LED lighting is great. The right bulb gives the same warm incandescent glow you love from a fraction of the energy. But there's a downside: while LEDs make cities look awesome, the most common type of LED lighting dims the ultraviolet trick laundry detergents use to make white clothes look whiter. The future is bright, but it's also kind of dingy.
There's some fascinating science going on here. Many laundry detergents contain fluorescent whitening agents, or FWAs, which absorb ultraviolet light and re-emit it as a visible blue wavelength. This slightly bluish tinge helps overpower the yellowish hue of, say, a well-worn undershirt, making that nasty old rag look radiant and white.
If you've ever done your laundry at a blacklight rave (and who hasn't?), you've seen FWAs in action:
Pass the fabric softener and crank up the Sandstorm.
Unfortunately, most of the commonly-available LED lighting today emits little or no light in the ultraviolet part of the spectrum. And as a research team led by Penn State's Dr. Kevin Houser discovered, that makes FWAs pretty much useless.
In a paper published in this month's LEUKOS (the journal of the Illuminating Engineering Society of North America), Dr. Houser and company asked 39 non-colorblind subjects to sort five pieces of identical material based on whiteness, with each item containing a different concentration of FWAs.
Under a normal halogen light, the subjects were able to order the items from dullest to brightest without issue, accurately sorting them from lowest to highest concentration of FWAs. Under typical LED light, however, things fell apart: since the LEDs emit no ultraviolet light, the FWA effect disappears. The subjects were reduced to moving the samples around at random.
There's hope, though! Dr. Houser and company found that "violet-pumped LEDs with a carefully chosen violet emission can emulate the behaviour of a halogen lamp." Meaning subjects were able to once again be fooled by the visual trickery of FWAs.