should be.

The RGB color space normally used in cameras and displays nowadays is even more limited in the possible color space so I assume these are the practical values that can be achieved from an (idealized) RGB source. Without color calibration however, your RGB data can not really be translated to an absolute color space model like CIELab without additional, substantial errors.