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Monday, March 16, 2015

From the Testing Lab: Size of Aperture

When making optical measurements, there are different apertures used for measurement of samples.

45° illumination     0° viewing
diffuse illumination    0° viewing
T 452
ISO 2469, T 525
T 524
ISO 5631, T 527
Aperture (viewing)
9 mm diameter
30 mm diameter
Viewed Area
63.6 mm²
706.9 mm²

The viewed area of the diffuse geometry is over 11 times the viewed area of the directional geometry. Therefore, the directional geometry will be more sensitive to small variations in paper or a handsheet. Most grades of machine-made paper are relatively uniform and satisfactory results can be obtained by making a few measurements at various points across the sheet and averaging the results, however, significant variations in brightness readings can be expected when using the directional brightness tester to measure pulp and other non-uniform materials.  The smaller measuring area results in much larger variation from reading to reading.

The main advantage of the diffuse geometry is that it averages non-uniformities in the sample and is, therefore, excellent for measuring pulp. However, the larger viewing area will not detect smaller variations when they exist and when they are important. In this case, the larger aperture can mask the actual variation in the sample.

In order to get better agreement in the measurements, one could make 11 measurements on the directional geometry for every 1 measurement on the diffuse geometry.  However, this may mask any true variation. In that case, the standard deviation from each device could be useful. The biggest problem here is that at most roll turn-ups only 1 measurement is made. If this is the case, the diffuse geometry will likely show an average value for the sheet and the directional geometry may show a higher, lower or similar value depending on where the measurement is made on the sample.

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