This part of ISO 13468 covers the determination of the total luminous transmittance, in the visible region of the spectrum, of planar transparent and substantially colourless plastics, using a double-beam scanning spectrophotometer. This part of ISO 13468 cannot be used for plastics which contain fluorescent materials. This part of ISO 13468 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 part 1 of this International Standard. NOTE 2 Substantially colourless plastics include those which are faintly tinted. NOTE 3 Specimens more than 10 mm thick may be measured provided the instrument can accommodate them, but the results may not be comparable with those obtained using specimens less than 10 mm thick.