Production and proofing substrates often differ in their white points. Substrate white points frequently differ
between reference and sample, for example between proof and print, or between a target paper colour and an
actual production paper. It is possible to generate characterization data for the printing process on the
production side to achieve an accurate colorimetric match but in many cases it is not practical to generate this
data empirically by printing samples and measuring them1. This approach however, does not account for any
degree of adaptation between the differing substrate white points whereas its acceptability may depend on
accounting for the change in paper colour such that appearance preservation of the original when printed on
the production substrate is attained.
The perceived colour of a stimulus depends on the conditions under which it is viewed. For colours employed as an
important cue or identifier, such as signage and brand colours, colour reproduction tolerances are critically important.
Typically, such stimuli would be judged using a known level of illumination but, in the target environment, the level of
illumination used to view the samples may be entirely different. The effect of changes in the viewing condition on the
perceptibility and acceptability of small colour differences should be understood when such tolerances and associated
viewing conditions, are specified.
A series of psychophysical experiments was conducted to determine whether changes in illumination level significantly
alter acceptability and perceptibility thresholds of uniform colour stimuli. It was found that perceived colour
discrimination thresholds varied by up to 2.0 ΔE<sub>00</sub>. For the perceptual correlate of hue however, this value could be of
significance if the accepted error of colour difference was at the threshold, thereby yielding the possibility of rejection
with changes in illumination level. Lightness and chroma on the other hand, exhibited greater tolerance and were less
likely to be rejected with illuminance changes.