While rarely found in nature, white is the color encountered most often in the human environment. It is the color of neutrality, of cleanliness and freshness. White provides contrast with other colors, and can increase the visibility of a logo or printed text. Conversely, yellow is the color of age, of things that are old and worn and uncared for. That’s why for product manufacturers that make white materials or use white packaging, it’s important to get a consistent color with high whiteness to convey to their customers that sense of freshness, that the product is new and clean.
The problem becomes how to objectively measure and quantify white colors so that a product is consistent. As with any other color, human perception of white will vary from person to person. Different people’s eyes perceive colors differently due to the different makeup of cones and rods that allow us to see color and lightness. Our brains also perceive color differently due to our learned experience. Our perception of how white something is can actually be influenced by society and culture. While people from North and South America prefer whites with a slightly bluish tint, European and Eastern cultures have a preference for reddish whites.
So how can we objectively measure something that is perceived differently from person to person, and culture to culture? First it’s important to understand the difference between the terms “white” and “whiteness”. The color white is characterized in CIEL*a*b* space as having a luminosity (L* value) greater than 70, and a* and b* values as close to zero as possible. The higher the L* value and the lower the a* and b* values, the “whiter” the color is. A theoretical perfect white would have an L* value of 100 and a* and b* values of 0, given as coordinates (100, 0, 0) in CIEL*a*b* space. A color with a* and b* values close to zero and an L* value of less than 70 would be considered grey.
So what is whiteness? Whiteness is how white we perceive a color to be, and is characterized by more than just how “white” a color is in CIEL*a*b* space. While white is a physical attribute, whiteness is a perception. Because blue is also considered a color of freshness, newness, and cleanliness similar to white, the colors we perceive as having the highest whiteness actually have an element of blue in them. White is characterized as a color having high luminosity, no saturation, and thus no hue. Whiteness is characterized as having high luminosity, finite saturation, and a blue hue. If we consider two white colors that are otherwise the same but with one having a slightly bluer tint (more negative b* value), the bluer color will appear “whiter” to the observer’s eyes. Thus, counterintuitively, a color having L*a*b* coordinates of, say, (100, 0, -2) will be perceived as being whiter than the theoretical perfect white having coordinates of (100, 0, 0). As we travel down the b* axis in the negative direction the whiteness of a color will appear to increase up until the point when the color becomes saturated enough that we perceive it to be blue rather than white.
Higher whiteness is sometimes achieved by adding blue pigment. This will decrease the luminosity of the color (L* value), yet the added blue will actually make the color appear whiter and can counteract yellowness, which is characterized as a color having a positive b* value. Another way to increase whiteness is by adding fluorescent pigments or by bleaching the material so it fluoresces. Fluorescence is a phenomenon where a material will absorb light at a particular wavelength and re-emit it at a longer wavelength. In this case we are talking about UV fluorescence, where invisible UV light is absorbed and re-emitted as visible blue light, thus decreasing the b* value and increasing the whiteness of the material.
This is why, when measuring how white a material appears, it is necessary to measure more than just CIEL*a*b* coordinates. There are ASTM methods with calculations that take the presence of blue into account when giving a value for whiteness, and some color measurement devices have the ability to do these calculations for you.