object such as for example an aircraft structure if a given coordinate on an object can be precisely located in space using the moving fringe concept then its motion in time can also be measured as described above the contour coordinate of an object point is determined by measuring the d c value of the phase if the object point is moving or vibrating then its interaction with the moving fringes acts like a carrier frequency which is phase modulated because this is a non contacting technique where the use of high fringe frequencies are possible both vibrational amplitudes and frequencies can be accurately measured over a wide frequency range one embodiment of a vibrational measurement system is shown in fig1 the laser acousto optic modulators and transmitter optics represents any of the transmitter configurations described above in general the transmitter is designed so as to have a minimal number of fringes across the beam one or two so that the total phase variation of the vibrating object would be less than 2 u03c4 radians as shown in fig1 a sighting telescope can be used to direct the transmitted beam onto a particular point on the vibrating specimen the reflected radiation from the vibrating specimen is picked up by a receiver which may be a simple lens followed by a detector and amplifier the detector may be a single silicon photo diode or other form of optical detector or an array of detectors the amplifier output will be a signal which oscillates at the transmitted fringe frequency and which has a phase modulated by the vibrational information the phase demodulator will.