A technique to determine flame surface density in a spark-ignition engine from planar laser-induced fluorescence (PLIF) images of the flame front is presented. To resolve the flame surface density in a three-dimensional fully turbulent flow field from two-dimensional images, the direction cosine (angle of intersection between the PLIF laser sheet and the normal to the flame front) must be known. To determine the direction cosine, images from orthogonal planes in the engine have been used to resolve the mean orientation angle along the line of intersection of the planes. The effect of engine speed and fuel-air equivalence ratio on the flame surface density are investigated. The flame surface density (∑) profiles and orientation angle are presented as a function of the progress variable. The direction cosines are almost identical in both orthogonal planes, indicating the isotropic nature of the flame front. As the engine speed is reduced, the direction cosine increases slightly. The Bray-Moss-Libby model coefficients obtained at maximum flame surface density in this work agree with previously reported experimental and DNS (direct numerical simulation) results and display an inverse correlation with the integral length scale when normalized by the laminar flame thickness.