High-quality white light produced by four-color laser source

Oct 26, 2011
Sandia researcher Jeff Tsao examines the set-up used to test diode lasers as an alternative to LED lighting. Skeptics felt laser light would be too harsh to be acceptable. Research by Tsao and colleagues suggests the skeptics were wrong. (Photo by Randy Montoya).

(PhysOrg.com) -- The human eye is as comfortable with white light generated by diode lasers as with that produced by increasingly popular light-emitting diodes (LEDs), according to tests conceived at Sandia National Laboratories.

Both technologies pass electrical current through material to generate light, but the simpler LED emits lights only through spontaneous emission. Diode lasers bounce light back and forth internally before releasing it.

The finding is important because LEDs — widely accepted as more efficient and hardier replacements for century-old tungsten incandescent bulb technology — lose efficiency at electrical currents above 0.5 amps. However, the efficiency of a sister technology — the diode — improves at higher currents, providing even more light than LEDs at higher amperages.

“What we showed is that diode lasers are a worthy path to pursue for lighting,” said Sandia researcher Jeff Tsao, who proposed the comparative experiment. “Before these tests, our research in this direction was stopped before it could get started. The typical response was, ‘Are you kidding? The color rendering quality of white light produced by diode lasers would be terrible.’ So finally it seemed like, in order to go further, one really had to answer this very basic question first.”

Little research had been done on diode lasers for lighting because of a widespread assumption that human eyes would find laser-based white light unpleasant. It would comprise four extremely narrow-band wavelengths — blue, red, green, and yellow — and would be very different from sunlight, for example, which blends a wide spectrum of wavelengths with no gaps in between. Diode laser light is also ten times narrower than that emitted by LEDs.

In the test setup, similar bowls of fruit were placed in a lightbox with a divider in the middle. In this photo, the bowl on one side was illuminated by a diode laser light and the other was lit by a standard incandescent bulb. The aesthetic quality of diode laser lighting (left bowl) compares favorably with standard incandescent lighting (right). (Photo by Randy Montoya).

The tests — a kind of high-tech market research — took place at the University of New Mexico’s Center for High Technology Materials. Forty volunteers were seated, one by one, before two near-identical scenes of fruit in bowls, housed in adjacent chambers. Each bowl was randomly illuminated by warm, cool, or neutral white LEDs, by a tungsten-filament incandescent light bulb, or by a combination of four lasers (blue, red, green, yellow) tuned so their combination produced a white light.

The experiment proceeded like an optometrist’s exam: the subjects were asked: Do you prefer the left picture, or the right? All right, how about now?

The viewers were not told which source provided the illumination. They were instructed merely to choose the lit scene with which they felt most comfortable. The pairs were presented in random order to ensure that neither sequence nor tester preconceptions played roles in subject choices, but only the lighting itself. The computer program was written, and the set created, by Alexander Neumann, a UNM doctoral student of CHTM director Steve Brueck.

Each participant, selected from a variety of age groups, was asked to choose 80 times between the two changing alternatives, a procedure that took ten to twenty minutes, said Sandia scientist Jonathan Wierer, who helped plan, calibrate and execute the experiments. Five results were excluded when the participants proved to be color-blind. The result was that there was a statistically significant preference for the diode-laser-based white light over the warm and cool LED-based white light, Wierer said, but no statistically significant preference between the diode-laser-based and either the neutral LED-based or incandescent white light.

Four laser beams — yellow, blue, green and red — converge to produce a pleasantly warm white light. Results suggest that diode-based lighting could be an attractive alternative to increasingly popular LED lighting, themselves an alternative to compact-florescent lights and incandescent bulbs. (Photo by Randy Montoya).

The results probably won’t start a California gold rush of lighting fabricators into diode lasers, said Tsao, but they may open a formerly ignored line of research. Diode lasers are slightly more expensive to fabricate than LEDs because their substrates must have fewer defects than those used for LEDs. Still, he said, such substrates are likely to become more available in the future because they improve LED performance as well.

Also, while blue diode lasers have good enough performance that the automaker BMW is planning their use in its vehicles’ next-generation white headlights, performance of red diode lasers is not as good, and yellow and green have a ways to go before they are efficient enough for commercial lighting opportunities.

Still, says Tsao, a competition wouldn’t have to be all or nothing. Instead, he said, a cooperative approach might use blue and red diode lasers with yellow and green . Or blue diode lasers could be used to illuminate phosphors — the technique currently used by fluorescent lights and the current generation of LED-based — to create desirable shades of light.

The result makes possible still further efficiencies for the multibillion dollar lighting industry. The so-called ‘‘smart beams’’ can be adjusted on site for personalized color renderings for health reasons and, because they are directional, also can provide illumination precisely where it’s wanted.

Colorimetric and experimental guidance was provided by the National Institute of Standards and Technology.

The research was published in the July 1, Optics Express.

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User comments : 19

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that_guy
5 / 5 (1) Oct 26, 2011
Ok, I am intrigued by laser lighting. I understand that single LEDs are relatively cheap, and that LED lighting can add up in price because LEDs suffer at higher voltages/amperage. I also understand that a laser diode is significantly more expensive than an LED.

But I have one really big burning question that hasn't been answered.

Lasers can definitely pump out much more light than an LED per diode. What is the ideal cost per lumen that we are capable of creating with laser light? It may be too expensive for home lighting solutions, but when it is scaled up, does it provide a good deal for other lighting solutions that need a great deal of light? Or is laser lighting destined only for tiny niches?
wiyosaya
3.3 / 5 (3) Oct 26, 2011
Personally, I don't like incandescent lighting and making a laser diode that mimics it is, IMHO, silly. Right now, I think that there are those in the solid-state lighting industry that seem to be mimicking incandescent lighting for no reason other than that is what the vast majority of humans are used to.

I thought that it is known that lighting with a spectrum that more closely matches that of the sun is a much better source for humans and the human condition especially since it helps people who are affected by seasonal affective disorder.

Perhaps, though, as is prevalent in industry in general, the reason for mimicking the incandescent spectrum is because its fast, cheap, and easy.

IMHO, give me 5000K or give me death in solid-state lighting.
Eikka
4 / 5 (4) Oct 26, 2011

IMHO, give me 5000K or give me death in solid-state lighting.


Give me 5000 K and give me insomnia.

The problem with the cheap "white" LEDs is that they produce light by only two wavelenghts, blue and yellow. Hence the poor CRI - but the further problem is that the 450 nm blue affects the circadian rythm - and that's something which the incandescent lamps don't have very much.

The ideal color temperature is dependent on the amount of light anyways. The less light there is, the lower the color temperature should be.

What goes for the test, such a cursory glance doesn't tell much, and they chose objects that have psychological impact. It is well known in research that if you view a piece of yellow paper and a banana under varying light, people report that the banana is still more or less yellow but the piece of paper changes color, because our brain interprets the view based on what it thinks it should be seeing.
that_guy
3 / 5 (2) Oct 26, 2011
Personally, I don't like incandescent lighting and making a laser diode that mimics it is, IMHO, silly.
...
I thought that it is known that lighting with a spectrum that more closely matches that of the sun is a much better source for humans and the human condition especially since it helps people who are affected by seasonal affective disorder.

Perhaps, though, as is prevalent in industry in general, the reason for mimicking the incandescent spectrum is because its fast, cheap, and easy.

Wiyosaya...
- The laser diode does not really mimic incandescent light. They are combining 4 pure colors to make a single, white, light source. Would you rather have all red or green lights?
- The closest consumer approximation we have to sunlight in the consumer market is the incandescent light. By giving off black body radiation, it emits a full spectrum of light, rather than specific wavelengths.

Continued...
that_guy
4.2 / 5 (5) Oct 26, 2011
The reason why we try to 'mimic' incandescent light is BECAUSE we are trying to mimic sunlight.
- The specific issue that makes flourescent, LED, or laser light less appealing to the eye is both

a) overall color temperature and
b)the specific wavelengths being emitted.

