A system and method electronically determines an edge of a copy substrate to be printed on. A copy substrate is inserted between a linear light source array and a linear sensor array and a light segment of the linear light source array is illuminated. An edge location of a shadow created by the copy substrate intersecting a light path between the illuminated light segment and the linear sensor array is measured, and a location of the edge of the copy substrate is calculated based on the measured shadow location. A second light segment of the linear light source on an opposite side of an expected copy substrate edge position of the light segment can also be illuminated, and a second edge location of a shadow created by the copy substrate intersecting a light path between the illuminated second light segment and the linear sensor array can be measured wherein this second measurement can be used with the first to calculate a location of the edge of the copy substrate. 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.