Abstract: An apparatus and method for determining the response and performance of a torsional vibration damper. Normally aligned optical patterns are placed on an axial face of the hub and on an axial face of the inertia member. Light is projected on and reflected from both the pattern on the hub and the pattern on the inertia member. Displacement of the hub and inertia member and the phase relationship between the displacements is sensed during rotation by reflected light passed to two photocells. The performance of the damper, including its dynamic response, is computed by comparing the outputs of the two photocells and may be read by meters.