The Color of White Light

The biggest lie in color photography is that you can accurately represent the colors of objects by simply recording the amount of red, green and blue in them. This technique - the only one in current mainstream use - gives good results only when the spectral sensitivity curves of the camera precisely match those of the human eye, and when the spectrum of the light used to make the photo is perfectly smooth, and no different kind of light will ever be used! It's clear that the first of these requirements is hard enough to meet, and that the second one is, simply and plainly, never true. And that's why photographers are always battling to get the right colors - and never do get them! Not only in photography is this matter an important one. In daily life it is, too. Lots of electronic technicians hate that stupid problem of not being able to correctly read resistors. It happens that old-style resistors, and some other parts too, are labelled with color bands or dots, instead of numbers. Under some lighting conditions it can be hard to tell a red from an orange, or a green from a blue. This leads to the wrong resistors being installed in equipment, and thus more troubleshooting work. Housewifes know the same kind of trouble, for example when trying to color-match a button to a shirt. Indoors under the electrical light they find the exact right button, that matches the others, sew it on, and when the dear hubby goes outdoors next day, !BANG!, that button sticks out like a sore thumb! Well, I have to admit that many housewifes these days don't know how to sew a button to a shirt, but they tend to have the same kind of trouble when getting their make-up just right, only that outdoors it doesn't look right any longer! And that's a big problem... To get a better grip on the problems of light color, I built myself a spectrograph a few days ago. It has been a lot of fun so far, so I'm making this colorful web page, both to bring my results into an orderly shape, and to let other people learn from them. The spectrograph is a simple attachment for my DSLR camera. Using my lathe, I made a two-part piece of plastic tubing, that assembles in an angle of 146.6 degrees. At the junction of the two parts, a diffraction grating is installed. It's an inexpensive foil-type grating that has 1000 lines per mm, which I bought on eBay. Each end of the angled tube screws into the filter thread of a lens. I used 50mm lenses on both sides, but other arrangements are workable too. The camera's lens stays focused at infinity, while the additional lens, uses as a collimator, has a narrow slit installed in the center of its focal plane. I made that slit by hot-gluing two pieces of hobby knife blade over a central hole in the cap, on its inside. The blue pipe is simply PVC water pipe, machined on the lathe to press-fit the lense's bayonet, and to have 42mm length. This allows using the lense's focusing ring to bring the slit into the exact position, and thus focus the entire spectrograph. Looks cool, eh? There are several web sites describing the construction of such spectrometers in greater detail. Many of them don't use the second lens, and instead use simply a long tube, placing the slit at a long distance from the diffraction grating, and focusing the camera's lens at that distance. The work being done, let's go and play.Let's start by looking at a full, pretty smooth spectrum of light. It's 1500 pixels wide, so it would be best to set your browser window wide enough. The left border must be roughly at 700nm wavelength, maybe a little lower, while the right one is slightly into the UV range, a little bit shorter than 400nm. I got this spectrum from a blue-tinted "daylight" incandescent bulb. The stage being set, let's look at some single-color LEDs. Here is a red LED dating from roughly 1988. It's pretty far down in the deep red range. It looks very dim to the eye, just enough to use it as an indicator light on a front panel. The following one is a modern, high efficiency red LED. It works much higher in the spectrum. Next in the spectrum comes an old (1988) yellow LED. The spectral range that looks yellow to the eye is pretty narrow, and those old LEDs had a fairly broad bandwidth. It spans from the red over the orange and yellow, well into the green. It looks a dirty yellow to the eye. This instead is a modern "orange" LED. It has a much narrower bandwidth, allowing a more precise color definition. It looks a beautiful golden color to the eye. Then comes an old green LED. As you can see, it's not cleanly green, falling into the lower half of the green range, and a lot into the yellow and orange. It looks green enough to the eye, partly thanks to a green tinted housing! Without that tint, I guess this LED would look rather yellow. Instead a modern green LED has a far narrower bandwidth, and is better settled in the green range. I'm not sure where the Moir interference in this spectrum came from. I don't think it's an