Problems in colour measurement of fluorescent paper grades

The paper industry's demand for high white paper products is increasing. This requires usage of well-bleached fibres and high dosage of fluorescent brightening agents (FBAs) often applied both in the wet end and at the size press. Specialty paper makers may also use fluorescent colourants for shaded grades, in combination with non-fluorescent colourants and sometimes FBAs. Thus there is an immediate need for reliable, repeatable and reproducible measurement of colour in fluorescent grades.

The paper industry uses a variety of spectrophotometers and colorimeters as their laboratory quality reference instruments. Unfortunately, even on non-fluorescent grades the agreement between different colour instruments is not at the level provided by measurements of properties such as weight or caliper. Furthermore, laboratory instruments measure a stack of sheets, while on-machine instruments measure a single layer, causing additional sources of instrument disagreement and quality deviations.

Colour target specifications and grade limits are usually supplied as colorimetric quantities. This is barely adequate for non-fluorescent grades and problematic for fluorescent grades. Moreover, the standard vocabulary for colour terms is ambiguous concerning certain phenomena relevant to measuring and controlling the colour of fluorescent samples.

This paper describes the special problems in colour measurement of fluorescent grades. Process and instrument issues are discussed. Ambiguities in standard terminology and practices are identified, and improvements are recommended. Examples are given with measurements made at several paper mills.