Edge Enhancement by Defocusing of Confocal Images

In coherent imaging, the position of an opaque edge may be determined by examining where the intensity has dropped to one-quarter of that far from the edge in the bright region, the precision with which this point may be found depending on the slope of the profile. This is true also for confocal imaging, for which it is found that the slope of the normalized edge image actually increases with defocus. Experimental confirmation of the effect is given. The effect has possible application in the measurement of linewidths in semiconductor microlithography.