Process > Background > Correct

Note: The Subtract command should be used instead of the Correct command for all images except those of transmitted light experiments which will be used to measure optical density.  For optical density applications, the Correct command should be used instead.  The Correct command accounts for the fact that optical density is not a linear function of the gray scale.

The Correct tool functions identically to the Subtract command except for two things:

• The Correct tool's options for the 'flatten' method of background adjustment (using, for example, the Dark Background or Bright Background option) use a different formula than the Subtract tool.  The Correct tool uses a formula based on division instead of subtraction.   
• Unlike the Subtract command, the Correct command allows you to specify the Black Level to be used as part of the background determination algorithm.  Black level is defined as the intensity value of the parts of the image that do not reflect or transmit any light.  With a camera, it can be obtained by grabbing an image with the lens cap on. The default value is 0.0.  To avoid a negative value, the value of Black Level must be less than the average of the background image.