The quality of a paper that causes it to appear shiny. When light hits a paper's surface, the orientation of the reflected light rays (or specular reflectance) determines a paper's gloss. A paper that has undergone extensive calendering, supercalendering, or coating, or has had its surface highly polished, will reflect the light primarily as parallel rays, or all in the same direction. This is what causes a paper surface to be shiny, or "glossy." The opposite of a glossy surface, a matte surface, is much less polished, so the light rays that strike it are reflected in different directions (or more diffusely) due to small surface contours. Gloss is related to paper smoothness, but there is no clear correlation between the two qualities. Glossy papers are used in some printing jobs to increase the gloss or color brilliance of the printing ink (glossy paper reflects light back through the ink), and less glossy papers are used in other printing jobs to reduce eyestrain (the high degree of reflected light makes text printed on glossy paper hard to read).
Paper gloss is measured using a glossmeter, which compares the amount of light reflected from a paper surface to the amount of light hitting it. Paper gloss may differ with grain direction, so separate glossmeter readings may be taken with the grain and against the grain and then averaged. Since papers also exhibit two-sidedness (i.e., wire side vs. felt side), an average gloss level may be provided for each side of the paper. "Perfect gloss," as determined by referencing a gloss standard (i.e., polished black glass), is near 100 gloss units. Matte papers register generally less than 20 gloss units, while dull-finish papers range up to 40 gloss units.
The term gloss also refers to the degree of shine of a printed ink. Certain inks, such as high-gloss inks, dry to a high degree of gloss. The key to glossy inks is the maximization of ink holdout, as it is premature absorption of the ink vehicle into the paper before it can dry by oxidation that decreases gloss.