Testing                                                                                                                                                                        OPACITY

Opacity

General

The method of test described below covers the procedure for measuring the opacity of all kinds of paper and paper products by determining the apparent light reflectance.

Apparatus

The apparatus shall be capable of measuring the apparent light reflectance as prescribed in this method. It may measure the values separately or give directly the ratio of the apparent reflectances.

Values of apparent light reflectance are relative to the apparent reflectance from magnesium oxide taken as 100 percent. The standard white backing shall have an apparent reflectance of 91. 5 percent and the standard black backing shall have an apparent reflectance of not more than 0. 5 percent.

Completely diffused illumination from incandescent lamps at a colour

temperature of 2400 to 2800 Kelvin shall be used. The. direction of viewing shall be not more than 20 from the normal to the surface of the specimen.

Observations shall be made visually or by equivalent means, such as photo­electric cell with a filter adjusting its sensitivity to that of the human eye.

The instrument shall be calibrated.

Calculation

The ratio of reflectance over black backing to that over white backing, expressed as a percentage, is the contrast ratio. Calculate the average contrast ratio from determinations on both sides of each test piece.

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