Tutorial Notes – An Introduction to Colour Measurement
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Colour test charts Figure 25. Three test targets designed for characterisation of digital cameras: (left) Macbeth ColorChecker; (centre) GretagMacbeth ColorChecker DC; (right) GretagMacbeth ColorChecker SG.
For digital cameras three colour test targets are commonly used, with colorants typical of natural scenes. The traditional target is the Macbeth ColorChecker, originally developed for film cameras and television cameras (McCamy et al, 1976). It has 24 patches in a 3:2 aspect ratio, including a 6‐ step grey scale. For digital cameras the more recent Gretag Macbeth DC and SG charts were developed (Fig. 25), with 12x20=240 and 10x14=140 colour patches respectively. The patches in the original ColorChecker and the DC chart are matte, whereas the SG chart is semi‐gloss. The SG chart includes the same 24 colours of the original ColorChecker plus 16 skin tones, making it good for characterising cameras for portrait photography. A miniature version of the Macbeth ColorChecker chart is also available, having credit‐card‐size dimensions of 82x57 mm, with each patch being 9x9 mm. The reflectance spectra of the 24 patches (Fig. 26) were measured three times with the eye‐one spectrophotometer, with data imported via the Key Wizard software utility directly into an Excel spreadsheet. Reflectance data were recorded from 380 to 760 nm at intervals of 10 nm. The corresponding colorimetric data for the CIE tristimulus values X,Y,Z, chromaticity values x,y and uniform colour coordinates L*,a*,b* were calculated using the D50 illuminant and 2‐degree standard observer. The colour accuracy of the mean of the three sets of measured data was assessed against the standard data provided by X‐rite (Pascale, 2005), and showed an average error (ΔE*ab) of 1.06, with the worst colours being purple (ΔE*ab =3.86) and blue (ΔE*ab =2.45).
Figure 26. Reflectance spectra of 24 patches in MiniMacbeth colour chart, measured by eye‐one.
Lindsay MacDonald, London College of Communication