Advances in Instrumental Colour Pass/Fail Analysis

The requirement to be able to make a reliable decision about whether two samples are a visual colour match is an important commercial consideration for most textile production companies [1,2]. Traditionally visual pass/fail analysis was carried out by trained colourists. However, these visual decisions are subject to a number of problems which can make them unreliable. Approximately 8% of the male population have a colour vision deficiency (known colloquially as colour blindness) which means that pairs of samples may be a visual match to them despite appearing to be very different to other so-called normal observers [3]. Some variation in colour vision also is found in normal observers [4] and the effect of the viewing environment can affect visual decisions. Although the use of high-quality viewing cabinets can reduce variation in pass/fail decisions even the colour of the background in the cabinet against which the pair of samples are viewed can greatly affect the magnitude of the visual difference (the ‘crispening’ effect) [5]. In 1953 an analysis of 287 pairs of samples were visually assessed by 8 trained colourists [6] and a later analysis of these data showed that 24.5% of the pairs that should pass were rejected and 13.3% of the pairs that should be rejected were passed [1]. Several such studies have since been carried out and it is widely understood that as many as 25% ‘wrong decisions’ are made by professional colourists when making visual pass/ fail decisions [7]. This variability in the pass/fail decision is potentially costly and instrumental methods have been available for at least 50 years. This paper considers the current state of instrumental colourdifference evaluation and highlights some best practice.