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1. Re: How can I use photoshop to compare photos (before and after) treatment of a material that may lighten the color to quantitatively measure color differences?
Mylenium Dec 2, 2014 8:49 AM (in response to geegs60)You need a reference chart and obviously you need a decent camera with a high sensitivity and standardized lighting conditions that allow you to use a high ISO. Then you can use Adjustments and blending modes to extract and visualize the differences...
Mylenium
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2. Re: How can I use photoshop to compare photos (before and after) treatment of a material that may lighten the color to quantitatively measure color differences?
geegs60 Dec 2, 2014 9:46 AM (in response to Mylenium)Mylenium,
Thank you for responding. As you suggest I have a decent camera and will standardize the lighting conditions. Would you please further explain the issue (in steps if possible) of using the reference chart, adjustment and blending modes. I will be staining egg shells as a surrogate for teeth with various materials (tea, coffee, etc) for 24 hours and then exposing the stained shells to various whiteners and taking photos of the treated shells. My hope is that by standardizing the position and lighting of the shell, the main difference will be that of a color change (if any). I would like to quantify this difference, preferably by assigning a number to the colors or color of interest. I was thinking of using the dropper and color picker approach. Possibly desaturating the image to simplify the data.
Is this a valid approach or is there a better way? I am only a early-intermediate level user of photoshop and am seeking more expert opinion on how to accomplish this.
Thank you again,
Geegs60
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3. Re: How can I use photoshop to compare photos (before and after) treatment of a material that may lighten the color to quantitatively measure color differences?
normfb Dec 2, 2014 10:38 AM (in response to geegs60)I suggest you use Lab Color for calibration of luminosity and color shift.
The L channel will provide a rating of 100% for the max whiteness reading.
The a and b channels will provide a variation from colorless at 0 for each channel, or a precise plus or minus number for variation from neutral.
In the a channel:
a plus number shows a shift toward magenta
a minus number shows a shift toward green
In the b channel:
a plus number shows a shift toward yellow
a minus number shows a shift toward blue
and in each case variation from 0 denotes a greater shift
In sum, your reading such as the one above, denotes a luminosity of 78 and a color shift toward blue with a small tinge of magenta
You may chart and compare your 3 readings for each sample in Excel or even graph them.
If you are interested in learning more about Lab Color, I suggest you pick up a copy of Dan Margulis’ book: Photoshop LAB Color and read the first few chapters.
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4. Re: How can I use photoshop to compare photos (before and after) treatment of a material that may lighten the color to quantitatively measure color differences?
geegs60 Dec 2, 2014 11:06 AM (in response to normfb)Thank you normfb, I will do so
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5. Re: How can I use photoshop to compare photos (before and after) treatment of a material that may lighten the color to quantitatively measure color differences?
normfb Dec 2, 2014 12:07 PM (in response to geegs60)Just one additional word of caution, Geegs60. You may find that differences in color or luminosity are so subtle that your camera cannot record them with enough precision even though you are shooting RAW. In that case, depending on the demands for absolute accuracy and/or the depth of your pockets you may have to opt for a Color Spectrodensitometer, the price of which may be in the $4,000 to $6,000 range. On the up side, it would eliminate the need for a camera and computer because you will be able to read actual sample and you will get a LAB readout that extends to decimal places rather than units. On the downside, as I mentioned, it can run into big bucks. It is certainly worth investigating if the Camera-Photoshop approach doesn't meet your needs. Good luck with the project.




