Need an app for that: color temperature

[kenw]

anything out there that can tell me the color temp of what available light I attempting to use? (ie, take a pic of something "white", do the numbers, tell me it's 5400k)

[Daniel W.]

Hmm, this would be an interesting task. The camera on say... a phone... has the ability to auto white balance based on what programming would make the camera "think" is neutral (usually the largest object with a high luminance value)...

Color meters operate in the sense that it takes a calibrated number (a control) and then compares it to the sample (i.e. light received through a diffuse layer). I imagine it'd require something like a ba-lens or expodisc, an object that reduces the light to a more even transmittance. Shooting a "white" object isn't always right. Clothing, paper, wall paint... they all may have optical brighteners that fluoresce and may throw off the reading. And "accurate" white isn't always "pleasing" white.

Best bet? If you're going through the trouble of metering color, you might as well shoot a Color Checker and balance in post (if you shoot raw). The only time I've used a color meter was to check the balance of flash tubes (new install vs. burnt in ones) for critical color accuracy using slide film and as a requirement in shooting a microscopy setup.

[groovyone]

There are color meters. They are not cheap though. You can use something like an expodisc to custom set your in cam WB based on the lighting. I use one and it is effective.

A phone app would be interesting.

[SCone]

Photo Tools by HCPL for android has lots of tools including a color temp chart

[texxter]

A color temperature meter will run you about $1,000 due to the electronics inside it. Not sure you can build an app without the three silicon photodiodes that actually measure the energy for each RGB band. But I have been surprised by what apps can do before!

[Dobick]

My sekonic meter came with a replacement dome. I would think that you could place that over the camera in the iphone (or even better than that would be a piece of frost filter from the Rosco sample pack) and then take a photo.

I can't imagine that the info isnt stored somewhere inside the raw image data that the camera uses to Auto whitebalance?

[texxter]

Originally Posted by Dobick

My sekonic meter came with a replacement dome. I would think that you could place that over the camera in the iphone (or even better than that would be a piece of frost filter from the Rosco sample pack) and then take a photo.

I can't imagine that the info isnt stored somewhere inside the raw image data that the camera uses to Auto whitebalance?

Calculating the Kelvin temperature of light and doing auto white balance are different things. The camera algorithms don't compute Kelvin from the light source, but they run algorithms to try to figure out the "correct" white balance. Different vendors use different algorithms. One of them, named Perfect Reflector Method, locates the reference white point by finding the pixel with the greatest luminance value. Other methods exist with different assumptions.

My point is that just because the camera can do some math to change RGB values around an assumed reference point doesn't mean it can spit out the absolute Kelvin number for an illuminant.

[kenw]

Originally Posted by texxter

A color temperature meter will run you about $1,000 due to the electronics inside it. Not sure you can build an app without the three silicon photodiodes that actually measure the energy for each RGB band. But I have been surprised by what apps can do before!

That's about what I was thinking, but since these app developers constantly surprise me, I thought it might be lurking out there unbeknownst to me....it wouldn't be the 1st time.

I think the simplified app could get away with some assumptions that:

1) the RGB values should be equal (if you have a decent white or grey target they should be as long as you aren't overexposing so much as to blow thing out)
2) then the math could start knowing that and computing the differences.

There would be some similarites to AWB but you're right, it isn't quite the same thing.

Obviously this isn't going to be a real meter killer, but just something handy for those weird gym light (et al) situations.

[fredkiana1972]

I got one for 200 bucks off of craigslist. I love it. It's a Minolta and does the job for me. Try craigslist all the time.

[Folkstedt]

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