| Résumé : |
The colour shade quality control scenario is ubiquitous within the cosmetic hair colouring industry. Visual inspection is nowadays the go-to standard for the evaluation of dye application outcomes, even although the increased availability of easily operated spectrophotometers has favoured the introduction of colorimeters in the manufacturing process. Human hairs, however, are very efficient scatterers, and this makes their instrumental measurements more difficult and less consistent. To assess 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 fibre hair swatches. First, we analysed results from two measuring modalities made available by instrumentation: Specular Component Included and Specular Component Excluded. Then, 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 analysed measurement variance. Here, we briefly discuss the qualitative visual inspection of spectra, before switching to ΔE to show the unreliability, at least as far as lighter coloured dyes are concerned, of the classic target-sample measurements which are employed to issue pass-or-fail verdicts. Despite quantitative measurements being an asset for quality control, we suggest that greater care should be placed in spectrophotometric readings when human hair is involved, as opposed to more traditional fields of application. |
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Consistency issues in hair spectral reflectance and colour measurements in COLORATION TECHNOLOGY, Vol. 136, N° 3 (06/2020)
