Ultraviolet Exclusion
Color measurement standards define a lighting condition that excludes ultraviolet energy to simplify color matching on fluorescent substrates. This specific configuration, termed illuminant m2, requires the measuring instrument to utilize a filter that removes light wavelengths below four hundred nanometers. It allows for the measurement of base sheet color and printed inks without the complicating effects of fluorescent emission.
By isolating the reflective properties of the pigments and fibers, it provides a stable baseline for comparing colors across different batches of paper. This method is particularly useful for measuring materials intended for indoor environments where ultraviolet light is absent.
Fluorescence Suppression
Optical brightening agents added to paper pulp require ultraviolet radiation to emit the blue light that makes the sheet appear whiter. When a spectrophotometer uses illuminant m2, the ultraviolet filter prevents these molecules from being excited, thereby suppressing their fluorescence. This suppression allows the operator to measure the true, unwhitened color of the substrate and the ink layers deposited on it.
It eliminates the measurement variation caused by differing levels of optical brighteners or varying ultraviolet outputs in different instruments. As a result, ink formulations can be adjusted based on the actual pigment reflectance rather than being distorted by fluorescent emissions. This capability is especially important when matching spot colors on recycled paperboards that have variable brightener content.
Metamerism Analysis
Discrepancies between visual and instrumental color matching often stem from differences in the light source used during evaluation. By comparing measurements taken under illuminant m2 with those taken under ultraviolet-included sources, technicians can calculate the exact contribution of the optical brighteners to the overall color. This comparison helps in predicting how the printed package will appear when moved from a retail store with fluorescent lighting to an outdoor setting.
