How to color calibrate to a set reference?

Question

My post can be considered a follow-up question to this post.

I have two sets of cameras (not DSLRs, rather industrial CMOS cameras), one a reference set and one a new set, and I want to calibrate them to produce the same color output. I have a light box and I have placed a Colorchecker card inside it, which I capture using my cameras. And I find that the colors produced are sometimes very different.

The lighting that the photos are taken in is the same - They are in a color calibrated lightbox.

The lighting is consistent - I do not see many over- or underexposed areas. Also, the photos were taken one after the other and the subject was not moved at all, whatever is overexposed, is always overexposed.

Exposure/ISO/Aperture settings - The exposure is same. These cameras do not allow me to change the ISO. And I manually fixed the aperture the same in both the lenses. (The lens have a manual iris). Maybe here it is worthwhile to note that the sensors have different resolutions. The new set has 4x pixel area compared to the reference camera sensor.

The white balance - This is where it gets a bit fuzzy. The reference camera has some historical data associated with it, when it was used without ever being color-calibrated. I am supposed to color correct the new camera set so as to get similar colors as the reference. I have access to both cameras, and I can produce RAW images in both, but I have to create a bmp (with colors similar to the historical data) at the end of the workflow.

How can I set the white balance the same in both the cameras?

Thanks.

Answer 1

Try setting both cameras to a reference rather than to each other.

If I had your problem, equipment and set-up, I would replace the Color Check-R™ with a 90% neutral grey card which can be found on the back of the Kodak 18% grey card. That's given that all other settings are as close to equivalent as possible.

(I would think that any "calibration" or other change of settings would over-write existing data in memory. Your CMOS model may have "sticky" settings as a feature necessary to keep settings for inconsistent power operation. In which case, there would be a reset to overcome that.)

Before I involved any software that could complicate evaluating the camera results…

• I would set the white-point of the camera with the lens set at infinity and the 90% card filling the frame.

• I would set the exposure of the test shot with the camera focussed on the 18% grey card.

• I would place the Color Check-R™ back into the shot, fill the frame, compose, and expose.

Repeat the above with the second camera. Rinse.

The result should be close-enough to compare very favourably. It certainly should be within the capability of any software to tweak