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Monday, September 1, 2014

Why do we calibrate TAPPI brightness instruments to paper and opal glass?

The effect of directional geometry on translucent materials should be understood since nearly all papers have some degree of translucency.  With reference to the figure above, note that the solid cone of the incident illumination strikes the first sheet of paper placed over the sample opening and all of the reflected illumination is viewed by the photocell.  Some of the light, however, penetrates through the first sheet and reflects off the second sheet, third sheet, etc. Notice that some of the light rays reflecting off the backing sheets are viewed by the photocell and some fall outside the viewed area.  Depending on the degree of translucency of the stack of sheets, more or less light will be viewed by the photocell.  

In order to overcome this possible discrepancy, standards are provided for the directional brightness tester by Technidyne which have high levels and low levels of translucency. Opal glass standards are used for high translucency and paper tabs for low translucency.  After the instrument is brought into calibration for both high and low translucency by adjustment of the lamp focus or photocell positioning, the instrument is considered to be in proper calibration for all levels of translucency.  By this means, the instruments are brought into agreement with one another.