A method of electronically determining the location of an edge of a copy substrate (710) to be printed on places the copy substrate (710) between a segmented linear light source (700) and a linear sensor array (712) and a light segment of the linear light source (700) is illuminated. An edge location (X co ) of a shadow created by the copy substrate (710) intersecting the light path between the illuminated light segment and the linear sensor array (712) is measured, and the location of the edge of the copy substrate (710) is calculated based on the measured shadow location (X co ). A second light segment of the linear light source on an opposite side of an expected copy substrate edge position of the light source can also be illuminated, and a second edge location of a shadow created by the copy substrate (710) intersecting the light path be measured wherein this second measurement can be used with the first to calculate the location of the edge of the copy substrate (710). Alternatively, the calculated location can be used to determine a light segment of the linear light source which corresponds to the calculated location. This determined light source is illuminated and the shadow measured. The new measurement is used to calculate the edge of the copy substrate (710).