Whenever a monochromatic collimated light beam illuminates a planar electrode surface parallel to that surface, a diffraction pattern characteristic of the beam’s coherence is obtained. The electrode edge position is generally determined utilizing a procedure which assumes either a totally coherent or an incoherent source. In this work, the image of the diffraction pattern was magnified and focused on a diode array, and the resulting intensity profile was compared with that obtained from a slightly defocused image. Perturbation theory was applied to predict the position of the electrode’s edge. Once the edge position was known, the source’s coherence was determined from the normalized intensity at the electrode’s edge. This technique can be utilized in situ with "parallel" spectroelectrochemical experiments. It does not depend on a priori knowledge of the light source’s coherence nor does it require an opaque-translucent interface. A simple model and experimental data employing this method are presented. Measurements of the experimental electrode positions were reproducible within the theoretical magnifier/array limit, 1 mu m