Problems in colour measurement of fluorescent paper grades

The paper industry's demand for high white paper products is increasing. Th is requires usage of well-bleached fibres and high dosage of fluorescent br ightening agents (FBAs) often applied both in the wet end and at the size p ress. 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 n ot at the level provided by measurements of properties such as weight or ca liper. 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 color imetric quantities. This is barely adequate for nonfluorescent grades and p roblematic for fluorescent grades. Moreover, the standard vocabulary for co lour 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 fluoresc ent grades. Process and instrument issues are discussed. Ambiguities in sta ndard terminology and practices are identified, and improvements are recomm ended. Examples are given with measurements made at several paper mills. (C ) 1999 Published by Elsevier Science B.V. All rights reserved.