The sensor features high speed measurement and resolution that provides process variability information not previously attainable. The Optical Properties Measurement uses three simultaneous measurements. The first is a 1 millimeter diameter bundle of optical fibers. The second is a concentric 30 millimeter-wide opacity window. The sensor calculates the root-mean-square relative optical density from these two measurements to determine a formation index. This solution enables the sensor to be insensitive to speed, dirt, alignment and varying weight. The third measurement is a reflectance measurement of sheet brightness. All information is displayed at the operator station on trend, profile and process summary video pages.
Formation
OP4255 provides a systematic equivalent of the subjective “look-through” test commonly used by many papermakers. Transmitted light is measured in a small area and then compared to the measurement of a larger, surrounding area. The difference between the two optical densities calculated as a rootmean-square indicates the formation. In this manner, direct spatial filtering is achieved. The measurement is totally machine-speed independent and does not require a separate input of machine speed to perform. On-board intelligence compares and processes the raw optical density measurements to simulate human evaluation. A resulting formation index number is communicated to the operator. The electronics preserve the fast raw data in a form that is immediately usable by the papermaker for high-speed frequency analysis of paper machine performance.
Opacity
Opacity is based upon the intensity of light transmitted through the sheet. OP4255 provides sheet opacity measurement in accordance with the TAPPI standard, independent of sheet composition and brightness effects. Proprietary optical techniques and signal processing modify the data to produce an accurate measurement of opacity. For example, an opal glass window provides a light-diffusing backing for the sample being measured to achieve a single calibration curve when grade-to-grade reflectance variations occur.
Brightness
Brightness is the light reflected from the paper entering the optical detecting unit which images the spot on the detector. OP4255 provides sheet brightness measurement in compliance with the TAPPI standard. The reflectance measurement of brightness is compensated for by the opacity measurement. Reflectance of an infinitely-thick stack of papers can be predicted by measuring a single sheet.