For example: in Eikka's example of the most common type of white LED (Blue led coated with a yellow phosphor) you are missing a big chunk of the middle of the visible spectrum. So when you look at something that is say, a pure green (Like the clorophyll in a plant), under those lighting conditions, it will look dark and sickly.

There's a lot of physiology about how we view light, versus the aspects of physics at play when analyzing light.
kaasinees
4 / 5 (4) Oct 27, 2011
But as said, this can be bad for your health, specifically the biological clock, can create serious illness that can influence your ability to work.
mortoo
5 / 5 (4) Oct 27, 2011
The light is created from components so maybe the color could be adjusted during the day - start out blue and fade to red in the evening - to keep the biology happy.
PinkElephant
3 / 5 (2) Oct 27, 2011
I find it incredible that the blue diode lasers -- the ones that were the hardest to achieve and the last achieved historically (relative to red, yellow, and green) -- are now actually the most energy-efficient of the bunch.

I also don't quite understand why they needed a separate yellow component. Just having R, G, and B ought to be enough to reproduce any color visible to the human eye (obviously, white included.) After all, that's the principle used in our finest professional computer monitors, not to mention digital flat-screen TVs and digitally projected movies...
Jimbaloid
4.7 / 5 (3) Oct 27, 2011
The light is created from components so maybe the color could be adjusted during the day - start out blue and fade to red in the evening - to keep the biology happy.


This is a great idea mortoo - these intelligent bulb units would be aware of the time of day and be heavy with the blue light component in the morning and then as the evening comes gradually tone it down toward the warmer colours of incandescent lighting. This doesn't just meet with users expectations of lighting, it avoids upsetting the our biological clocks, and could probably be marketed as having medical benefit. I for one would buy them.
antialias_physorg
5 / 5 (1) Oct 27, 2011
The real application here is in LED laser beamers. I'm using beamers exclusively at home (for TV and computer display). The DLP beamers are too large/loud (because they need cooling) and the bulbs are not long lived and also very expensive.

The LED beamers I use are much better in that respect (no cooling fans and the LEDs last basically forever), but have a low intensity (i.e. you need to make the room really dark to get a picture with good contrast)

Diode-Laser beamers would be the best of both worlds.

I also don't quite understand why they needed a separate yellow component.

This is not so much physics as physiology. Many women have a fourth type of color receptor in the eye that is particularly sensitive to the yellow part of the spectrum (this is the reason why many women can distinguish more shades of color than men - and why 'avocado', 'peach' and 'eggshell' are colors to them where men only see green, yellow and white)
wiyosaya
3 / 5 (2) Oct 27, 2011

IMHO, give me 5000K or give me death in solid-state lighting.


Give me 5000 K and give me insomnia.

Perhaps the problem is using artificial light too much of the time.

I've been using "full spectrum" florescent bulbs for a long time and I far prefer them to the incandescent alternatives. I also have a 20W 5000K LED on my bench that I find very closely approaches the color of the fluorescent bulbs I have.

As I understand it, you do need to have a good mix of all three primary colors in order to get something that the eye perceives as natural sunlight.

Cut up my post if you like, but 5000K is referring to what should be an attempt to match the black-body spectrum of the sun which is that of a black-body radiator at 5000K.

wiyosaya
1 / 5 (1) Oct 27, 2011
The reason why we try to 'mimic' incandescent light is BECAUSE we are trying to mimic sunlight.
- The specific issue that makes flourescent, LED, or laser light less appealing to the eye is both

a) overall color temperature and
b)the specific wavelengths being emitted.

For example: in Eikka's example of the most common type of white LED (Blue led coated with a yellow phosphor) you are missing a big chunk of the middle of the visible spectrum. So when you look at something that is say, a pure green (Like the clorophyll in a plant), under those lighting conditions, it will look dark and sickly.

There's a lot of physiology about how we view light, versus the aspects of physics at play when analyzing light.


A glowing wire is significantly more red than even the sun is as it lacks higher frequency components. That is likely why the physiology of some people reacts so poorly to that type of light. Like I said above, match the black-body spectrum of the sun.
wiyosaya
2 / 5 (1) Oct 27, 2011
But as said, this can be bad for your health, specifically the biological clock, can create serious illness that can influence your ability to work.

I guess that part of my post was just not heard.
that_guy
5 / 5 (1) Oct 27, 2011
A glowing wire is significantly more red than even the sun is as it lacks higher frequency components. That is likely why the physiology of some people reacts so poorly to that type of light. Like I said above, match the black-body spectrum of the sun.


Incorrect. A glowing wire will emit a spectrum based on the amount of energy that is put into it. That's how we get color temperatures. A color temerature is the idealized peak emission given off of an object due to black body radiation.

That means, if you heat the wire up more, it goes from infrared, to red, yellow, etc.

The incandescent light is a glowing wire, and yet, it is not red.

The reason why black body radiation is more appealing to the human eye is because it emits a fuller spectrum of light, on a bell curve. This is also why the incandescent light looks white-ish - because it is emitting significant amounts of light above and below the yellow spectrum in addition to it's peak.
that_guy
5 / 5 (1) Oct 27, 2011
Also, floursecent bulbs do not emit the full spectrum - they emit 3 or 4 peaks based on the included phosphors.

@anti-alias: this is physiology and physics. We percieve light one way (Physiology) which is why we can see white light from just a few colors. Also why we see pink/magenta, even though it doesn't have a counterpart as a single wavelength (It is what we see when our brain tries to close the circle of color when see see violet at the top of the spectrum and red at the bottom).

Physics is involved in the ways we try to reproduce light. In non-black body light sources, it can look white, but still under saturate/over saturate some colors unnaturally - which is one reason why flourescent light seems so harsh to many people (in addition to color temp and 'buzzing' or flickering)

So I believe the science of lighting depends heavily on an understanding of both.
Eikka
5 / 5 (1) Oct 30, 2011
I also don't quite understand why they needed a separate yellow component. Just having R, G, and B ought to be enough to reproduce any color visible to the human eye (obviously, white included.)


The RGB color system works for additive color, but the color of objects around you is subtractive color.

All the colors are "made" by taking something away from the ambient light, not by adding to it, so you can't take away the red and the green and the blue to have a yellow color if the yellow wavelenghts aren't there. You'd simply have black.

Eikka
5 / 5 (1) Oct 30, 2011
This is not so much physics as physiology. Many women have a fourth type of color receptor in the eye that is particularly sensitive to the yellow part of the spectrum (this is the reason why many women can distinguish more shades of color than men - and why 'avocado', 'peach' and 'eggshell' are colors to them where men only see green, yellow and white)


Now that is just bullshit.

The phenomenon is real, but it only affects one eye at a time by slightly shifting the sensitivity curves of the rods and the cones. In other words, one eye sees slightly different colors than the other. It has some limited use in more accurate color perception when viewing things with both eyes, but it only affects a small minority of women.

In fact all people have four color receptors in the eyes. One of them only works when the light levels drop too low for the other three to work, though. It's called rhodopsin, or visual purple and it's sensitive to a slightly greenish blue wavelenght
Eikka
5 / 5 (1) Oct 30, 2011
Incidentally, rhodopsin is most sensitive to the peak wavelenght of sunlight, and therefore the peak wavelenght of moonlight which has mostly the same spectral characteristics but lower intensity.

So, under moonlight we literally switch to night vision mode. Under sunlight when there's more than enough light to see, we switch to seeing the color of the sky and the grass and the red and yellow berries.
Eikka
5 / 5 (1) Oct 30, 2011

Cut up my post if you like, but 5000K is referring to what should be an attempt to match the black-body spectrum of the sun which is that of a black-body radiator at 5000K.


But that's only right if you look directly at the sun at midday. Otherwise the color temperature goes all over the place all the time from dusk to dawn.

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