This document covers the determination of the total luminous transmittance, in the visible region of the spectrum, of planar transparent plastics and substantially colourless plastics, using a double-beam scanning spectrophotometer. This document cannot be used for plastics which contain fluorescent materials. This document is applicable to transparent moulding materials, films and sheets not exceeding 10 mm in thickness. NOTE 1 Total luminous transmittance can also be determined by a single-beam instrument as in ISO 13468-1. NOTE 2 Substantially colourless plastics include those which are faintly tinted. NOTE 3 Specimens more than 10 mm thick can be measured provided the instrument can accommodate them, but the results cannot be comparable with those obtained using specimens less than 10 mm thick.