It is useful to be able to apply some post-processing brightness and contrast adjustments to the images. Photoshop can be used to interactively adjust the brightness and contrast but I like to use the free Imagemagick open source software to process images in batch. Imagemagick has a convert utility which can perform brightness and contrast adjustments on images from the command line.
To make an intensity image brighter, type in the following at the command line prompt with the brightness-contrast option.
c:\> convert -brightness-contrast 50x20 input.tif out_lighter.tif
where the 50 tells the utility to increase the brightness by +50 and 20 is to increase the contrast by 20.
To make an intensity image darker, type in the following at the command line prompt with negative brightness and/or contrast values.
c:\> convert -brightness-contrast -30x10 input.tif out_darker.tif
where -30 is to darken by 30 and +10 is to increase the contrast by 10.
More information about the convert utility is available on this link.