Laser Light, Laser White

November 8, 2011

I'm sure that nearly every reader of this blog is computer-savvy enough to know that the sixteen million colors on their display screen (actually, 2²⁴ = 16,777,216) are formed from just three colors - red, green and blue. These three colors were once generated by phosphors excited by electrons in the cathode rays tubes (CRTs) formerly used as display devices. Liquid crystal displays (LCDs), the present replacement for CRTs in desktop computing, use optical filters to derive these three colors from a white backlight. Many mobile devices use red, green and blue organic light-emitting diodes (OLEDs).

The human eye is most sensitive to green light (520–570 nm), and much less sensitive to red light (630–740 nm) and blue light (440–490 nm). This is shown quantitatively in the figure below. The human eye, integrating over this spectrum, sees white light. As Isaac Newton so ably demonstrated in his prism experiments, white light is just a combination of colors, and a proper choice of red, green and blue light intensity will give an overall appearance of white light.

The CIE Photoptic Luminosity Curve (1931) that quantifies human color vision. (Via Wikimedia Commons).

This additive principle of colors is usefully employed in the manufacture of household fluorescent lamps, where ultraviolet light from a plasma excites red, green and blue phosphors painted on the inside of the glass envelope. As can be seen in the figure, the actual light output from such a lamp is a series of colors of different intensities that add to give the appearance of white light to the human eye.

Spectral output of a typical fluorescent lamp. Red, green and blue emissions come from excitations of Eu, Ce and Tb ions. Some Hg lines are also present. (Via Wikimedia Commons).

There's often a disconnect between technical precision and aesthetic reality. An engineer can design a lamp that gives you a precise simulation of white light, at least as far as the mathematics guides him, but the consumer thinks the light is "too harsh." This is all subjective, but the customer is always right. "Too harsh" was a common complaint about halogen lamps when they were first introduced. Aesthetics is why you can buy "soft white" lamps.

Lasers, unlike other light sources, produce light in a very narrow bandwidth. This is an advantage in many applications, such as fiberoptic communications in which the object is to put as many signal channels in the fiber as possible without their interfering with each other. This has always been considered to be a disadvantage in a room illumination application, although the price of semiconductor lasers in the visible color range has been too high for anyone to even consider using lasers for room illumination.

It's not that the inadequacy of lasers for illumination was established by a scientific study. It was just folklore based on the fact that such sharp emission lines, ten times sharper than those for LEDs, would not be blended by eye into a suitable white spectrum. Jeff Tsao, a member of the Sandia Labs that decided to test this hypothesis, said that the reception of this idea was something like, "Are you kidding? The color rendering quality of white light produced by diode lasers would be terrible."[1]

The Sandia team produced a white light source from four laser sources, as shown in the table.[2] The positions of the light sources on the The CIE 1931 color space chromaticity diagram are shown in the figure.

Color	Wavelength (nm)	Laser Type
Red	625	800 mW AlGaInP laser diode
Yellow	589	500 mW sum frequency generation of 1064 nm and 1319 nm from 808 nm laser-diode pumped Nd:YAG
Green	532	300 mW frequency doubled 1064 nm from 808 nm laser-diode pumped Nd:YVO4
Blue	457	300 mW frequency doubled 914 nm from 808 nm laser-diode pumped Nd:YVO4

The The CIE 1931 color space chromaticity diagram marked with the wavelengths of the laser light sources. White light is readily accessible to this color gamut. (Via Wikimedia Commons).

The test, published in Optics Express,[2] involved forty human subjects at the University of New Mexico's Center for High Technology Materials who were presented with two nearly identical still life arrangements in adjacent chambers (see photo). These scenes were illuminated at random by various light sources, including a white light mixture of laser colors, and the subjects were asked which scene they preferred.[1] Such "A-B" tests are common for evaluation of audio signals.

Still life scenes under different white illumination. In this photograph, the scene on the left was illuminated by a diode laser light and the scene on the right was illuminated by a standard incandescent bulb.(Sandia photo by Randy Montoya).

Five subjects were found to be color-blind, so their results were excluded from the dataset. The other participants were presented with eighty random pairings over the course of 10-20 minutes. Surprisingly, there was a statistically significant preference for the semiconductor laser white light over warm and cool LED white light. However, there was no statistically significant preference of laser over neutral LED or incandescent white light.[1-2]

One problem that appears in laser illumination that does not appear in other light sources is speckle. The Sandia team found that speckle detracted from image quality, especially for younger subjects with high visual acuity. To make an acceptable laser light source, they added considerable diffusion, which resulted in a 75% reduction in light intensity.[2] A different solution would be needed in a practical system.

The main stumbling block is that semiconductor lasers are still not sufficiently efficient and inexpensive to be used for illumination. The research team received colorimetric and experimental guidance from the National Institute of Standards and Technology. The work was supported by the Solid-State Lighting Science Energy Frontier Research Center, which is funded by the U.S. Department of Energy Office of Science.[1]

References:

1. Neal Singer, "High-quality white light produced by four-color laser source," Sandia Labs Press Release, October 26, 2011.
2. A. Neumann, J. J. Wierer, W. Davis, Y. Ohno, S. R. J. Brueck and J.Y. Tsao, "Four-color laser white illuminant demonstrating high color-rendering quality," Optics Express, vol. 19, no. S4 (July 4, 2011), pp. A982-A990.