Flat fields to correct for uneven illumination -- has anyone tried this?

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Mohib
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Flat fields to correct for uneven illumination -- has anyone tried this?

Post by Mohib »

There is a well known process -- flat fielding -- that is used to correct for uneven illumination in images in scientific and astronomical images. See here:

http://imagej.net/Image_Intensity_Proce ... correction

I'm wondering if anyone has tried this method to eliminate or reduce illumination gradients that many lighting configurations? I'm sure it is a very compute intensive process but is something that could be useful when it's important to get evenly lit images and lighting is not optimal.
mera461
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Re: Flat fields to correct for uneven illumination -- has anyone tried this?

Post by mera461 »

You could try CLAHE (Contrast Limited Adaptive Histogram Equalization). With OpenCV it is 6 lines of python: https://opencv-python-tutroals.readthed ... ation.html
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daniel_reetz
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Re: Flat fields to correct for uneven illumination -- has anyone tried this?

Post by daniel_reetz »

Yeah, in the very first scanner, Aaron Clarke and I would take a picture of a blank piece of paper, and then subtract the inverse of that from the page image. It works, as long as your cameras have fixed settings and the camera-platen relationship doesn't change. If it changes at all, you need a new reference image. If you want to try this, it's easy to do in Photoshop with just two layers.

If you're only interested in correcting for vignetting or low-frequency falloff, you can also blur an image of a blank piece of paper and subtract that.

Here are some other weird ideas around lighting: http://www.danreetz.com/blog/2011/01/05 ... -scanning/
Beni
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Re: Flat fields to correct for uneven illumination -- has anyone tried this?

Post by Beni »

We did this for years in Capture One software. It's called LCC. Works really well.
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