Incandescent Bulbs May Not Be Dead Yet

If you thought that incandescent bulbs were dead and buried, well, you need to think again. Although incandescent bulbs had many things going in their favor such as a warm glow, dimming capability and low cost, efficiency was not one of them. Most of the energy that went into an incandescent bulb was wasted as heat and only a little was converted into visible light. Now, scientists at MIT and Purdue University are developing an ultra-efficient new incandescent light bulb. It reuses the heat it gives off by converting the heat into light.

Traditional incandescent bulbs heat a tungsten filament, causing it to glow. This also creates both visible and infrared light. While the visible light is useful, the infrared wavelength is dissipated as heat and is hardly of use. In the new type of incandescent bulb, scientists have coated the filament with a structure called photonic crystals.

Photonic crystals are made from abundant elements and applied on the filament using conventional material deposition technology. Although the crystals allow visible light to pass thorough unimpeded, they reflect the infrared wavelengths back into the filament. This heats the filament further, keeps it glowing and emitting more of the visible light, while the bulb itself uses much less electricity than it does otherwise.

According to the scientists, the bulb can have a high luminous efficacy, a measure of how well a light source produces visible light – a ratio of luminous flux to consumed power. For instance, regular incandescent bulbs show a luminous efficacy of 2-3 percent, CFLs come in at 7-15 percent (excluding ballast loss) and LEDs at 5-20 percent. The new, two-stage incandescent, once developed further, would be able to manage greater than 40 percent luminous efficacy.

For those who perceive luminous efficiency in lumens per watt, the maximum luminous efficiency of 100 percent, is 683 lm/W. That means, incandescent bulbs have a luminous efficiency of 13-20 lm/W, CFLs of 47-103 lm/W and LEDs are 34-136 lm/W. Comparatively, it is expected the new incandescent bulb would show a luminous efficiency of 273 lm/W.

To make the concept successful, scientists had to design the photonic crystal such that it worked for a very wide range of wavelengths and angles. They had to make the photonic crystals in the form of a stack of thin layers, which they deposited on a substrate. The efficient tuning of how the material interacts with light depends on the right thickness and sequence of the layers, according to the scientists.

The photonic crystals cover the filament, allowing only visible light to pass through. The crystals reflect infrared light just as a mirror would, adding more heat to the filament. As only the visible light goes out, the heat waves keep bouncing back into the filament until they can come out in the form of visible light.

Although at present the luminous efficacy reached is only about 6.6 percent, it is rivaling that of the commercial LEDs and CFLs. However, it is too early to say the two-stage incandescent will be able to beat the LEDs, because research on LEDs is also progressing very fast.