Enlightened About Blue And White Light

The article describing the awarding of the Nobel Prize for the development of the blue light-emitting diodes suggests that white LED light is created from a combination of light from red, green, and blue LEDs (C&EN, Oct. 13, page 7).

The vast majority of white LEDs are based more simply on the use of a phosphor that generates yellow light from a portion of a blue LED emission. Changing the relative amounts of blue and yellow produces different correlated color temperatures—that is, warmer- or cooler-appearing lights.

Martin Bide
Kingston, R.I.

Regarding the well-deserved Physics Nobel for the blue LED created by the Nichia Chemical group, I can offer a personal story.

In the autumn of 1984, I decided to take a break from my graduate work at the University of Cincinnati chemistry department to research analytical methods for developing countries. Exiting the library after reading about LED-based spectrometry, I was excited to buttonhole the nearest person, who happened to be Thomas Ridgway, whose research focuses on chemical instrumentation and sensors. I had never met him before. Silent and apparently utterly disinterested, he quickly walked away when I stopped talking, leaving me feeling rather ridiculous.

So I was surprised when he came up to me a few weeks later and put in my hands a copy of Electronics Weekly reporting a blue light-emitting diode by Siemens using a silicon carbide (SiC) substrate. A few days later I happened to attend a talk by a young doctor, Rob McConnell, a National Institute for Occupational Safety & Health associate working on pesticide poisoning in Latin America, who was voicing frustration with methods of testing for pesticide exposure. Roger Messick, head of computer engineering, was a friend of Richard Day’s, my graduate adviser. Messick introduced me to his graduate student, Patrick Eberly, who was intrigued by the idea of building a field spectrophotometer for pesticide exposure using a generously donated Siemens blue LED.

Despite an output of only 2 millicandela, Eberly’s instrument was rugged and accurate. I asked a scientist at Siemens if the blue LEDs would get any cheaper ($60!) or brighter and was told, unfortunately, no and no, because they were rather useless. Luckily for us the Nichia Chemical group recognized the commercial potential for using a blue LED to make white light in combination with other colored LEDs.

Ralph Magnotti
San Diego