A photoelectronic apparatus has been developed which makes possible the continuous simultaneous measurement of the depth of penetration, the speed, and the deceleration of a nondeforming small‐caliber projectile dring armor penetration. The basic operating principle of the apparatus is to have the flight path of the projectile pass perpendicularly through a thin parallel light beam of uniform intensity which activates a vacuum‐type phototube. The output of the phototube is proportional to position or the depth of penetration under appropriate geometry. A voltage signal derived from the phototube circuit is preamplified and differentiated to give voltage signals proportional to speed and deceleration. Permanent records of the signals are made by photography of cathode‐ray tube traces and analyzed. The apparatus is capable of measuring real transient accelerations or decelerations as great as 108ft/sec2(3×109cm/sec2) with average error of 2 percent. Possible applications of the system to mechanical problems as an accelerometer, speed, and displacement measuring device are indicated.