| Tikalon Blog is now in archive mode.
An easily printed and saved version of this article, and a link
to a directory of all articles, can be found below: |
|
This article |
| Directory of all articles |
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, 2
24 = 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:
- Neal Singer, "High-quality white light produced by four-color laser source," Sandia Labs Press Release, October 26, 2011.
- 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.
Permanent Link to this article
Linked Keywords: Computer literacy; computer-savvy; red; green; blue; phosphor; electron; cathode rays tube; liquid crystal display; desktop computing; optical filter; white; backlight; mobile device; organic light-emitting diode; human eye; green light; nanometer; nm; red light; blue light; Isaac Newton; Opticks; prism; CIE Photoptic Luminosity Curve (1931); Wikimedia Commons; fluorescent lamp; ultraviolet light; plasma; europium; Eu; cerium; Ce; terbium; Tb; mercury; Hg; aesthetics; engineer; mathematics; halogen lamp; laser; spectral linewidth; bandwidth; fiberoptic communications; signal channel; illumination; scientific method; scientific; folklore; Jeff Tsao; Sandia Labs; hypothesis; The CIE 1931 color space chromaticity diagram; Optics Express; University of New Mexico; Center for High Technology Materials; still life; randomness; "A-B" test; incandescent bulb; color-blind; speckle; visual acuity; diffuse reflection; diffusion; efficient; colorimetric; experiment; National Institute of Standards and Technology; Solid-State Lighting Science Energy Frontier Research Center; U.S. Department of Energy; Office of Science.