This program is part of Netpbm.
ppmflash reads a PPM image as input. It changes the color of each pixel to bring it a specified amount closer to white. It generates a PPM image of the result.
flashfactor is a real number between 0 and 1, inclusive. ppmflash increases the intensity of each RGB component by the fraction flashfactor of the difference between the current value and full intensity. So if a pixel contains 60% full red, 10% full green, and no blue and you specify 0.5 (half), ppmflash increases the red to 80% (because it was 40% from full intensity, so it adds half of 40% to the original 60%), the green to 55%, and the blue to 50%.
If flashfactor is zero, the output is identical to the input. If flashfactor is one, the output is all white.
pambrighten does a more normal kind of brightening. pamfunc does a very simple brightening. Both pambrighten and pamfunc can reduce brightness as well.
pnmgamma is another way people do a similar brightening, though it isn't really intended for that.
There are no command line options defined specifically for ppmflash, but it recognizes the options common to all programs based on libnetpbm (See Common Options.)