EET

What would a ten million dollar light bulb look like?

Surprisingly, it will appear very similar to a standard A19 medium-base incandescent lamp.

You may see one soon. The Energy Independence and Security Act of 2007 established a $10M prize for the best LED (light emitting diode) based replacement of a common 60-W incandescent bulb. The contest is also known as the “Bright Tomorrow Lighting Competition” and the “L-Prize.” The US Government's intent, as described in Subtitle E, Section 655 is to encourage the development and rapid deployment of new energy-efficient lighting products. There is a catch, however — the winning device must “…perform similarly to the incandescent lamps they are intended to replace in terms of color appearance, light output, light distribution, and lamp shape, size, form factor, appearance and operating environment. They must be reliable, available through normal market channels, and competitively priced.”

The physics of light emitting diodes actually makes this quite a challenge. LED die are constructed from a junction of p-type and n-type semiconductor materials. P-type materials have an excess of positive charge carriers (holes), and n-type materials have an excess of electrons. When voltage is applied to the LED, electrons from the n-type material move across the junction and “fill” holes on the p-side, transitioning into a lower energy level. Ideally, the excess energy from each transition results in the emission of a photon. In practice however, not all of those electron-hole recombination events are productive. Foreign atoms, lattice dislocations and other defects in the semiconductor material give rise to non-radiative recombinations, realized as heat. As the intensity of an LED increases, so does the heat. High brightness LED die can get very hot, well over 120 °C if not properly managed.

Of course LEDs are much more efficient than traditional light bulbs. To produce 900 lumens of light, an incandescent bulb will require about 60 watts of energy; a well-designed LED solution might use 10 to 12 watts.

Note that almost three quarters of the energy supplied to a traditional bulb heat radiates outward in the form of infrared (IR). This is why they were used in the “Easy Bake Ovens” of your childhood. The remaining heat is safely isolated from the lamp fixture through a plastic or ceramic socket. But this thermal insulation is exactly opposite of what you need for an LED. White LEDs cannot conveniently shed their excess heat by radiating IR — it's not part of their output spectra. Nor is ultraviolet.

Heat must be conducted away from the LED die in order for it to function for any reasonable length of time, and replacing lamp sockets across the nation to make them thermally conductive is obviously not an option. Like most sources of light, the output of an LED diminishes gradually over time. Maintain a typical LED's die temperature at 60 °C and it will emit at least 70% of its initial light output after 100,000 hours. But if the die temperature is allowed to reach 100 °C, this time could be shortened to 30,000 hours or much less.

The L-Prize is unique because it is federally funded, but there are other privately funded awards aimed at improving the quality of life and accelerating innovation. The Progressive X Prize offers a $10 million purse for a viable automobile with an equivalent fuel economy of 100 MPG or greater — currently 111 teams from 25 US states are in the competition. Google is sponsoring the $30M Google Lunar X Prize for the first privately funded robotic lunar exploration, and the Archon X Prize offers $10M to the first person or team that can sequence 100 human genomes within 10 days or less.

How interesting to consider that forty years ago, man stepped onto the surface of the moon. One might make the argument that conquering space was a logical progression from conquering the sky, and a great impetus for doing that was the Orteig Prize. In 1919, Raymond Orteig offered $25,000 to the first aviator who could fly non-stop from New York to Paris. Charles Lindbergh won that prize in 1927 in his Spirit of St. Louis. The spirit lives on.

If you have any questions or comments, please feel free to contact me at [email protected]

TYPICAL ENERGY DISTRIBUTION OF VARIOUS LIGHT SOURCES

White LED Fluorescent 60W Incandescent
Visible light 30% 21% 8%
Infrared ~0% 37% 73%
Ultraviiolet 0 % ~0% 0%
Total Energy Radiated 30% 58% 81%
Remaining Heat 70% 42% 19%
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