actual ripple in the spectral response. What you can see here is a historical device, straight out of my museum: The very first blue LED I bought! It was very expensive, and gives a very dim glow, but with some goodwill it looks blue indeed! I bought it just to show off with it, in a time when most people still thought that blue LEDs were impossible to make! I understand it's a silicon carbide LED. As you can see, like all old LEDs it's pretty broadbanded, and is centered in the cyan range rather than the blue. This instead is a modern, true blue LED. And what follows is a LED that was sold to me as "ultraviolet". I can't tell how much actual UV output it has, because my camera and my lenses will cut off any higher UV. But it looks promising that I had to lengthen the crop of my spectrum image, to fit the far violet part of the spectrum, which I had cut off before because I though my camera wouldn't see anything there! Note that it also has some faint green and red output. As you can see, there are LEDs for pretty much any part of the visible spectrum. In my collection I don't have any clean, nice, narrowband cyan LED, but it surely exists too! Let's go to actual lamps now. The above LEDs are intended mostly as indicators, having strong colors. Lamps instead are normally designed to produce the whitest possible light, or let me say, a light as close as possible to daylight.This is because the standard for color vision and reproduction is how it looks in nature, under the open sky. This can be either a sunny day, with the proper mix of yellowish sun and blue sky shining down, or a smooth layer of white clouds, which mixes together the light. Of course, clouds do absorb some wavelengths more than others, and so the color of cloud light is _not_ the same as well mixed light of a sunny day. Matters ain't ever simple! The smoothest spectrum comes from blackbody radiators. Plain incandescent glow bulbs are such blackbody radiators - only they aren't hot enough! To produce the holy grail in perfect lighting, we need a blackbody heated to about 5000 kelvin, which is pretty close to 4700 degrees Celsius. The problem is: What material can be used, that survives such temperature while remaining solid? So we use tungsten, the metal with the highest melting point of all, and heat it up as much as we possibly can, without it vaporizing too fast. That's the humble glow bulb. What we get is a smooth spectrum, but poorly balanced, with intense red and weak blue, and almost no violet: The above is a 25W bulb, while the one below is a 100W one. The spectra of these two are almost identical. Both look very yellow to the eye. Engineers searched desperately for ways to increase the filament temperature, to improve the blue end output. This led to the development of halogen glow bulbs, which use a high pressure halogen gas to recover tungsten that went off the filament, and re-deposit it where it belongs. This allows running the filament of a halogen bulb slightly hotter than that of a common bulb, leading to slightly stronger output in the blue, and thus an overall better balance: And there was also a brute-force approach to getting whiter light from incandescent bulbs, which was simply to add a blue filter, by tinting the bulb blue. This works not by increasing blue, but by _decreasing_red! So it does achieve the goal of producing a well balanced light spectrum, but the efficiency, already very low for any glow bulb, simply gets ridiculous! This spectrum, the same I displayed for reference at the beginning,  is of a Philips 60W blue-tinted "daylight" glow bulb: As you can see, it has a little bit more blue light than the halogen bulb, making it the smoothest light source in my collection, with the best color rendering and visual impression - but also the least energy-efficient one! For comparison, here is the spectrum of true daylight - the sun, plus blue sky, at mid afternoon, so it should have an average daylight balance. It can be seen that the "daylight" lightbulb, even with its blue glass, is significantly redder than real daylight. The dark stripes in the sun spectrum are the Fraunhofer lines. Now that we have seen the spectrally smoothest light sources, let's see the most "adventurous" ones: Gas discharge lamps! They are essentially pure spectral line radiators. This is the spectrum of neon, obtained from one of those little neon indicator bulbs, that look - can you guess it? - red/orange: And here comes the spectrum of of a discharge through mercury vapor, which is used in many lamps. It looks green/blueish: The spectrum above was obtained from a metal halide lamp, right after switching it on, while it was still cold. The other metals in it don't get excited by the electrical arc, so they don't glow yet. But a few minutes later the lamp has warmed up, and the other metals in it get thermally excited, adding a lot of spectral lines of their own. That's when the light of a metal halide lamp turns white to the eye, and very intense: