Eliminating unwanted color casts (Photoshop)
If your scanned image contains an unwanted color cast, you can perform a simple test to determine whether the cast was introduced by your scanner. If it was, you can use the same test file to create a color-cast correction for all images scanned with the scanner.
To identify and correct a color cast introduced by a scanner: - Make sure that your monitor has been calibrated. (See Creating an ICC monitor profile.)
- Open a new Photoshop file, and use the linear gradient tool to create a blend from pure black to pure white.
- Choose Image > Adjust > Posterize, and posterize the blend using 11 levels.
- Print the 11-step gray wedge on a black-and-white printer, and then scan it into Photoshop.
Note: You can also perform this test using an 18-percent neutral gray card or an 11-step gray wedge from a photography store. - Open the Info palette, and read the RGB values on-screen for each of the gray levels. Uneven R, G, and B values indicate a color cast.
- Use Levels or Curves to correct the color cast, and then save the dialog box settings. (See 4. Adjust the color balance..)
- Open the scanned image you want to correct, reopen the dialog box you used to correct the cast in step 6, and load the saved settings.
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