Color matching with standardised light D50

GRAPHIC COLOUR MATCHING TO ISO 3664:2009

The D50 light source referenced in the standard ISO 3664:2009 “Graphic technology and photography – Viewing conditions” is defined by the CIE with a specific spectral energy distribution between 300 nm and 780 nm in increments of 5 nm. Other conditions to be fulfilled by a standardised viewing device in accordance with this standard are chromaticity coordinates (x and y for the 10° observer) with a tolerance of max. 0.005°, a general colour rendering index of Ra > 90, and metamerism indices of MIvis < 1.0 and MIuv < 1.5. The metamerism index further defines the quality of the light source simulating the D50 illuminant in the visible (VIS) and UV ranges. The UV aspect of this standard is particularly important given the prominent use of production materials containing optical brightening agents (OBAs).

UV COMPONENT IN D50 STANDARDISED
VIEWING DEVICES

Natural daylight with certain characteristics contains a given amount of ultraviolet radiation, and the D50 illuminant is designed to approximate “horizon” midday natural daylight. UV radiation excites optical brightening agents (OBAs) present in a wide variety of printing papers and substrates.
As a result, both the UV content of light sources and OBAs present in papers influence colour rendering. To make this effect controllable in reprographic processes, and to achieve better matches between standardised light sources and the D50 illuminant, the 2009 version of ISO 3664 implements a far tighter metamerism index tolerance for UV in the 300–400 nm range (MIuv < 1.5).
This amendment meant that the UV component in standardised lamps had to be substantially increased to achieve the standard and make OBAs in substrates visible and thus controllable using traditional colour management methods.