Metamerism (Colors, Fluorescent whitening)
Metamerism problems involve the matching of paper's appearance to a color standard, particularly when one has to make sure that the paper will look the same under different types of illuminant. The classic example is when the papermakers make a product that appears to be an exact match to the standard provided by the customer, based on either direct observations or instrumental measurements at one particular condition of illumination. Then, later on, the customer can view the same paper under a wide range of illuminants. Examples of common illuminants including daylight (which depends on the weather and the position of the sun), fluorescent bulbs of various kinds, or old-fashioned incandescent lighting. Let's suppose that the customer views the paper with a different type of light, observes that "this is a terrible match!", and rejects the product.
To avoid the occurrence of metamerism problems, the best approach is to use reflectance curve data corresponding to the visible range of light wavelengths. An excellent match can be expected under all lighting conditions if the diffuse reflectance of light matches the standard throughout the visible spectrum AND there are no significant differences with respect to fluorescent whitening agents (FWAs) or fluorescent dyes. By contrast, it is inherently impractical to solve metamerism problems if one has only data related to a three-parameter color coordinates (e.g. the L* a* b* system). Three-parameter color coordinates are commonly used for process control and for specification of paper colors.
If the product contains broke or waste furnish that may contain variable amounts of FWAs, then you might want to look at the related section of this guide. In white grades one can get a preliminary idea about whether whitening effects are highly variable by illuminating samples of paper with an ultraviolet light or by comparing brightness or color measurements in the presence and in the absence of a UV cut-off filter in the incident beam of light.
In many other cases, especially those involving colored paper products, metamerism problems can be overcome by selecting dyestuffs that have similar hues to those that were used when making the standard samples for the grade. Suppliers of papermaking dyes usually can perform the needed tests of the standard paper and do the calculations for you to develop one or more recommended combinations of dye products that will reduce or eliminate metamerism issues.
Jay, S. L., "Color Control for the Paper Producer in the 90's," Proc. 1991 Papermakers Conf., 93.
Lips, H. A., "Dyeing," in Casey, J. P., Ed., Pulp and Paper Chemistry and Chemical Technology, 3rd Ed., Vol. 3, Ch. 19, 1627 (1981).
PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (firstname.lastname@example.org). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.