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Monday, December 23, 2013

What's a reasonable color tolerance?

A customer recently asked, "What are visually perceptible color differences in L* a* b* coordinates?" The answer is not so easy.

Generally speaking in the Paper Industry we are normally dealing with near-white colors. In that case, we often use the rule of thumb that +/- 0.3 in L*, a* or b* is visually perceptible. However, when we start to look at more saturated colors the question becomes much more difficult.

L*a*b* color tolerance
Color tolerance vs. Visual acceptability
If we set a tolerance based on L* a* b*, the color space we are looking at is cubical. However, when we plot actual visual acceptability it is more ellipsoidal-shaped.  See at the right, the black shading represents numerical acceptance, but visually unacceptability.

Obviously, there is a difference here. If we look at the ΔEcmc tolerancing which is based on ellipsoidal tolerances, this does a much better job of matching visual and numerical acceptability.  Looking at a particular cross-section of the a* b* space (below), we can see that the visually acceptable ellipses vary in size depending on the position in color space. The ellipses in the orange area of color space are longer and narrower than the broad and rounder in the green area. The shape of the ellipses are larger as the color increases in chroma (away from a*=0, b*=0).
This means that visually acceptable differences in L*, a* and b* differ depending where we are in color space.  A Δa*=0.5 would be noticeable on a near-white, but Δa*=5.0 on a red may not be noticeable.

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