The recommendations of ISO 13655 should be followed when measuring transmitting media, with the exception of the source of illumination, which presents the same issues as in the case of reflecting media.
ISO 13655 specifies that the measurement geometry for transmitting media should be either 0E/diffuse or diffuse/0E. If an opal glass diffuser is used, it should conform to that defined in ISO 5-2. The procedure for the calculation of tristimulus values should be the same as for reflecting media, by using the CIE 1931 standard colorimetric observer (2 degree), with the CIE D50 illuminant. The ISO 13655 spectral weighting functions, derived from this observer and illuminant, should be used when the measurement is made with a spectrophotometer or spectroradiometer in which the spectral sampling interval is coarser than that specified by CIE – namely 1 or 5nm.
