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Optical measurements

The Optical Properties Measurement combines measurement of three visual paper properties – formation, opacity, brightness – in one compact head package. Continuous and repeatable measurement of sheet formation online, allowing progressive papermakers to capitalize on this dimension of process visibility. In addition, it simultaneously measures opacity and brightness, resulting in substantial economic and quality benefits.

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.

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 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 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.


  • Reduces wet-end breaks, improves product uniformity and opacifier usage
  • Provides continuous, repeatable and objective formation measurement
  • Ultra-fast measurement — 50,000 to 100,000 measurements/second. Measures every 1 mm (1/25th inch)
  • Machine-speed independent
  • Measurement principle and signal processing simulates human perception of formation
  • Bi-user output: formation index for operators and fast raw data for machine frequency analysis
  • Meets or exceeds TAPPI opacity standard T425om-81
  • Meets or exceeds TAPPI brightness standard T452os-77
  • Lower head lightly contacts sheet to stabilize the moving web and eliminate effects of flutter
  • Linear response and sensor stability permit factory pre-calibration minimizing the need for on-machine dynamic correlation
  • Information displayed as a trend, profile orprocess summary page
  • Ideal for rawstock applications or twin-wire machines
  • Can be used to optimize process variables
  • Can be linked to off-line spectral analyzers and used as a trouble-shooting tool
  • Speed, dirt, head alignment and weight insensitive
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