We present an approach to real time direct aerial image monitoring which utilizes the information contained in an alignment signal generated by scanning the image of a mask grating over a corresponding wafer grating and detecting the backscatter electron signal. The basic principles of this measurement technique are described. The effect of noise and other common errors, such as magnification and rotation on the signal quality, are derived and used to set requirements on signal contrast and noise level for obtaining blur values accurate to approximately I nm. The application of this approach to measuring space charge blur is described and preliminary data illustrating the concept is presented. The potential for this technique to Form the basis of an automated self-calibration system on SCALPEL tools is clear.