Tuesday, June 03, 2008

Toward Reasonable Light Sources (not an idea)

Is it just me, or is the yellow-orange tint of in-door incandescent lighting totally depressing? It's like one step away from hell (which would, itself, be red). For most of my life I must have denied it - it was hard to notice because the eye adjusts anyway, and when I did, I must have figured, maybe it was subjective. But now I've come to realize that the light simply is inferior. It IS yellow. It's not white; it's yellow. I don't know about you, but I find this unacceptable.

Light from the sun, or any other source that emits light by being heated up, follows a spectral envelope (that is, an intensity vs. frequency graph) called a blackbody spectrum. (The very existence of this curve directly led to the invention of quantum mechanics.) The shape of the curve depends on (and ONLY on) the temperature to which the body is being heated up. The average temperature of the surface of the sun (the photosphere) is 5780° Kelvin, hence the "color temperature" (the blackbody spectrum for a given temperature) of direct sunlight is 5780K. That basically defines what we see as white. The color temperature of an incandescent bulb is 2600K to 3300K. That's this color. Halogen and typical fluorescents are higher, but not by a whole lot.

Fortunately there now exist fluorescent bulbs that emulate the color temperature of sunlight, or even overcast (6500K). No other common household lighting will do that, but fluorescents aren't perfect either: I say "emulate" because the spectrum doesn't actually follow a blackbody spectrum. They use photoluminescence, a form of cold body radiation. That means, since the spectral output doesn't naturally adhere to a blackbody curve, they have to simulate it, by combining multiple colors. The light itself may *appear* white (just as white as the sun), but colors that it reflects off of will render differently; they will have less variation between them. This applies to any fluorescent lighting - it's unnatural light. Hence a 5000K fluorescent light isn't *really* 5000K; 5000K is the Correlated Color Temperature (CCT), which basically just means that it's no bluer or redder than white light.

If you're wondering why two light sources that appear equal in color could render reflected colors differently, it's because the eye doesn't see colors according to their full spectral envelope. If it did, the experience would be unimaginable. The eye perceives three dimensions of color; there are three specific cell types that resonate with three different wavelengths of light. They don't resonate ONLY with those wavelengths, but generally speaking, the closer a frequency is to a cell type's resonate frequency, the more that cell type will respond to it. Of course light is (usually) composed of many frequencies; they all add up and result in the three respective stimulus levels for any particular point of color. So the same-looking 'white' can be made using an unlimited number of different combinations of frequencies. It can be 5000K blackbody radiation, for example (emitting light on all visible frequencies), or it can be just as few as two different frequencies: blue and yellow. But in addition to that, reflected colors have their own frequency distributions. Cyan, for example, could actually be reflecting cyan, or it could only be reflecting a combination of blue and green. Both of these look the same to the eye, at least under a given light. But what if our light source were (for example) a combination of monochromatic red, blue and green, and the color reflected only cyan? Then you wouldn't even see it, because cyan is none of those. So between the spectral envelope of the light source, the spectral envelope of the reflective surface, and dimensionality of color perception, you get the same color looking differently under different lights, even if the lights themselves appear the same. Full-spectrum light is just better for rendering color.

Some stores purposely use fluorescent light to, for example, make their tomatoes appear redder. (Some colors may appear more vivid; the only thing that's lessened is the differentiability between colors.) While it may look good for their tomatoes, I don't think it's overall a good thing. The environment just appears sterile and it has a deadening effect on the feelings.

There is a measure of color-rendering ability, called the Color Rendering Index (CRI), used to guage various light sources. A true blackbody profile, which includes incandescent lights, has a CRI of 100 ("perfect"). Note that CRI is independent of color temperature or CCT. A fluorescent bulb can have a CRI as low as 49, although they've gotten better over the years. 85 and above is considered decent; 90 and above is considered excellent. A fluorescent bulb with a CCT of 5000K to 6500K and a CRI of 90 or above is technically considered a "full-spectrum" light source, but I'm not so sure I agree. What I have now are 2 fluorescent bulbs with a CCT of 5000K and a CRI of 92, called Homelight Natural Sunshine, by Philips. The color temperature is fine - it really does look like daylight, but the color rendering just seems off to me. It doesn't seem natural. It could just be my imagination, but I don't think so. And the CRI is an out-dated measurement. It uses an inferior color map (which is a way of mapping the entire spectrum of perceivable colors using a number, usually 3, of particular dimensions), and there are better methods available. It's considered not very good for visual assessment, but light manufacturers still use the CRI, so that's what we have to go on.

So, I plan to get a better light light source than fluorescent, one of these days. The only better light source I KNOW OF is the xenon arc lamp, which seems to have - depending on whom you ask - a color temperature of 4500K (still higher than halogen or typical fluorescents) to 6500K and a CRI of 95 to 99+. But xenon arc lamps (and their power sources) are not cheap, nor even easy to attain for that matter. Also they take about a minute to warm up, as far as I can ascertain. White LEDs, by the way, can also render light at 5000K and above (and they last longer), but they have even lower CRIs than fluorescent. (They use phosphors, just like fluorescents do.. there's no such thing as a white-light-emitting diode.)

But they're also working on something called a Quantum Dot White LED (QD-WLED). An LED outputs light in very narrow frequency curve, according to the size of its "band gap", and different band gap sizes have hitherto been discovered only with the development/use of different materials. Quantum dots, on the other hand, allow light emission of arbitrary colors, with the same given material, depending on the sizes of the dots (using many different dot sizes on one QD-LED). This could lead to white LEDs that are extremely close to a natural light curve. Guess I'll just have to wait and "see."

No comments: