Throwing some (bluish) light on the subject

© 2013 Robert Sekuler

Neodymium (Nd) is a shiny silvery-white metallic chemical element, with atomic number 60. Discovered in 1885 by C. A. von Welsbach, Neodymium is one of the Lanthanides series of related elements (whose atomic numbers run from 58 to 71). When it's heated in the presence of air, Nd reacts vigorously to form Nd2O3, an light blue oxide. That last sentence is crucial to this essay, so let me restate it in a form that is more correct technically: To a normal human eye, under daylight viewing conditions, Nd2O3 looks light blue. And that's an important fact.

Spectrum of Neodymium bulbBecause of its color, Nd2O3 is a useful inner coating for light bulbs. When such bulbs are not lit, they have a telltale powder-blue color. When they are lit, their special coating filters out much of the dulling yellow cast produced ordinary light bulbs, producing a light that seems purer, more like sunlight. The diagram at the left shows the physical basis of these color effects. The black line shows the power spectrum of an ordinary tungsten filament light bulb. Notice that as the wavelength of light increases (left to right), the power emitted by the ordinary bulb steadily increases. The colorful spectrum in the diagram represents the power emitted by a bulb with a Nd2O3coating. The notch in the spectrum (from about 560 to 590 nm) represents the light that is filtered out (absorbed by) the bulb's coating. To a healthy, normal human eye such wavelengths look yellow or yellowish.

For more than a decade such bulbs were available only from small companies such as Verilux. But these bulbs have remained a well-kept secret --because of their cost and because there's never been a significant advertising campaign to educate consumers. All that may change now. The proverbial 600 pound gorilla of the light bulb industry entered what was a small, niche market. With a huge, sophisticated advertising campaign, General Electric (GE) introduced its Reveal© product line. An excerpt of a GE press release tells us

"Americans spend $149 billion on home improvements every year. Why should they settle for anything less than the best quality home lighting? The richness of the clothing, fabrics, walls and artwork in their homes will appear truer in color with GE Reveal lighting. The design and technology of GE Reveal bulbs make colors "pop" in a way they don't with ordinary incandescent bulbs.

"In focus group research, consumers preferred GE Reveal bulbs for ambient lighting, citing the "crisper" nature of the light, making it ideal for tasks such as hobbies and other close-up work.

"Users indicated that the light from Reveal bulbs makes colors and home environments richer and more textured, and that it makes task-related details appear crisper... more highly defined. In fact, a number of test users described the light as a "100% improvement" over the light from ordinary bulbs."

I'm not sure that I know exactly what "100% improvement" means, but it's not surprising that these bulbs do alter the way we see. After all, the bulbs' effects, and whatever financial gain they bring to GE, depend on old reliable human color vision.

closetIn their television and print advertising, GE uses many different images to convey the benefits of their Reveal© bulbs. The figure at the left shows one GE's images, a side by side comparisons of a scene rendered under normal artificial illumination and under Reveal© illumination. The differences seem obvious, particularly for certain surfaces in the image --note which ones.

How can we know that any new bulb actually performs as advertised? In particular, how can we tell that it really would make objects look more natural, more like they would in normal daylight? The answer lies in a measure called the Color Rendering Index (CRI). A value of 100 on this index signifies that the illuminant (light source) will cause objects to look exactly as they would in natural outdoor sunlight. A lower CRI signifies that an illuminant "distorts" or shifts objects' colors.

To calculate the CRI for some new illuminant, that illuminant's effect on a set of standard surfaces is measured. Ordinarily, the samples comprise the eight standardized surfaces shown at the left. The samples are intermediate in saturation, and are spread throughout the range of hues that humans see. Sometimes, a more extensive set of samples may be used, such as samples that include "typical" Caucasian skin, or leaves. samples used for ordinary CRI

Color rendering evaluates how the colors of surfaces will look when they are illuminated by a given light source. For example, depending upon the characteristics (spectrum) of the light source illuminating a "red" surface, that surface may be rendered (made to look) pinker, yellower, lighter or darker. As you know, the spectal distribution of light that is reflected from some surface depends upon two quantities:

the spectral reflectance of the surface, and
the spectral power of the source that illuminates that surface.

The computation of the CRI uses a mathematical model of a "standard" human eye's response to a spectral distribution of light. This model is based on a lot of psychophysical data, and essentially predicts the color that would be seen. The model characterizes the resulting color by a pair of numbers. These are coordinates in a color space developed by CIE, Commission Internationale De l'Eclairage (in English, the International Lighting Standards Commission).

Basically, for each sample surface, the difference is calculated between the predicted perceived color under some illuminant (such as a new light bulb) and the perceived color under natural daylight. Because the color space has been grounded on psychophysical measurements, each of these numerical differences corresponds to the perceptualdifference between colors.

Those resulting eight differences --one for each surface-- are summed and scaled to produce the index, which runs from 100% (for perfect match to colors seen in daylight) to 1%.

A lamp that generates a CRI of 80 renders colors more faithfully --that is more like daylight would-- than a lamp whose CRI is only 50. The Reveal© bulbs produce CRIs in the neighborhood of 92-93, considerably better than ordinary incandescent bulbs' CRI of 80-85. And, as you saw in the images of the closet (above), this improvement is especially signficant for particular "color" surfaces.

Here's a question that's worth pondering.: If the Reveal© bulbs actually make a difference as large as the one between the pair of closets (above), why --when you use a non-Reveal© bulb-- don't you notice how yellow it makes things appear?