Effect of microdensitometer scan slit misalignment in MTF determinations

To determine the modulation transfer function (MTF) of a radiographic film or film-screen system, it is common to expose the system to a line source and scan the result with a microdensitometer. The question arises as to how the MTF determination is affected by such factors as microdensitometer slit width and possible misalignment of the slit with the line pattern being scanned. In any experiment, some degree of misalignment can be expected. This paper analyzes the resulting problem and derives an expression for the MTF of slit misalignment and slit width: MTF = [sin[2pif(L/2)]/2pif(L/2)] X (sin[2pif[H sin(theta/2)[[/2pif[Hsin(theta/2)[[), where L is the slit width, H the slit height, theta the angle of misalignment, and f the spatial frequency variable. The left-hand factor of this product is the MTF of a finite slit width, while the right-hand factor is the MTF of misalignment. It is shown that, for radiographic film-screen combinations, small-angle misalignment up to 0.25 degrees is not serious with 1.5-cm-long slits, while such a misalignment may become serious in highbandwidth systems such as nonscreen films.