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Tuesday, March 11, 2014

How do you measure transmittance?

Transparency is the characteristic of a material allowing it to transmit light.  Transparency is applicable to papers which are intended to be seen through (such as glassine or tracing paper) and is distinguished from opacity, which is applicable to papers that are intended to hide objects placed immediately behind them.

Transmittance is not simply the inverse of opacity.  Because of the way opacity is defined in the paper industry, a sheet may measure 100% opaque but when held up to a bright light the observer will see some light passing through the sheet.

A transmittance value may be calculated based on the following equation as referenced in ISO 22891 & DIN (German) Standard 53147.

                        T       =       [(YW – Y0)((10,000/Y(W)) – Y0)]1/2




                        YW       =        Y value for sample backed by a white body

                        Y0         =      Y value for sample backed by black body

                        Y(W)   =       Y value for white body

This is a very simple calculation. The Color Touch spectrophotometer was used to make measurements in determining the repeatability and reproducibility of this measurement for ISO 22891.

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