Lovibond
®
PFX
i
Series Operator’s Instruction Manual V1.6
Page 36 of 53
Understanding Off Hue
When using a visual system to compare samples with glass standards, it is possible to see
when the sample becomes too different in colour to make accurate colour comparisons. With
automatic instruments, the nearest scale point is often displayed with no consideration of the
colour difference between the sample and the colour scale. If the colours are not good
matches, this approach increases the variability between visual and automatic methods.
Most colour scales such as Gardner, ASTM D1500 and Pt-Co are progressions of colour,
most often from a pale yellow through to deep amber. The diagram below shows a
representation of the Gardner scale.
If the measured sample has a hue similar to that of one of the colour standards, then the user
will find it easy to make a match in the visual system. This can be seen with the diagram
below, where the different colour standards are shown on top and the sample to be measured
below. It is quite apparent that the standard with the arrow above it is the best match.
If the hue of the sample is dissimilar to the colour standards, making a match visually
becomes far more subjective. In the diagram below, the nearest colour to the sample is
shown by the arrow and it is this value that would be predicted by automatic instruments.
Visually, because of the hue difference, the value selected might well be the next standard
along to the right. The PFX
i
instrument would report this sample with an Off Hue factor of 5.
