Advanced Spectrophotometers Optimize the Value of Whiteness Indices in the Presence of Optical Brighteners


Posted on January 12, 2018

White may appear to be the most simple of hues. It is the color of wedding gowns and unpainted canvases, lab coats, and freshly fallen snow, suggesting cleanliness, freshness, and purity. It’s a blank slate, a new start, an untainted surface. However, producers of products like paper, textiles, and plasticsknow that whiteness can be complicated, requiring precise manufacturing in order to obtain the ideal shade. From the selection of raw materials to the monitoring of processing methods, creating the perfect white is often an exacting endeavor.

Across industries, whiteness indices are an invaluable part of determining what that perfect white is and ensuring that it is consistently produced. However, as modern manufacturers increasingly turn to optical brighteners to enhance their products, the accuracy of standard spectrophotometric whiteness assessment may be compromised. Choosing sophisticated color measurement instruments and methods that can account for the effects of optical brighteners in paper, textiles, and plastics is essential to capturing accurate, meaningful data.

The Subjective Nature of Whiteness Indices

A number of whiteness indices have been developed over the years that offer standardized ways of quantifying whiteness using instrumental color analysis. Although it may seem logical to assume that there is a static color we can objectively identify as “white” based on 100% reflectance values across the visible spectrum, this is in fact not the case; what the human eye perceives as “perfect white” is typically different than a theoretical “perfect white.” Richard S. Hunter and Richard W. Harold write:

The earliest studies [on quantifying whiteness] merely used average reflectance through the visible spectrum as the scale of whiteness. This was hardly satisfactory, since whitening by addition of blue dyes lowers average reflectance but increases visual whiteness. We now know that individual preference for whites makes optical criteria of whiteness variable from one observer to another.1   

It is important to note that these individual preferences are not simply a matter of “liking” one shade more than another, but a difference in perception of what constitutes a true white in the first place. As Hunter and Harold note, for example, materials with blue dye added to counteract yellowness tends to be perceived as whiter, even—and especially—when the blue dye creates a bluish tint rather than simply neutralizing yellowness. As such, modern whiteness indices attempt to quantify perception of whiteness rather than measuring “objective” whiteness, which explains why there are multiple whiteness indices and ongoing changes to existing indexing formulae. By using these indices, you can evaluate your products according to standards that take the psychophysical process of color perception into account, making them more meaningful than a theoretically objective standard that ignores how humans process color information.2   

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