Calibration between two images.

FM
Posted By
Fresh_MD
May 22, 2007
Views
364
Replies
4
Status
Closed
Hello all,

I’m trying to get two gray scale images to match gray scales exactly. Each image is a scan of an X-ray and on the side of each X-ray is a set of 10, gradually increasing in darkness, squares that I had hoped to use as my calibration. I am trying to use Levels -> Set White Point/Set Black Point to calibrate them but one of my scans always looks darker than the other. Am I doing something wrong or is it a problem with my X-rays?

Thanks in advance.

-FMD
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Andrei_Doubrovski
May 22, 2007
Hi FMD,
Perhaps, it’s just viewing problem. Even on a good monitor, brightness may slightly vary. Have you tried to measure the colors exactly?
If not, activate the Color Sampler tool (under the Eyedropper button), set samplers in the supposedly matching spots of your images, and compare "#1K" values in the Info palette.

SimplePhotoshop.com <http://simplephotoshop.com/>
FM
Fresh_MD
May 22, 2007
Thanks Andrei,

When I measure with the Color Sampler tool, two spots that should have an equal % black (on my calibration strip) are not equal. I threw a Levels Adjustment Layer on to equalize the 40% square but that doesn’t adjust the other levels to match. Adding more adjustment layers just keeps throwing other numbers off.

The more I play with it, the more I think that my calibration scales on the X-ray plates are off somehow. Maybe there is something else I’m missing?

-FMD
LH
Lawrence_Hudetz
May 22, 2007
Can you scan both together? This eliminates one possibility of error. Get both strips as close together as possible.

I rather doubt that both strips are the same. X-ray development isn’t as well controlled as regular film. In fact, i would start with a couple of pieces of film shot together and developed together if at all possible. Then trim off the step tablets, scan them together and compare.
FM
Fresh_MD
May 25, 2007
I found my problem.

I didn’t realize that distance played a part in how X-rays turned out. My calibration strip was always at the same distance but the item of interest always varied. A quick experiment showed me that, for the strip to work, it needs to be in the same plane as the item. (Or use points on item itself to calibrate between to X-rays.)

There were are couple of other variables that may be affecting the scans but they will be removed so that everything will be even.

Thanks for your help Andrei and Lawrence.

-FMD

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