In the latest revision of the ISO 13655 standard (Graphic technology -- Spectral measurement and colorimetric computation for graphic arts images) four measurement modes, the "M" series, were precisely defined in order to help standardize the settings for measurements of color. Sun Chemical's customers are particularly concerned with the measurements of white substrates that exhibit a presence of optical brightening agents (OBAs). It is known that these substrates can have a significant influence on the measurements of lighter colored inks, especially transparent process inks, and produce noticeably different results based on the instrument used to capture the spectral radiance factors of the specimen. The main purpose for revision of the ISO 13655 standard was to eliminate the inconsistencies in measured data from press prints and contract proofs arising from the variation in the UV content between instruments and to allow for effective inter-instrument data comparison.
A set of white and near-white specimens was selected for an experiment in which the agreement level between three different commercial instruments in M1 mode was assessed. The data indicate that there are still discrepancies in measurements between the instruments when various manufacturers' M1 measurement conditions are used. These variables can directly affect any other indices or values derived from the measured data based on M1 conditions. In an attempt to improve the agreement of the three instruments the spectral data were normalized to a non-fluorescent material. The results indicate that differences in geometry play a large role in the inter-instrument agreement in the characterization of optically brightened paper substrates.