White panel thermometer

The temperature of exposed samples is dependent on both the air temperature in the cabinet and the absorbance of direct radiation. Temperature is usually measured with a black panel thermometer, which gives the surface temperature of a perfectly absorbing material. White panel thermometers are also commonly used which measure the other extreme. The actual temperature reached by a test piece depends on the material and its colour. It will also depend on the air temperature and velocity so that both the air and black panel temperatures should be controlled. ISO 11403-3 [23] defines three sets of conditions in air with the black standard temperature at 65 °C (ISO 4892-2 Method A [27]), behind glass at the same temperature (ISO 4892-2 Method B [27]), and behind glass at 100 °C. 

Samples with different colored or even white surfaces show different deviations to the black standard temperature. A so-called white standard thermometer (WST), indicating the lowest surface temperature of a sample whose surface is directly absorbing and reflecting (also described in the basic ISO standard), can be used. Alternatively, a white panel thermometer (WPT) may be used. The temperature of samples with colored surfaces ranges between the black and white standard temperatures. Constant black and white standard temperatures under given exposure conditions during a test indicate constant surface temperature of any exposed sample — even if, in the field of pharmaceutical substances, the real surface temperature of specifically prepared samples may be above or below the characterizing standard temperatures. 

The absorption ratio of various materials differs greatly. However, knowing the upper and lower limit is sufficient for making an estimate. Under otherwise identical conditions, an object whose absorption ratio is 1 will exhibit the greatest rise in temperature compared to its surroundings, while one with an absorption ratio of 0 will heat up least. In materials testing, black and white panel thermometers have proved to be useful in... 

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