Some materials change colour with temperature.


This thermochromism occurs with some red, orange, and yellow pigments that contain selenium and it has been shown that, in such cases, it is necessary to standardise the temperature at which the measurements are made.


For this purpose, ‘room temperature’ is often a sufficiently precise definition, because significant changes usually occur only when the sample is quite hot.

For purposes of standardisation however, it is more correct that measurements be made at the temperature (and relative humidity) that were used by the calibration laboratory.
These values should be found on the calibration certificate supplied by the laboratory.
Although instruments are normally operated at ‘room temperature’, the samples may warm up under other circumstances.

If measurements are made on samples illuminated with ‘white’ light, then there may be a sufficient rise in sample temperature for some significant thermochromism to take place. This can be checked by taking a series of measurements with the sample left in the measurement position in the instrument; if the measurements show a steady drift with time, thermochromism may be taking place. It can be noted that many modern integrating-sphere based spectrophotometers use a pulsed xenon lamp to provide the illumination, and the very short duration of the flash precludes any significant rise in temperature.

The table below shows the changes in CIELAB values, L*, a*, b*, Cab* and Hab* for a 10 C° rise in temperature ( Δ= value at 25 C° minus the value at 35 C°); these data are applicable to any geometry and are calculated using CIE Standard Illuminant D65 and the CIE 10 Standard Observer.