Engineers seeking to fit ever-brighter LED bulbs into residential light sockets have to work around a tough legacy design problem. Created for incandescent bulbs, the sockets (and the bases that screw into them) are good at conducting a lot of electrical current while minimizing thermal conductivity. Those are benefits when your light source is a filament that can vaporize, melt, or ignite most materials in its vicinity. But they are liabilities when your light source is a semiconductor that requires almost no current at all and works at a mere 50 to 125 kelvins above room temperature.
LEDs like to run cool: Even a few degrees of extra warmth can play havoc with color, brightness, and life expectancy, so the poor heat-transfer properties of existing sockets delayed the advent of 100-watt-equivalent LED bulbs. But they’re finally here. So how have bulb makers reached—and transcended—1600 lumens of brightness, the minimum required to call a bulb the equivalent of a 100-W incandescent? With a little trickery, a little brute force, and a little thinking outside the bulb.
General Electric’s entry, for example, relies on an active cooling system developed by Nuventix, in which a vibrating diaphragm blasts tiny puffs of air through the fins of a metal heat sink, significantly increasing its efficiency. Switch Bulb Co. uses liquid instead of air or metal to convey heat more rapidly from the LED junctions to the outside of its bulb. Osram Sylvania and others have chosen an A21 form factor for their bulbs (A21 is the larger size and shape commonly used for three-way incandescents),which allows significantly more area for heat sinks, even if it means the bulb won’t fit all fixtures. And Philips Lighting and Intematix Corp. (whose design is sold by Feit Electric under the Utilitech label at Lowe’s home improvement stores) have gone with “remote phosphor” designs, in which the phosphors that temper the LEDs’ harsh blue light are part of the typically opaque bulb envelope that surrounds the light source, rather than bonded to the light-emitting chip itself. This strategy lets engineers control the temperature (and thus the color) of the LED and the phosphors separately.
Oh, and NanoLeaf (formerly called NanoLight), whose crowdfunding effort on Kickstarter yielded preorders for more than 5000 bulbs, has gone with a bare-bones—or rather bare-board—approach. Instead of diffusing the light through an envelope to mimic a standard frosted bulb, the company simply bonded the circuit boards together to create a bulb with its LEDs open to the world. You can either put it in a fixture that conceals the bulb or put your efficiency on display for all to see. The designers also chose phosphors that are more efficient at turning the LED’s output into broad-spectrum light. However, these phosphors may make some colored objects look unnatural (hence the lower color-rendition index, which is a scale from 0 to 100 that measures how accurately a light source can produce the colors seen under natural light).
Selected 100-watt equivalent bulbs
CRI (color rendition index): 80
TRICKS: Active cooling pushes air through heat-sink fins rather than relying on convection.
CRI (color rendition index): 90
TRICKS: Remote phosphor. High color temperature lets designers use more efficient phosphors. The LED’s very large size offers more room for heat sinks.
CRI (color rendition index): 94
TRICKS: Efficient driver circuitry minimizes the amount of heat generated in the bulb. A large heat sink maximizes heat transfer. It can be used only where room temperatures are 40 °C or less.
CRI (color rendition index): 70
TRICKS: An exposed circuit board maximizes heat convection. High color temperature and a low color-rendition index let designers use more efficient phosphors.
CRI (color rendition index): 81
TRICKS: A21 size offers much more room for heat sinks. Internal convection passages guide air closer to the hottest parts of the lamp. It can be used only in open fixtures where air can circulate and external temperatures are 45 °C or less.
CRI (color rendition index): 80
TRICKS: A21 size. A remote phosphor allows the LED and phosphor to operate at different temperatures for better efficiency.
CRI (color rendition index): 80+
TRICKS: Liquid cooling transfers heat away from the LEDs. Clear envelope maximizes the number of lumens getting out of the bulb. High color temperature allows more efficient phosphors.
This article was updated 3 September 2013 1:00 p.m. EST: Corrected reference power rating of Switch 100 to 20 watts.
About the Author
Paul Wallich is a frequent contributor to IEEE Spectrum. In June 2013 he wrote about the small number of often little-known companies that control much of the world’s industrialized agriculture.