The modulation transfer function of an image intensifier is a measure of the output amplitude of dark and light lines on the display for a given level of input from lines presented to the photocathode at different resolutions. It is usually given as a percentage at a given frequency (spacing) of light and dark lines. For example, if you look at white and black lines with a MTF of 99% @ 2 lp/mm then the output of the dark and light lines is going to be 99% as dark or light as looking at a black image or a white image.