FAQ: ""We have conducted a study involving hair with very low L* values (between 0.5 and 3). We were wondering if the ColorFlex EZ unit suffers from any sort of non-linearity in a* at these low L* levels? I appreciate that color theory dictates the two are orthogonal and so a* should be unaffected by L*."
When your customer standardizes the CFEZ, they set the bottom of scale with a black glass standard. This is a physical reference for 0% (with corresponding values of L* = 0, a* = 0, b* = 0) and the CFEZ is forced to match that point by the standardization process. So, L* values of 0.5 to 3 are measurably significant.
Even at the black glass level, the signal counts are not 0 but typically are still a few hundred signal counts to there is measurement sensitivity and linearity right down to 0% reflectance.
I would disagree with the statement that a* is unaffected by L*. It's exactly the opposite. While L*, a*, b* are orthogonal to each other as color scale coordinates, they are also highly correlated. That is, in theory, as all colors become darker and darker with increasing absorbance, the L*, a* and b* values all approach 0, a perfect black of 0% reflectance.
So, measurement down to L* = 0, a* = 0, b* = 0 is possible but the question to your customer is, are they getting meaningful results in terms of product quality in this dark and murky area of color space in the L* = 0.5 to 3.0 range? Is this the normal range of their dyed hair lots (natural black hair is seldom this dark)? Are poor lots higher in L*? We can measure in that dark area of color space but are the values meaningful in terms of product quality?