The Color of White Light
The Color of White Light
The biggest lie in color photography is that you can<br>accurately<br>represent the colors of objects by simply recording the amount of red,<br>green and blue in them. This technique - the only one in current<br>mainstream use - gives good results only<br>when the spectral sensitivity curves of the camera precisely match<br>those of the human eye, and when the spectrum of the light used to make<br>the photo is perfectly smooth, and no different kind of light will ever<br>be used!
It's clear that the first of these requirements is hard enough to meet,<br>and that the second one is, simply and plainly, never true.
And that's why photographers are always battling to get the right<br>colors - and never do get them!
Not only in photography is this matter an important one. In daily life<br>it is, too. Lots of electronic technicians hate that stupid problem of<br>not being able to correctly read resistors. It happens that old-style<br>resistors, and some other parts too, are labelled with color bands or<br>dots, instead of numbers. Under some lighting conditions it can be hard<br>to tell a red from an orange, or a green from a blue. This leads to the<br>wrong resistors being installed in equipment, and thus more<br>troubleshooting work.
Housewifes know the same kind of trouble, for example when trying to<br>color-match a button to a shirt. Indoors under the electrical light<br>they find the exact right button, that matches the others, sew it on,<br>and when the dear hubby goes outdoors next day, !BANG!, that button<br>sticks out like a sore thumb!
Well, I have to admit that many housewifes these days don't know how to<br>sew a button to a shirt, but they tend to have the same kind of<br>trouble when getting their make-up just right, only that outdoors it<br>doesn't look right any longer! And that's a big<br>problem...
To<br>get a better grip on the problems of light color, I built myself a<br>spectrograph a few days ago. It has been a lot of fun so far, so I'm<br>making this colorful web page, both to bring my results into an orderly<br>shape, and to let other people learn from them.
The spectrograph is a simple attachment for my DSLR camera. Using my<br>lathe, I made a two-part piece of plastic tubing, that assembles in an<br>angle of 146.6 degrees. At the junction of the two parts, a diffraction<br>grating is installed. It's an inexpensive foil-type grating that has<br>1000 lines per mm, which I bought on eBay.
Each end of the angled tube screws into the filter thread of a lens. I<br>used 50mm lenses on both sides, but other arrangements are workable<br>too. The camera's lens stays focused at infinity, while the additional<br>lens, uses as a collimator, has a narrow slit installed in the center<br>of its focal plane. I made that slit by hot-gluing two pieces of hobby<br>knife blade over a central hole in the cap, on its inside. The blue<br>pipe is simply PVC water pipe, machined on the lathe to press-fit the<br>lense's bayonet, and to have 42mm length. This allows using<br>the<br>lense's focusing ring to bring the slit into the exact position, and<br>thus focus the entire spectrograph.
Looks cool, eh?
There are several web sites describing the construction of such<br>spectrometers in greater detail. Many of them don't use the second<br>lens, and instead use simply a long tube, placing the slit at a long<br>distance from the diffraction grating, and focusing the camera's lens<br>at that distance.
The work being done, let's go and play.
Let's start by<br>looking at a full,<br>pretty smooth spectrum of light. It's 1500 pixels wide, so it would be<br>best to set your browser window wide enough. The left border must be<br>roughly at 700nm wavelength, maybe a little lower, while the right one<br>is slightly into the UV range, a little bit shorter than 400nm. I got<br>this spectrum from a blue-tinted "daylight" incandescent bulb.
The stage being set, let's look at some single-color LEDs. Here is a<br>red LED dating from roughly 1988. It's pretty far down in the deep red<br>range. It looks very dim to the eye, just enough to use it as an<br>indicator light on a front panel.
The following one is a modern, high efficiency red LED. It works much<br>higher in the spectrum.
Next in the spectrum comes an old (1988) yellow LED. The spectral range<br>that looks yellow to the eye is pretty narrow, and those old LEDs had a<br>fairly broad bandwidth. It spans from the red over the orange and<br>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<br>bandwidth, allowing a more precise color definition. It looks a<br>beautiful golden color to the eye.
Then comes an old green LED. As you can see, it's not cleanly green,<br>falling into the lower half of the green range, and<br>a lot<br>into the yellow and orange. It looks green enough to the eye, partly<br>thanks to a green tinted housing! Without that tint, I guess this LED<br>would look rather yellow.
Instead a modern green LED has a far narrower bandwidth, and is better<br>settled in the green range. I'm not sure where the Moir�...