The grey scale consists of nine pairs of non-glossy neutral grey coloured chips, which illustrate the perceived colour differences. These are used along side the blue wool samples to determine the corresponding fastness rating (as described in ISO 105-A02).
The problem with the blue wool scale and the grey scale is that it is a subjective test i.e. it depends on an experienced operator comparing changes in test swatches versus reference swatches. Hence it can lead to some variability. Secondly, significant changes on the blue wool scale are not particularly onerous with respect to outdoor performance of most moulded or fabricated products. Whilst it is a useful reference for printed medias and the textile industry, products that require significant outdoor exposure will require a rating significantly greater than the maximum of 8.
Most modern testing now uses colourimeters or spectrophotometers to accurately measure any colour change or colour differences and report these as a Delta Eab reading. At Materials Technology we will typically use the colourimeter measurement method using the L*A*B* colour space model along with Delta Eab readings. A full explanation and interpretation of this technique is supplied with our reports as standard.
A brief summary of the LAB colour space is also provide below. The diagram to the right demonstrates the L, A and B coordinate systen.
The L*a*b colour space system defines the colour of an object as a 3 dimensional coordinate within a colour sphere. The L coordinate represents lightness and runs from 100 (complete lightness) to 0 (complete darkness), a is the red direction –a is the green direction, +b is the yellow direction and –b is the blue direction. L*a*b readings taken are an average over a 2mm spot size in each case under standardised lighting conditions.
Delta E* represents the difference in each of the L, a and b readings compared to the initial readings and hence show the significance of change in each element. In quoting overall colour difference values it is common to produce a single value referred to as Delta E*ab, this is produced using Pythagoras theorem and gives an absolute value for the size of the colour difference (magnitude) , but not the direction of change.
A value of about 1 is just about perceptible to the human eye under normal lighting.