Paper smoothness, one of the most important printing characteristics measuring air leakage rates across an area of paper. Parker Print-Surf (P.P.S.), a 1965's development by John Parker in the U.K., was well received by the paper industry, to test the quality of paper for gravure printing. In 1985, this procedure was adopted as an British standard (BS) and in 1992 as an ISO standard.
The principle is as follows: an extremely thin measurement ring is pressed over the surface of a specimen with a certain level of pressure, and air of low and stable pressure is fed from the inside of the ring. The air leaks out from the surface at the extremity where the measurement ring and the paper contact each other. The leak air volume varies with the roughness of a paper sheet, and the amount of leaked air volume is represented as an indicator of paper surface roughness in micron meters.