Most incandescent lights I've looked at the RAW data from, have almost equal red and green levels for the white square in the GM color checker. Blue is about 1.5 to 2 stops weaker.
...following values for RGB: 200, 243, 86.
Thanks for correction and clarification. But I was dealing more with the general issue, than with exact weighting of each cahnnel. But as orange is a mixture of red and green, it makes perfect sense, that the green channel is more up to the red, rather than to the blue.
In your above example you've described a DR benefit resulting from use of a blue filter to suppress the warm (yellowish) glow of tungsten lighting. Couple that with a magenta filter to correct the imbalance of the camera's RGB channels and you have, in effect, an even stronger blue filter which I guess you might call purple, if there's still some red coming through.
STOP! I think, here is a misunderstanding! I did not mean to stack the magenta filter to the blue filter. There is ONE light temperatur (maybe with cast in the purple-green-axis? But let's ignore that for simplification), where a specific sensor captures exactly equal values in all three channels in the Raw file. For example in
the lower picture of this posting from bjanes, you'd have a value of (180,180,180)*. Now, according to the color of your light source, you have to filter it with a special CC filter. A blue one for Tungsten light and a magenta one for day light, simplified. I did not mean to use the magenta filter in any case as kinda standard filter.
I'm having trouble with the logic of the above statement. Unless you are pointing the camera directly at a light source, the camera is recording light that has been reflected off your subject and which has therefore been 'modified' in some way by the 'light reflecting' characteristics of the subject
Correctly, and that's exactly what you want to eliminate, if you measure the light using a grey card.
Instead of fitting a blue filter to reduce the amount of red and green entering the camera, you could paint the subject an appropriate color which could have the same effect (if it was done properly).
No, that wouldn't change the
light, only the
object color. What you want to do, is to
calibrate the sensor to the light color, so that it renders any object perfectly w/o regarding its color. That's what a part of CM is about: To neutralize the influence of different light colors to get to the 'real' colors. Again, if that is necessary or not, depends on personal or professional needs.
It therefore doesn't seem to be true to say This balancing has nothing to do with colors in the scene, like green foliage or red skyes.
An imagier works perfectly, if a grey card would give equal amounts of load in each colored pixel, right? So, a grey is a grey, and a red is a red. But that's not the case, as you can see in bjanes pictures: For the sensor, a grey is a greenish color with unequal amounts of 'light' in each channel. And so is a red not a red, but biased in the same way. So, you now want to balance the light, in order to get equal amount of load in each photo site. That's, where a CC filter comes in: If it's the right color, it just blocks the right amount of light of the accordingly color, so that each photo site get's the same amount of light. If this is done, a grey is a grey and a red is a red again. It does
not depend on obejct color, but only on light color. This is the balancing, I am talking about.
Consider the following image, taken with a 5D without use of filters of any sort. I'm struggling to understand how a magenta filter would have helped this scene.
A nice picture! But strange blue? Anyway, you didn't get the point: A magenta filter would be useful in daylight condition at about 5500 K or so. This scene here has a much lower light temperature, more like 3500 K or so (just guessing). So you'd need another filter (more like the 80B), if you ever
wanted to correct the light here, what's clearly not the matter, since the scene 'lives' from the wonderful warm light.
As it is, the scene is way beyond the dynamic range capabilities of the camera so any filter that could have increased the DR of the camera would have been appreciated.
You can see the problem with this image in your ACR conversion: Massive and ugly noise banding. This is due to the fact, that the blue channel is poorly exposed, and so has to be amplified strongly, which brings up the noise. To get the perfect result, you had to do the following: Measure the light temperature** and placing the accordingly filter in front of the lens. Of course, the exposure time might then come to an unacceptable range - bad luck! But if it worked, you had a 'perfectly' balanced pictured
without the beautiful warm cast, more like a boring day light picture. But that picture would suffer
much less noise problems. What you had to do now, is to change the color temperature in ACR, the get the beautiful warm color cast again. And that's, were the ultimate trick is: Now, you have to
reduce the blue channel. This is not critical regarding noise. Before, you had to
bring up the blue channel, and this is very critical with noise!
So I agree perfectly with you, that it's very impractical to meter the light temperature each time and choose the right filter out of hundreds. But I think something like "use a 80C filter for sunsets" could bring the sensor data in the right place, and would be very practicable. It's up to you to verify, if this could be useful for
your work.
Best Regards,
Dennis.
* Wouldn't it be interesting, to figure out this light temperature? Has it anybody done so far?
** It actually is very simple to measure the light temperature with a digital camera: You do it every time you use a manual WB. But unfortunately, the camera only uses the computed settings for internal Raw conversion, rather than showing the values on the display. Or is there meanwhile a camera, which does it?
