Under some LED bulbs whites aren't 'whiter than white'

Apr 18, 2014

For years, companies have been adding whiteners to laundry detergent, paints, plastics, paper and fabrics to make whites look "whiter than white," but now, with a switch away from incandescent and fluorescent lighting, different degrees of whites may all look the same, according to experts in lighting.

"Retailers have long been concerned with the color-rendering qualities of their lighting, but less aware how light sources render white," said Kevin W. Houser, professor of architectural engineering, Penn State.

Not long ago, the only practical choices for home, office or commercial lighting were incandescent or fluorescent bulbs. More recently, , which use less energy than incandescent bulbs, became popular, but compact fluorescents are not always accepted by consumers because of poor color rendition, lack of dimability, slow warm-up to full output and because they contain mercury.

The most recent popular entry into home or commercial lighting are light-emitting diode (LED) bulbs, which while currently expensive, are often even more energy-saving than compact fluorescents.

While some LED bulbs will make colors pop, the vast majority do not showcase or differentiate the appearance of white products, according to Houser, because all white light is not the same.

Different light sources contain different combinations of the wavelengths of light. A broad variety of wavelengths will create light that appears white to the human eye, but different mixtures of wavelengths will affect how colors are rendered. When it comes to seeing the color white, the is very important because of how product manufacturers make white products appear white using whiteners.

Whiteners contain fluorescent materials that glow under violet and . Sunlight, fluorescent light and incandescent light all produce some light in the violet and ultraviolet range. The whiteners used in consumer products work under those conditions, resulting in a bright white perception.

However, most current LED bulbs use blue LEDs to excite a phosphor that then glows white, but produces no violet or ultraviolet light.

Houser, working with a Penn State student and researchers from Soraa Inc. of Fremont, Calif., asked 39 participants to observe various combinations of light sources and white objects to see how the light source affected perceptions of white. They report their results in a recent issue of Leukos, the journal of the Illuminating Engineering Society.

The participants completed three tests—selection, forced choice and sorting—using five different light sources—a blue-pumped LED, filtered halogen lamp and three violet-pumped LEDs with differing levels of violet emissions.

In the sorting experiment, the researchers placed six calibrated whiteness cards of varying whiteness on a table in a booth enclosed on three sides. They asked participants to arrange the cards in order of whiteness under each of the five light sources.

Under the halogen light and violet-pumped LED lights with 7 and 11 percent violet emission, the order was correct. Two of the cards were flipped under violet-pumped LEDs with only three percent violet emissions.

"With the LED but only blue pumping the phosphors, the order became random," said Houser. "People simply couldn't tell the difference between the cards. Under the blue-pumped LED, which is notable because blue-pumped LEDs are by far the most common type for general lighting."

In the forced choice test, two nominally identical cards were placed in each of two booths containing different light sources. Participants were asked to choose the card that was whiter under all of the permutations of each of the five light sources.

"The sources with higher violet component permitted the best discrimination between the targets," said Houser.

In the selection test, researchers asked the participants to look at a reference card in one booth and rank the cards in a second booth as either as or whiter than the reference card. Again the blue-pumped LEDs did not fare well.

The researchers note that "engineering of an LED source's spectrum is necessary for an accurate rendering of whiteness."

Other researchers on this project were Minchen Wei, graduate student in architectural engineering, and Aurélien David and Michael Krames, Soraa Inc.

Soraa Inc. funded this study.

Explore further: Tech review: The future is bright for LED bulbs — and your wallet

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Eikka
3 / 5 (1) Apr 18, 2014
Yet the luminous efficacy (lm/W) which is used to compare lighting efficiency is harmed by the addition of violet, because spreading the spectrum wider puts much of the light outside of the optimum wavelenghts for light intensity perception.

In other words, LED lighting typically achieves higher efficiency by reducing the quality of the lighting.

Drjsa_oba
5 / 5 (1) Apr 18, 2014
In the sorting experiment, the researchers placed six calibrated whiteness cards of varying whiteness on a table in a booth enclosed on three sides. They asked participants to arrange the cards in order of whiteness under each of the five light sources.

Read more at: http://phys.org/n...html#jCp


Who is to say which white is whiter? Are we saying that the whiteness was calibrated under sunlight or calibrated under incandescent light?

And since whiter than white is an artificial construct of luminescence as described that does not make it true except under those same conditions.

