| 摘要 |
Specifically disclosed is a method and apparatus for measuring the opacity of sheet material, more particularly for measuring the opacity of paper according to the TAPPI contrast ratio definition. A moving sheet of paper from a paper making machine or coater is passed through the gap between a light source and a photodetector. The light transmitted through the paper is passed through a window of opal glass and a bandpass filter before it is received by the photodetector. The opal glass window constitutes a partial reflector having an effective reflectivity between about 20 and 70 percent. This produces multiple reflections between the opal glass and the paper next to it, with sufficient magnitude to compensate the transmittance measurement for the reflectivity of the paper, and thereby automatically corrects the instrument for changes in the composition of the paper. Because of the bandpass filter, the photodetector responds substantially only to light in the visible portion of the spectrum. According to another disclosed arrangement wherein the opal glass window is not used, the instrument is compensated by the use of a second photodetector responsive to light reflected from the side of the paper where the light source is located. Signals from the two detectors are then combined in a simple computer arrangement to obtain a signal which is compensated for the reflectivity of the paper. The compensated signal provided by either of the disclosed arrangements is correlated with opacity in conformance with the TAPPI standard. |
