Remote phosphor expands reach of LED light
Margery Conner, Technical Editor - March 17, 2011
The 100-year reign of the incandescent light bulb will end in 2012 as the Energy Independence and Security Act of 2007 mandates the phase-out of most incandescent bulbs in favor of more efficacious forms of lighting. New forms of lighting will also need to meet consumers’ expectations for light color, dimming performance, lifetime, and the light pattern that a bulb casts.
To evaluate light patterns, I used a home-made jig that’s basically a slanted rim—a conical section of light cardboard—that encircles a light bulb, reflecting its light over the 360° around the bulb.
1. Here’s the familiar global-light distribution of an incandescent bulb.
2. The CFL (compact fluorescent light) also has a fairly universal light distribution, although slightly less than that of the incandescent.
3. The snow-cone LED light was less than half of a semispherical pattern, which you’d expect: The LEDs rest on a flat surface, pointing up with little to no secondary optics to direct the light to the sides or down.
4. Now, take a look at the Philips light. It is virtually the same as the incandescent light.
5. How does the Philips bulb create such a spherical light pattern? Let’s take a closer look at the bulb. Rather than mounting the LEDs horizontally on the base of the bulb, the LEDs mount vertically with their light shining out and striking the yellow case that surrounds each LED section.6. The yellow plastic is a remote phosphor. White LEDs comprise blue LEDs with a dollop of phosphor directly on top of and touching the blue-LED emitter. By using a remote, or secondary, phosphor, the bulb can use the remote phosphor’s characteristic of
unidirectional and uniform light emission, rather than as a point source. After carefully removing the yellow plastic remote phosphor and powering up the bulb, you can see the blue LEDs.
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