Making a WB/Tint sensor?

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p.1 #1 · Making a WB/Tint sensor?

Does anyone know the theory behind this?

The design I'm using right now has 4 individual photocells, each behind a different color filter (orange, blue, green, pink).

The photocell has a load resistor that is in the 400K to 2200K range. This generates a voltage across the load of between 0.008V to 0.130V; values above 0.130V are outside of the linear region of the photocell, so they are thrown out; values below 0.008V are under precision of the ADC so they are also thrown out (for now, this number will likely farther decrease).

Periodically, 10 times a second, all four photocells are sampled by the ADC and normalized based on a white calibration. Then a ratio is computed from this, orange/blue to determine white balance and pink/green to determine tint. This ratio is used against another calibration table to find the approximate white balance (in EV) and tint.

1) Is there a difference between using O-B-G-P vs. R-G-B photocells? I figured the tradeoff here was the complexity of deriving white balance and tint from R-G-B data instead of O-B-G-P.

2) In most cases I am using gray card in front of the sensors to catch the light I am trying to sample. Otherwise it seems like it gets thrown off by background colors (even if they aren't reflecting a lot of light). Still experimenting here, is it better to use a semi-translucent white materiel like some of the commercial models do?

3) Converting between an exponential (2^n) scale like EV and the way white balance is normally expressed (in KV)?

Feb 25, 2013 at 06:40 PM

Roland W
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p.1 #2 · Making a WB/Tint sensor?

Are you re-inventing the wheel? There are currently available color temperature meters that can characterize the incident light from light sources, to allow you to adjust or gel the light, or to use the reading as a guide for your digital camera. If you happen to be talking about film, the existing meters should work out well.

And of course, the auto white balance systems in current digital cameras do quite fancy calculations to come up with an estimate of the required color temperature setting for each shot, and pass that estimate along with other information with the RAW, so that you can manage things in post. If a user insists in "bakeing in" the color temperature by shooting in JPEG format, the auto white balance is going to be a good guess for what is correct, or you can override the auto system with a test shot that produces a custom white balance value.

Feb 25, 2013 at 09:15 PM

jcolwell
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p.1 #3 · Making a WB/Tint sensor?

I have a Gossen Sixticolor colour temperature meter. It's almost a museum piece, but it works great! OTOH, I haven't used it for many years.

Feb 25, 2013 at 09:22 PM

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p.1 #4 · Making a WB/Tint sensor?

Roland W wrote:
Are you re-inventing the wheel? There are currently available color temperature meters that can characterize the incident light from light sources, to allow you to adjust or gel the light, or to use the reading as a guide for your digital camera. If you happen to be talking about film, the existing meters

I can find plenty of light meters for very inexpensive prices, but a color temperature meter (especially one that measures tint) can be quite expensive. At least the ones I'm finding can be. Maybe I'm just looking in the wrong places?

I also want something with a lightweight and removable sensor assembly that can be manipulated using a number of servos.

The problem I have isn't with white balance in general, it's with identifying and avoiding mixed scenarios as well as being able to identify good areas quickly in the field.

Feb 25, 2013 at 09:34 PM

jcolwell
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p.1 #5 · Making a WB/Tint sensor?

Have you looked here?

http://www.ebay.com/itm/Gossen-Sixticolor-Color-Meter-Works-Looks-Great-/400413933296?pt=US_Light_Meters&hash=item5d3a87bef0

(item 400413933296)

Feb 25, 2013 at 09:50 PM

Roland W
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p.1 #6 · Making a WB/Tint sensor?

I have a Gossen Sixticolor also, but it is burried somewhere with my "old junk". I would have mentioned it above, but it has been out of production for a long time. My main use for it back in the 1970's was to add color correcting gel filters in the little slot on my Bolex 16mm movie film, especially if the lighting was mixed sources of color temperture.

Feb 25, 2013 at 09:59 PM

Roland W
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p.1 #7 · Making a WB/Tint sensor?

The Sixticolor meter is not real sensitive, so expect problems with it not working in modern light levels. It is also an old design, so its accuracy may not be all that great, expecially if you end up with one that is 30 to 50 years old. I know the one I have was purchased new in the early 1970's.

Feb 25, 2013 at 10:03 PM

jcolwell
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p.1 #8 · Making a WB/Tint sensor?

The Sixticolor worked fine for me, when I was using medium and large format film cameras, whatever they were.

It's definitely a simple and inexpensive first step into color temperature measurement. If it's not adequate, then there are other solutions.

Feb 25, 2013 at 10:10 PM

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p.1 #9 · Making a WB/Tint sensor?

jcolwell wrote:
The Sixticolor worked fine for me, when I was using medium and large format film cameras, whatever they were.

It's definitely a simple and inexpensive first step into color temperature measurement. If it's not adequate, then there are other solutions.
Okay thx, I will see if I can get one of those cheap and work from there.
But if it's old, I don't know how useful it will be because it's hard to get the old parts to reverse-engineer it. If it's an older phototransistor design it might not have the sensitivity for what I'm trying to do.

Feb 25, 2013 at 11:46 PM

mpmendenhall
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p.1 #10 · Making a WB/Tint sensor?

Can you provide a little more info on what application this is for?

A quick-and-easy method, if you can work within USB cable range of a computer, might be to tether a cheap web-cam (covered by a diffusing filter), and write your own software to average the image color and extract white balance parameters. I suspect it would be difficult / expensive to build your own device from components with higher light sensitivity and readout accuracy than what you already get pre-packed in a cheap camera. Of course, this doesn't work so well if you need a self-contained device without a computer; writing a color balance program for an iPod Touch or similar camera-equipped Android device --- surely something like this already exists --- would give you a self-contained version.

Feb 26, 2013 at 03:22 AM

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