John Seymour is an applied mathematician and color scientist. He is a professor at Clemson University, teaching color science and process control in the Graphic Communication school. He has worked as a consultant since 2012 under the name “John the Math Guy”. John currently holds thirty US patents, has authored over thirty technical papers, has presented at thirty conferences, and has keynoted at six conferences. He is an expert on the Committee for Graphic Arts Technologies Standards and ISO TC 130. He writes a blog which is described as “applied math and color science with a liberal sprinkling of goofy humor.”

Between 1992 and 2016, John was responsible for advanced product development for QuadTech, a manufacturer of process control equipment in the print industry. Prior to working with QuadTech, John worked as a scientific programmer in medical imaging, satellite imagery, electron microscopy, and spectroscopy. He holds bachelor’s degrees in mathematics and in computer science from the University of Wisconsin-Madison.

Coated Versus Uncoated – Why the Difference In Density?

John Seymour, John the Math Guy LLC and Clemson University

It is well known in the industry that you can get richer colors (higher density) on coated stock than on uncoated stock. One common explanation is that ink more readily soaks into an uncoated stock. The paper fibers, in effect, hide the ink so that some of the pigment is lost. Another common explanation is that there is an incomplete lay-down of ink on uncoated stock. According to this explanation, the uneven topography of the paper fibers results in pinholes where white paper shines through.

A few simple experiments will be presented which show that, while both of these may have an effect,neither of these fully explains the phenomena. A thin coat of clear varnish will be shown to make CMYK solids darker and richer in color. How can varnish atop ink pulls the pigments out from behind paper fibers? A microscope image of a black solid appears to show minute pinholes that would reduce the density. Rotation of the light demonstrates, however, that the “pinholes” move around as the direction of the lighting is changed. These “pinholes” are actually points of specular reflection.

The nature of surface reflection is described according to Fresnel’s law and “Billiard’s” law. The smooth surface of a coated stock will not appreciably scatter directional light. Thus, when ink on a coated stock is measured with a 45:0 spectrophotometer, the surface reflected light will reflect at 45°, and will not be captured by the detector. When we look at a sample of print on a coated stock, there is an appreciable amount of surface reflected light which we call glare. Just as in the spectrophotometer, we do not include that surface reflection as part of our assessment of the color.

For an uncoated stock, the surface reflection of direct lighting is scattered due to the rough surface. Thus, a 45:0 spectrophotometer will capture at least some of the surface reflected light. As a result, the reflectance is augmented by the introduction of perhaps 5% white light. When we look at a sample of print on an uncoated stock, we do not have the opportunity to differentiate between glare and bulk color –light from the two paths have been mixed at all locations on the page. Just as in the spectrophotometer, we necessarily include this surface reflected light in our assessment of the color.

This demonstrates that the main reason that ink on a coated stock is richer than the same ink on uncoated stock has to do with how the surface reflected light is scattered from the two surfaces.