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TAPPI T 425 om-11 Opacity of paper (15/d geometry, illuminant A/2*, 89% reflectance backing and paper backing)
standard by Technical Association of the Pulp and Paper Industry, 2011
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1.1 Opacity is a fundamental optical property of paper as a whole, yet the measurement of opacity isdetermined by a ratio of reflectance measurements. The opacity of the sheet is influenced by thickness, the amount and kind of filler, degree of bleaching of the fibers, coating, and the like. The utility of bond, writing and book papers is enhanced by a high opacity.1.2 The essential principle of this method for determining the opacity of paper is as follows: The reflectanceof paper when combined with a white backing is higher than that of paper when combined with a black backing because, in the former case, light transmitted through the imperfectly opaque sheet is largely reflected by the white backing, and a portion of the light is transmitted through the paper a second time thus increasing the total reflection.
1.3 The determination of opacity is of vital importance to both the manufacturer and the consumer. Whenwhite pigment is added to a sheet, it scatters more light and thus increases opacity; however, it is also possible to increase opacity of a sheet by adding dark pigment or dye which absorbs light. This being so, it is of value to the manufacturer, in meeting an opacity specification, to be able to predict whether a sheet which does not have desired opacity can be brought up to specification by raising or, alternatively, lowering the reflectivity within permissible limits. To the consumer, opacity measurements are used to evaluate some of the characteristics of appearance. The user is interested in the comparison of samples under identical conditions. When comparisons are made, one sample with another, very small differences can be identified visually. For this reason, small measured differences between similar samples represent actual differences in appearance.
1.4 This method employs 15/d geometry, illuminant A/2 degrees and 89% reflectance backing or paper backing. For the measurement of opacity with d/0 geometry, illuminant C/2 degrees and paper backing, see TAPPI T 519.