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[icon] Tell me if I said something stupid - Patti
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Subject:Tell me if I said something stupid
Time:09:43 pm
This is primarily directed at my friends who actually showed up for freshman physics when they were in college.

In a photography discussion forum, someone was asking where to get a good bulb for a lightbox, so that colors were rendered correctly. A respondent said not to worry about it-- to just use any old light, shoot raw, and then correct the colors later. This seemed like a bad idea to me, since you ran the risk of losing some of the colors. As I was typing, though, I realized I was only about 95% certain of what I was saying so I sprinkled caveats liberally around my message. Tell me if I'm right or wrong.

You might be able to find a daylight bulb at Home Depot. Try looking for things that are intended for plants.

It's entirely possible that I'm wrong about what I'm about to say. If so, I hope that the people who correct me do so diplomatically. :-)

It seems to me that even if you can correct the white balance later using RAW processing, it makes sense to use the best spectrum light you possibly can, so that the camera can see all the colors. Imagine for example that you had a bulb that produced a lot of red and yellow light, but almost no blue. If you use that to take a picture of something that had the full range of colors, it seems like the blues and greens would come out too dark since little to no light would reflect off of them.

I believe this is the reason that red cars look black under sodium lights in parking lots.

In reality, you're probably not going to find a bulb that's completely deficient in one area, but rather one that has strengths and weaknesses. Still, it seems like it makes sense to start with the best possible light source, rather than one that is known to be deficient in a particular area. I'd rather use RAW processing to solve problems that I couldn't otherwise fix, rather than relying on it as a primary tool.

I was googling around as I was writing this, and found this potentially useful page: www.assumption.edu/users/bniece/Spectra/Complete.html It shows the spectra of various light sources. It's interesting to see how much more red there is than blue in an incandescent bulb.

How about it, lazyweb?
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whitebird
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Time:2006-11-02 06:41 am (UTC)
Yup, it's much better to use a full spectrum light source if you can.

Things you can do to mitigate a bad light source is to use a standard color checking chart, and a standard white balance chart.

Examples are:

Color Checker

Three Step Grayscale Checker

Colorchecker White Balance

Digital Color Checker Semi Gloss

And if you want to go whole-hog in not buying a proper light source, well, you can get the GretagMacBeth EyeOne spectrophotometer device and the digital camera profiling module.

But, I bet you can get a nice full spectrum light source for a lot cheaper than most of these other ideas.
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dmorr
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Time:2006-11-02 03:45 pm (UTC)
I was all set to write a post about this, but Patri got it right (as usual). His explanation differs a little from mine, which would have involved compressed ranges not expanding into all the possible values, but the noise idea makes sense to me too.

You can think of using a colored light as lossy compression of the less well-represented colors. Sure, you can uncompress them, as long as you don't mind the lossiness.
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abostick59
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Time:2006-11-02 08:33 pm (UTC)
Red cars look black under sodium lights because sodium light is pretty close to monochromatic -- it is spectral emission from high-temperature sodium, and most of the emission is from one spectral line.

Your original querent will have only a small problem using an incandescent light bulb for her light box -- tungsten-filament light bulbs emit a black-body spectrum with a color temperature of about 2800K, which looks yellowish to the human eye. Bright daylight has a color temperature of 5500K, and that's what the human eye sees as white.

Correcting RAW files for color temperature is the moral equivalent of having a graphic equalizer as part of your stereo sound system. By beefing up the blue end of the spectrum by a few dB, you raise the noise floor by the same amount, but this amount isn't very much -- no more than a bit or two of dynamic range.

If the original querent doesn't want to have to correct for color temperature, she should get a photoflood bulb for her lightbox -- something with a color temperature of about 3400K oughtt to be good enough.
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stevecohen
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Time:2006-11-03 07:15 pm (UTC)
To answer your subject line: almost certainly at some point in your life.

On to the problem of photographic lighting. While Mr. Friedman is correct that you can run into SNR issues with a poor lighting choice, for all practical purposes just about anything incandescent will work just fine.

You want to avoid fluorescent lights as they tend to have phosphors that are bright in narrow spectral lines - much like white LEDs. If these lines don't match up well with the sensitivity of your CCD (they will, but there is *some* risk here) you could end up with the SNR problem to which Mr. Friedman alluded.

Mr. Bostick's reply was similarly accurate from a slightly different perspective bringing in the issue of "color temperature" and black body radiators.

On the whole, just about any incandescent bulb will do a *fine* job, requiring only post-processing to get the colors correct. On the other hand, if you actually want to be able to see what you are shooting in something like the correct colors, high-quality photo-floods are required.

Given the relatively low cost of high-quality photo-flood bulbs, I am at a loss to explain the desire to use cheep bulbs.
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[icon] Tell me if I said something stupid - Patti
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