SO what I am getting at is that it is all relative , we need to describe the parameters and assumptions as well as the results and understand that we spend more time under artificial lights than real sunshine and this will probably increase so the base line is not necessarily located where the story implies.
Drjsa_oba
4 / 5 (1) Apr 18, 2014
With LED's capability to provide light in a range of specific wavelengths and fluorescent lights being graded often by temperature ranges but not made up of the same combinations of wavelengths as solar light. White is not white. There currently is not such thing. When you say white from this day forth you are only talking about an approximation open to interpretation.
fulwild
5 / 5 (1) Apr 19, 2014
Regarding the whiteners in laundry detergent. The 'whiteners' in at least Cheer laundry detergent were actually blue dye which was introduced as market research indicated that a very slight blue tint was perceived as whiter than plain white. This info comes from a relative who was an engineer and patent attorney for P&G in the '60. This was also about the time clothes dryer came into wide spread use and I suspect that women were there for looking at their whites under incandescent (yellower) light instead of under sunlight on the clothes line.
alfie_null
5 / 5 (2) Apr 19, 2014
To some extent, whiteners in laundry detergent are there to obscure dirt the detergent failed to clean. I'd rather not have the whiteners, thank you.

Regarding broad spectrum emissions and faithful rendition of colors, it's probably been an issue since fluorescent lighting became popular, back in the 1930s. I bet those early fluorescent tubes were harsh.
antialias_physorg
5 / 5 (1) Apr 19, 2014
it's probably been an issue since fluorescent lighting became popular, back in the 1930s.

Incandescent bulbs also don't give off the same spectrum as true sunlight. So we have never had a light source that was perfect in that regard. And with the spectrum being variable over the course of a day (and depending on weather conditions) there is no 'perfect' light, in any case.

With LEDs we have the chance to tune/combine them to get arbitrarily close, though.
jakeflamingau
not rated yet May 13, 2014
Considering about the whiteness, it means to get sun light as the purest form of light. The whiteness in which eyes feel comfortable.
This led to the discovery of mimic led lights with the scattering of the nano particles by by Paolo di Trapani at the University of Insubria.
Ref - http://autopal.ne...cts.aspx
Eikka
not rated yet May 13, 2014
Who is to say which white is whiter? Are we saying that the whiteness was calibrated under sunlight or calibrated under incandescent light?


There is a thing called the Kruithof curve that relates the absolute light intensity to the percieved whiteness of different color temperatures.

But that's slightly different because for a light to appear white is different than for an object under a light to appear white because the color temperature is simply the average peak wavelenght.

With LEDs we have the chance to tune/combine them to get arbitrarily close, though.


As with fluorescent lights. It's actually easier to achieve better color rendering with fluorescent materials than LEDs, and currently more efficient, which is why all the top of the line LED bulbs use simple red and blue/violet or UV diodes and shine them through a plastic fluorescent/phosphorescent shell which fills in all the other wavelenghts.

But the problem still is that the spectrum isn't continuous
Eikka
not rated yet May 13, 2014
And the problem of discontinuous spectrum is that your eye recieves light at all wavelenghts, and tries to measure some sort of weighted average out of them. It isn't looking at just particular reds, greens and blues but everything in between as well, despite the fact that the signal output to the brain is reduced to those three.

The eye sees equal amounts of red and green light combined as yellow, so a light that has red and green but no yellow still appears yellow. A yellow object on the other hand does not reflect just red and green but the yellow in between as well, and if the yellow wavelenght is missing it can return unequal amounts of red and green, and will look more reddish or greenish than it would if the yellow wavelenght was present. It will also look darker and muted because it isn't returning as much light.

So in this way the lack of all the intermediate wavelenghts distorts colors, and the worse the discontinuity, the more difficult it becomes to tell colors apart.
Eikka
not rated yet May 13, 2014
And that color discrimination problem is what causes the effect described in the article.

"Whiteness" means technically that the object returns equal proportion of light at all wavelenghts. It's essentially just returning the same spectrum of light as it recieves.

Perceptually white depend on psychological effects, contextual cues, the intensity of lighting and other effects that are subjective to the viewer, but that's not the question here. The cards are calibrated to be slightly off-white so that one is a bit bluish, another a bit reddish, one might be a bit magenta etc. and you should be able to put two cards side by side and see a difference.

But with poor CRI lighting that's not happening. There's too many and too large gaps in the spectrum, which makes discrimination of hues of not only white but of all colors more difficult.

The effect is more or less the same as with people who have mild cataracts. Everything turns a bit dull and grey.
Eikka
not rated yet May 13, 2014
Incandescent bulbs also don't give off the same spectrum as true sunlight. So we have never had a light source that was perfect in that regard.


They can with filters.

Because incandecent bulbs have continuous spectrum for most intents and purposes.

Direct sunlight has a color temperature of roughly 4800 K while halogen bulbs reach 3500 K so all one needs is a red filter to bring the color temperature up and there you have it.

This is often done in studio lights for movies to simulate outdoors scenes because again, fluorescent or LED lights, or different sorts of arc lights don't give a continuous spectrum and it would interfere with the colors.

Eikka
not rated yet May 13, 2014
You can buy 5000 K H4 halogen bulbs for car headlights as well.

For people who like to pretend that they have HID lights without going through the expense of fitting in the system. They're just ordinary xenon bulbs with a blue tinted glass:

http://litbimg.ri...7417.jpg