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Using a standard grey scale for colour change in a multi-angle colour-assessment cabinet

AutorMelgosa, Manuel; Gómez-Robledo, Luis; Cui, G.; Li, C.; Ferrero, Alejandro ; Bernad, Berta ; Campos Acosta, Joaquín ; Richard, N.; Fernández-Maloigne, C.
Fecha de publicación6-sep-2016
EditorInternational Commission on Illumination
Citación4th CIE Expert Symposium on Colour and Visual Appearance (2016)
ResumenIn recent colour-difference experiments involving solid and effect samples [M. Melgosa et al. Optics Express 22, 3458-3467, (2014); Journal of Physics: Conference Series 605 (2015) 012006; Proc. AIC 2015, pp. 1180-1185 (2015)] the standard grey scale for colour change of the Society of Dyers and Colourists (SDC) has been positioned inside a multi-angle bykospectra colour-assessment cabinet. In these experiments the grey scale was illuminated by a light source close to CIE illuminant FL11 at a fixed angle of around 45° with respect to the perpendicular to the sample, and observed at 6 different viewing angles of -15°, +15°, +25°, +45°, +75° and +110°, measured with respect to the direction of the specular reflection (ASTM E2194.01, 2011). In this situation, the colour differences of the 9 colour pairs of this grey scale exhibited some variability, which may be mainly attributed to the lack of uniform and directional illumination on the samples in this cabinet and the goniochromatism of the samples. This paper focus on the measurement of such variability and potential best practices for future experiments using this kind of multi-angle colour cabinets and standard grey scales. First, we performed 3 independent spectroradiometric measurements of each sample in the SDC grey scale placed in the byko-spectra cabinet (plus a PTFE reference white placed on top of each sample) for the 6 viewing angles mentioned above. We employed a Konica Minolta CS 2000 spectroradiometer (1° visual field, CIE 1964 colorimetric observer) placed at the same position that the head of the observer performing visual experiments in the cabinet. On the average, for the 9 colour pairs, the standard deviation of the measured colour differences at the 6 viewing angles was 0.81 CIELAB units (range from 0.49 to 1.39 CIELAB units), around 4 times higher than the standard deviation of the 3 independent acquisitions made with our spectroradiometer, where it was found an average standard deviation of 0.22 CIELAB units (range from 0.12 to 0.43 CIELAB units). These results suggest that the lack of uniformity in lighting cabinets and the goniochromatism of the samples are relevant factors to be taken into account. In addition, we noticed that for colour pair #5 (two identical physical samples) the average colour difference for the 6 viewing angles was 1.60 CIELAB units (range from 0.79 to 2.07 CIELAB units), considerably higher than the expected null value. This result can be attributed to the lack of uniform illumination when the colour pairs are not positioned just in the center of the multi-angle cabinet, and the problem can be partly solved by using a chromatic adaptation transform to the average reference white of the two samples in each colour pair. Second, in order to analyze only the goniochromatism of the samples, we decided to perform additional BRDF measurements for each of the samples in the SDC grey scale, and also for each of the samples in the grey scale provided by the American Association of Textile Chemists and Colourists (AATCC), which seems visually identical to the SDC scale. Such measurements were made using the gonio-spectrophotometer available at Instituto de Óptica, CSIC, Madrid (A. M. Rabal et al., Metrologia 49, 213-223, 2012). For comparison purposes, we only considered in our next analyses the illumination angle of 45° and the 6 viewing angles used in the spectroradiometric measurements described in previous paragraph. From the experimental measurements of spectral reflectance factors, we assumed the CIE FL11 illuminant and CIE 1964 colorimetric observer for the computation of colour coordinates and colour differences. On the average, for the 8 colour pairs (now the colour pair #5 with two 4th CIE Expert Symposium on Colour and Visual Appearance - Abstract Booklet
DescripciónPrague, Czech Republic, September 6 - 7, 2016 ; http://div2.cie.co.at/?i_ca_id=985
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