Free Surface Velocity Measurement of an Impacted Projectile by Optical Doppler Shift

An optical technique for measuring the velocity of an accelerating, specularly reflecting surface from a position on the axis of motion has been successfully applied to a free surface velocity measurement of an impacted projectile in a pneumatic accelerator. The technique combines a laser light source, a surface tilt-insensitive, lensless optical configuration, and a Fabry-Perot spectrometer to obtain a velocity-time profile of the impacted projectile. The technique involves unique features in addition to surface tilt insensitivity. These include a mode matching scheme which permits the use of high power, multilongitudinal mode lasers where formerly a single frequency laser was required. Experimental results are presented and compared with independent accelerometer measurements.