Consistency issues in hair spectral reflectance and color measurement

The color shade quality control scenario is ubiquitous within the cosmetic hair coloring industry. Permanent hair dyes are an especially marketable product, so their diffusion is dependent on stringent quality controls that guarantee product safety and adherence to expected results. To this day, visual inspection is the go-to standard for the evaluation of dye applications outcomes, even though the increased availability of easily operated spectrophotometers has favored the introduction of colorimeters in the manufacturing pipeline. Human hairs, however, are very efficient scatterers 1 and this makes their instrumental measurement more difficult and less consistent. In order to test the degree to which this intrinsic property compromises the reliability of spectrophotometric measurements, we tested a variety of samples of human hair locks (swatches), both dyed and undyed, and those of nylon fiber hair swatches. First, we analyzed results from two measuring modalities made available by instrumentation: Specular Component Included (SCI) and Specular Component Excluded (SCE). Then, in order to modify macroscopic spatial orientation, we arranged swatches so that hair strands would cross at varying angles. Measurements confirmed the expected dependencies, namely the clear influence of the scattered component and of sample orientation on instrumental reading outputs. Having settled on a preferred measuring setup, we analyzed measurement variance. Again, we evaluated two specific aspects, which tie directly into industrial quality assessments. We briefly discussed qualitative visual inspection of spectra. We then switched to ΔEs to show the unreliability, at least as far as lighter colored dyes are concerned, of classic target-sample measurements employed to issue pass-or-fail verdicts. Despite quantitative measurements being an asset for quality control, we suggest that greater care is placed in spectrophotometry readings when human hair is involved, as opposed to more traditional fields of application.