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In the last half century, there has been an increase in the manufacture and use of opaque fluorescent materials and with this has come the need to measure the colour of these materials. The different types offluorescent materials are considered and the methods which can be used to specify the colour of a fluorescent sample are described in this paper.

The difficulties in simulating CIE Standard Illuminant D65 are considered and an interior daylight illuminant is described, Illuminant ID65, representing exterior daylight attenuated by average window glass. The benefits of Illuminant ID65 are that it represents the daylight source which most commonly occurs in the viewing of critically coloured merchandise, and that it can be practically simulated using a filtered tungsten‐halogen source. Its applicability to measurements, particularly to the assessment of fluorescent samples, is considered. The results of measurements on the two common classes of fluorescent samples are discussed and indicate that an Illuminant ID65 simulator can be used validly for the measurement of fluorescent materials, even including materials intended specifically for exterior use.