Definition
Magnitude scaling (also: magnitude estimation) is a psychophysical method in which the colour assessor must not only determine whether two colours differ, but also how largethat difference is. The assessor gives a numerical or visual estimate of the perceived distance between two colour stimuli.
Background
In the context of ASTM E1499-16, magnitude scaling is described in §6.4 as a supplementary qualification test. The standard states that the ability to correctly estimate the magnitude of colour differences represents a higher competency than simply detecting a difference.
§6.4.2 specifically describes the magnitude scaling module of the Japanese Color Aptitude Test: the colour assessor is shown colour pairs and assesses the magnitude of the difference on a scale. The correlation between the assessor's estimate and the actual ΔE value determines the qualification score.
This is particularly relevant for professional colour assessors who make pass/fail decisions: they must not only see a difference, but also correctly estimate whether the difference falls within or outside the acceptance tolerance.
Relevance to ColorAptitude™
ColorAptitude™ implements magnitude scaling directly in the platform. The colour assessor evaluates colour pairs across three dimensions (hue, lightness, chroma) and provides an estimate of the difference magnitude. The scoring calculates the correlation between the estimate and the actual OKLCH-based perceptual distance. Cross-comparisons with industry partners that report deviations in CIEDE2000 are supported.
By measuring magnitude scaling across all three colour attributes — not only hue as in the original Japanese Color Aptitude Test — ColorAptitude™ provides a more complete picture of assessor competence.