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572nm Opacity is the characteristic ability of a material to hide from view a non-active backing material. When reading a book, one prefers to read one page at a time, rather than read all of the pages at once. Each page must have sufficient opacity to obscure the printing on the subsequent pages. If the intended use of the paper is to allow the objects placed behind it to be seen, such as glassine, then transmittance measurements should be utilized rather than opacity measurements.

To simulate viewing conditions a ratio means of opacity measurement has been adopted by the paper industry and has been used for many years. ISO (Printing Opacity) = R 0 / R ∞. ISO 2471 specifies a response of Y C, which has an effective wavelength of 557 nm. In TAPPI T425, the specified response is Y A, which has an effective wavelength of 572 nm. Unlike brightness, which is measured in the blue portion of the spectrum, opacity is predominately measured in the green or greenish yellow portion of the spectrum with some response throughout the entire visible spectrum. The objective is to measure opacity using a spectral response that correlates closely with human observation. FOOTNOTE Many of the geometric and spectral variables are less critical for opacity measurements than for brightness and color because opacity is a ratio measurement. Haunted Horror 15 Cbr 1000rr on this page.

For example if a sample being measured is glossy, nearly the same amount of gloss will be present in the numerator and denominator of the ratio causing most of the gloss effect to be canceled out. Similarly the effect of fluorescence on opacity is minimized by the ratio calculated that is inherent in the determination of opacity.