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TAPPI T 579 Brightness

Standard:

TAPPI

Method:

TAPPI T 579

Title:

Diffuse brightness of paper, paperboard and pulp (d/0)
(ultraviolet level D65)

Scope

TAPPI T 579 determines the brightness of white, near-white, and naturally colored pulp, paper, and paperboard. Brightness is a commonly used industry term for the numerical value of the reflectance factor of a sample with respect to blue light of specific spectral and geometric characteristics. This method requires an instrument employing diffuse illumination and 0° viewing geometry.

This method is applicable to all naturally-colored pulps, and papers and board made therefrom. The measurement is not suitable for paper or paperboard containing added coloring matter (such as yellow or green dyestuff) which appreciably absorbs light in that part of the spectrum extending from about 400 to 500 nm. This brightness method is not applicable to colored papers.

Pulps to be tested are made into handsheets prepared according to TAPPI T 218 “Forming Handsheets for Reflectance Tests of Pulp (Büchner Funnel Procedure).”  This method may also be used to measure the brightness of machine-dried sheets.

This method utilizes an integrating sphere to provide diffuse illumination and perpendicular (0°) observation geometry (1) designated in optical terminology as d/0. With this geometry, specimen surface structure and azimuthal orientation have a negligible effect on brightness.

The instrument has a relatively large specimen aperture for the purpose of averaging small area variations in reflectance, making it possible to obtain a reliable average value with only a few individual measurements.

This method is not intended for use with colored materials.

Summary

Diffuse reflectance is measured in the wavelength range of 400-520 nm with an effective wavelength of 457 nm by using a suitable filter set or an equivalent device for modifying the spectral response and an instrument having diffuse illumination and perpendicular observation geometry. The measurements are made in terms of absolute reflectance factors.

Significance

Blue-light reflectance measurements were originally designed to provide an indication of the amount of bleaching that has taken place in the manufacture of pulp. The higher the blue-light reflectance, generally the whiter the products will appear. In recent years, the method has been extended to white and near-white paper and paperboard, and is suitable for that purpose. The method provides a simple, single-number index useful for comparing similar white materials; however, colored materials are better identified by using a standardized three-dimensional color space [see TAPPI T 524 “Color of Paper and Paperboard (45/0 Geometry)” and T 527 “Color of Paper and Paperboard (d/0 Geometry)”].

Because the instrument geometry of this method is different from that of TAPPI T 452 “Brightness of Pulp, Paper and Paperboard (Directional Reflectance at 457 nm),” there is no simple relationship between the two brightness scales.

Specularly reflected light (gloss) is excluded from the measurement of diffuse brightness by the use of a gloss trap (specular reflectance absorber).

 

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