The M Factor…What Does It Mean?

By Raymond Cheydleur and Kevin O’Connor

Many modern printing and proofing papers contain Optical Brightening Agents [OBA]. These agents are added to enhance the brightness of the papers and improve the appearance of the printed product. The presence of these agents in contemporary papers creates challenges for successful color management, so new standards have been defined to help manage and communicate color for papers containing these brightening agents.

OBAs operate through the process of fluorescence. They absorb invisible ultraviolet (UV) radiation at wavelengths below 400 nanometers (nm) and, through an electrophysical change, emit light mostly in the blue end of the visible spectrum at about 400 to 450 nm. When this light is emitted from papers using brightening agents, they are perceived as having a color that is “whiter than white,” since the observed light from the paper will be the total of the reflected and emitted (due to fluorescence) light when illuminated by a source containing a large UV component. You can often see this effect if you illuminate a paper containing brightening agents with an ultraviolet light source, such as a “black” light.

The perceived color of a piece printed on a substrate containing OBAs will look different, depending whether the light source used to view the print contains UV or not. Older graphic arts measurement standards (except for density standards) specified D50. It was assumed that all substrates would measure the same using a D50 illuminant, and OBA content was not a concern. In practice, colors viewed under real viewing conditions containing UV sometimes were notably mismatched, and failed to meet expectations. These mismatches created serious challenges for people trying to measure and manage color consistency in a variety of workflows.