Lighting Research Center Lighting Research Center


Simulated and measured beam patterns for the bare LED array and the LED array with 305-mm-long square light guide.

Optical Elements for Mixing Colored LEDs to Create White Light (2003)

At present, there are two distinct approaches for creating white light with colored Light Emitting Diodes (LEDs): combining a blue LED with a down-conversion phosphor and mixing monochromatic LEDs in appropriate proportions. Mixing monochromatic LEDs eases manufacturing and provides more light output for the same input power. One of the challenges for this method in LED system packaging is mixing the light from the LEDs to produce both uniform illumination and uniform color without an unacceptable reduction in efficiency. This experiment examines whether a light guide can serve as an optical element for a colored LED illumination system producing white light.

EXPERIMENT

Researchers systematically analyzed two types of light guides—one with a square cross section and the other with a circular cross section—for color mixing. The use of light guides allows for color mixing within the light guides on the basis of total internal reflection (TIR) of the rays from the colored LEDs. Researchers used a computer simulation with a commercial ray tracing software package and verified the results in an experimental study. The measured results matched well with the simulation.

CONCLUSIONS
  • The light guides provided no significant improvement in illuminance, color, or beam uniformity compared to bare LED array.
  • The light guides reduced system efficiency as a function of the length of the guides.
  • Circular and square light guide geometries did not show a difference in performance, contrary to suggestions from previous literature.
  • A diffuser at the output end of the light guide significantly improved illuminance and uniformity of color, resulting in a small additional loss of 6 percent on the light output.

Light guide efficiency as a function of length



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