This week, the lighting start-up company vu1 is beginning to ship a new type of lightbulb that could displace compact fluorescents and LED lamps as the energy-saving bulb of choice. The technology, known as cathodoluminescence or electron-stimulated luminescence (ESL), offers similar energy savings, but provides a more natural quality of light.
My life in the past several years has been a living lightbulb joke. I’ve changed, re-changed and re-re-changed the bulbs in my house trying to find replacements that save money but don’t make our skin look like zombie flesh. Compact fluorescents and LED lamps save energy, last longer, and emit less heat than incandescent bulbs. But their light is ickier, versions advertised as "dimmable" often dim only over a limited range, and CFLs take maddeningly long to come to full brightness. No single type works everywhere. LEDs are great for desk lamps, but their narrow beams fail to fill larger spaces. In darkly painted rooms, I went with cold-cathode fluorescents with a low brightness temperature. I’ve filled a big box in the basement with all the bulbs I’ve tried and rejected. So much for saving money.
The color quality has to do with how these bulbs work. The white(ish) light you see is given off by a phosphor coating. In a CFL, the phosphor glows when backlit by ultraviolet light from mercury vapor; in an LED bulb, it soaks up light from a pure-blue LED. (The mercury is why you can't just throw a CFL in the trash when it dies.) On the left is a graph showing the spectra of incandescent, CFL, and LED bulbs; the x-axis is the wavelength in nanometers. Although this particular graph was provided to me by vu1, which has a certain self-interest in showing its competitors in their worst light, it matches information from other sources such as the lighting control company Lutron and the RPI Lighting Research Center. The CFL spectrum is a series of spikes, reflecting the combination of phosphors used to approximate white light; CFLs therefore accent certain colors and fail to render others.
LED bulbs, which use a different type of phosphor, have a smoother spectrum, but the blue LED that drives the phosphor creates a sharp peak in the blue, short-wavelength range of the spectrum, which might pose a "blue light hazard." Ignacio Provencio of the University of Virginia will have an article in our May issue about how our eyes have a special class of photoreceptor that doesn't form images; instead it absorbs blue light to synchronize our body clock with the day/night cycle. Too much blue light could disrupt your sleep cycle. I've also seen claims that excessive blue light might fry your retina and increase your chances of developing macular degeneration. The French counterpart to OSHA issued a report last November warning that children are at particular risk, although Physics World quoted other experts who thought the claims overblown.
The new ESL bulbs use a phosphor, too, but one that does not absorb light at all—instead, it absorbs electrons. Roughly speaking, ESLs are cathode-ray tubes repurposed as lamps. The electrons stream off a metal cathode plate and are pulled by an electric field toward an anode, a thin layer of metal on the backside of the phosphor. Charles Hunt of University of California at Davis, who helped to develop the phosphor, explained to me that ESLs differ from the CRTs used in old TV sets by virtue of their lower electron densities and energies.
Because they use a different phosphor, ESLs provide a somewhat more natural light; see the spectrum at left. The company claims all sorts of other advantages, too: the bulb turns on faster, shines omnidirectionally rather than in a narrow beam, dims over a wider range, and contains no mercury. I've been trying out a demo, an R30 spotlight for a recessed fixture in my kitchen. When you flip the switch, it comes to full brightnesss in a second or two. My wife judges the light to be yellower and milder than a CFL's. Measured with a Kill-a-Watt, the bulb draws 16 watts, about as much power as a CFL of the same brightness, roughly equivalent to a 60-watt incandescent. I'm able to dim it down to, or up from, 20 percent of maximum brightness. The main downside is that the bulb is much heavier than any other I've ever tried—nearly a pound.
Since vu1 first announced the bulbs last year, people on Internet discussion groups have worried about x-ray emission, since the technology is similar to that of an x-ray tube. Hunt said ESLs do produce some x-rays, but at levels below the ambient background dose. The company says that UL-certification included x-ray safety testing.
The R30 begins shipping this week, for $20. The company says it'll introduce an A19 standard bulb shape in the summer, followed by decorative and vanity lighting.
Image and graphs courtesy of vu1