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Processing RAW images for accurate color measurements in scientific applications

efisher16

US
6 posts

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efisher16 Registered since 05th Sep 2010
Wed 10-Oct-12 11:56 AM

Hi all,

I am new to Nikonians, but happy I joined. I have a question for everyone about how to correct the color cast and curves within a RAW photo so that colors in RGB color space can be measured accurately. Now, having said that I need to clarify what I mean by 'accurate' as I have already read the post on 'accurate' vs. 'perfect' color and I don't want to wade into that debate!

I am doing some scientific research where color measurement from photos is very important (geological strata, archaeological artifact colors, etc). The photos are not for print, thus I am not using CMYK color space. I also don’t need the measurements to be insanely accurate, like to 1 or 2 RGB points, but I would like to get it as close as possible, to the point where there are negligible visible differences between the image and real-life. It would be nice to even be able to quantify the range of variation between the colors in an image compared to real-life standards so that I can discuss how well the image colors match.

Currently, I am taking my photos under consistent lighting conditions. My predominant light source is a Nikon D800 strobe. Due to field conditions I can never restrict all other light sources, so there is always some component of background sun light, but I try to minimize it. I am using a D300s with a Nikon Nikkor DX 18-200 mm f3.5-5.6GII lens. The white balance is set to flash so that it remains a constant 5500 degrees. I place white, black, and 18% grey cards from Digital Image Flow in the scene, which I am able to use later in Photoshop. The white and black cards help me to adjust exposure while the grey card adjusts for the color cast. After I download my photos I use a personalized lens calibration file specific to my lens and camera to minimize geometric distortion, vignetting, and chromatic aberration. My monitors are calibrated using a Spyder Pro.

Lately, I have been taking photographs of a Munsell color chart, correcting my photos, and then comparing the RGB values of the actual Munsell colors against my image results. The DGK grey card seems to very accurately remove color cast and my RGB values only vary by 1 or 2 points. The problem is that the entire curve is typically skewed positive or negative, which means that the colors are accurate relative to each other in the photos but not accurate to absolute colors. I have been toying with using the brightness to shift the curves up or down so that my measured RGB value of the grey card matches its real-life values. This method produces decent results and my color comparisons with the Munsell colors are all within ~20-30 RGB points (mean = 17, sd = 12.4), which is a barely perceptible difference in hue to the human eye on a neutral background. However, by adjusting the brightness of the image it displaces the whites and blacks.

I keep thinking though that there must be some better method to accurately correct the colors in photos? Is there a way to correct each band (R, G, and B) individually? Does anyone have any experience with this or know of any papers, etc where some methods are described? I would really appreciate any feedback.

Best,

Erich

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