The use of an image derotator in hologram interferometry and speckle photography of rotating objects

An image derotator is described that consists of a folded Abbé inverting prism built into the center of a hollow-shafted torque motor. The alignment and operation of the unit is discussed, and resolution of the derotated image in excess of 15 cyc/mm is shown. Its application to vibration analysis of rotating objects is demonstrated via double-exposure holograms recorded with a Q-switched, double-pulse, ruby laser. The necessity for using such a derotator is to maintain image alignment between pulses, rather than for stopping image motion during the pulses. The technique is shown to apply to contoured objects, to resonant and nonresonant vibrations, and to speeds up to 9200 rpm. In addition, application of the system for recording large-vibration amplitudes via double-exposure, speckle photography is demonstrated. Finally, concomitant observation of vibration with stroboscopic illumination synchronized to the rotation is described, where the vibration is manifested as a streaking of defocused speckles.