Colour perception testing

A commonly-used device for testing colour perception is the Isihara test. This consists of a number of plates composed of numerous spots of different colours and sizes. Incorporated into this random arrangement are a number of dots, tracing out numerals, which are of a colour distinguishable from the remainder. The colour sensitivity of the observer s eye is assessed by his ability to read these numbers. It is also possible to detect different kinds of colour blindness with the plates. Four of them, for example, each contain two numbers one in scarlet and the other in purple. Those who suffer from panatropy cannot see the scarlet spots and only read the purple ones, but the deuteranopes read only the scarlet numerals those with normal vision see both numbers. (For further information see JORDINSON AND MlNSHALL, J. 5.D.C., 1959, 75, 585.)... 

The ision ])rocess is tlierefore prone to various influences which can affect the perception of detail, briglitness, contrast and colour. Through tests such as those used to examine for colour blindness, the Camouflage Section was able to identify optical illusion-type effects and other methods of visual confusion utilising particular mixes of colours, all of which were incorporated in systems of protective painting for ships. 

A solution of iodine in aqueous iodide has an intense yellow to brown colour. One drop of 0.05M iodine solution imparts a perceptible pale yellow colour to 100 mL of water, so that in otherwise colourless solutions iodine can serve as its own indicator. The test is made much more sensitive by the use of a solution of starch as indicator. Starch reacts with iodine in the presence of iodide to form an intensely blue-coloured complex, which is visible at very low concentrations of iodine. The sensitivity of the colour reaction is such that a blue colour is visible when the iodine concentration is 2 x 10 " 5 M and the iodide concentration is greater than 4x 10 4M at 20 °C. The colour sensitivity decreases with increasing temperature of the solution thus at 50 °C it is about ten times less sensitive than at 25 °C. The sensitivity decreases upon the addition of solvents, such as ethanol no colour is obtained in solutions containing 50 per cent ethanol or more. It cannot be used in a strongly acid medium because hydrolysis of the starch occurs. 

There are other minor nonequivalencies of ISO test procedures compared to ASTM standards, but for color difference the ISO Test Procedure No. 105 is unique. Those who use Colour Measurement Committee (CMC) procedures—particularly CMC 2 1 Lightness to Color ratio—claim that it facilitates a uniform description for acceptability decisions that is better than any other system in existence. These equations permit the use of a single number tolerance, DEcmc, in a nearly uniform color space. The CMC formula is a modification to the perceptibility CIELAB formula. It is fully described elsewhere in this book, but it deserves some brief notice here because, after all, it is an ISO procedure. The CMC developed the basic British Standard No. 6923, Calculation of Small Color Differences. Soon afterward, in 1989, the American Association of Textile Chemists and Colorists (AATCC) adopted AATCC Test Method 173, CMC Calculation of Small Color Differences for Acceptability. Ford Motor Company indicated a preference for using CMC 2 1 ratio color difference for plastics weathering data for plastics interior trim materials. 

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