Whiter than white? Not with LEDs

I have bought your paper with a high percentage of post-consumer recycled content for several years. I don’t typically notice that anything is different until I see my paper up against someone else’s; that’s when I see that white is not always white. But it turns out that manufacturers of things like paper and laundry detergent have been taking advantage of the environment to create a perception that something can be whiter than white.

Consider the last time you were at the bar, the black light comes on and everything white including your teeth starts to glow. This is due to fluorescence. Your teeth have it naturally but your clothes a lot of that is from your detergent.

In 1928, the journal Science published a brief note on the fluorescence of teeth in which they noted that dentin fluoresces more than enamel and dental caries (cavities) do not fluoresce at all. This occurs when teeth are exposed to UV light.

Paper and clothes however, will glow based on the quantity of fluorescent whitening agents (FWAs). FWAs absorb light on the short end of the visible spectrum – ultraviolet and violet wavelengths. They then re-emit blue light. This gives the appearance that objects are glowing and we perceive the blue light as whiter then objects that are not emitting light.

I can think of several laundry detergent ads that tell me that their detergent will get my whites whiter than any other detergent will. However, these companies have a new challenge. As many people will tell you LED light tends to be a different colour then incandescent and fluorescent lights we are used to.

Unlike incandescent and fluorescent lights, LEDs generally do not emit light that is shorter than 430 nm. This means that LEDs are not emitting ultraviolet light. As a result, the clothing or paper or other white material cannot absorb these shorter wavelengths. If the materials can’t absorb the required wavelengths they also can’t emit the blue wavelengths. As a consequence, the FWAs do not have any impact on our perception of the colour under LED lighting.

The one exception is when Violet LEDs are used. In this case, the wavelengths are shorter and can be absorbed by the white objects and readmitted by the FWAs, thereby making the objects appear whiter than white.

Houser, K., Wei, M., David, A., & Krames, M. (2014). Whiteness Perception under LED Illumination, LEUKOS, 10(3), 165-180. DOI: 10.1080/15502724.2014.902750