Color Ratios LRGB in other Photoshop versions

Because not all versions of Photoshop compute the Grayscale image the same way, here are the instructions that should work with versions of PhotoShop other than 6.0.  

 

Pay particular attention to steps 2 and 3. They are critical to getting this method to work properly:

 

Step 1. Duplicate the original RGB image to a new document RGB copy

Step 2. Convert RGB copy image to grayscale as follows:

        2.1 From the Channels window, use the right arrow for additional menu commands and select Split Channels

        2.2 Activate the R channel image produced in previous step

        2.3 Select Image/Apply Image menu item

        2.4 Change Source to point to the G channel image from step 2.1

        2.5 Set Blending mode to Screen at 100%, and click OK

        2.6 Select Image/Apply Image menu item

        2.7 Change Source to point to the B channel image from step 2.1

        2.8 Set Blending mode to Screen at 100%, and click OK

 

Step 3. Invert grayscale  image in step 2.8 by selecting the Image/Adjust/Invert command

Step 4. Copy the grayscale  image and paste it as a Layer 1  into the original RGB image

Step 5. Set the combine method for Layer 1 to Color Dodge: 

Step 6. Paste the new Luminance image as Layer 2 into the original RGB image

Step 7. Set the combine method for Layer 2 to Multiply:

Step 8. You are done! You should have three layers in your final image: 

  1. Layer 0 = Background is your original RGB image

  2. Layer 1 = Grayscale, Inverted copy of your original RGB image

  3. Layer 2 = Your Luminance frame taken, presumably with a clear filter

         

 

Copyright 2001,02 Paul Kanevsky