PHASE MODULATING IN ESPI FOR VIBRATION PATTERN MEASUREMENT

The conventional method of electronic speckle pattern interferometry (ESPI) for vibration pattern measurement is to freeze reference beam.This method enables to sweep over a wide frequence range and to identify resonant modes.However,the obtained fringe contrast is not too fine.In order to eN_hance the visibility of fringe pattern,it is possible to subtract two time average pattern at any given frequancy.In result,the fringe pattern is obtained with high visibility but only valid at the given frequency.A phase shifting technique is adopted to overcome this shortcoming.A time average image is put into processor and frame store(renewed and put into processor per 40ms),then a subsequent image with a phase shift of ΔΦ resulted from sinusoidal modulation in object beam is recorded and subtracted from the first image.This method is similar to modulation of light intensity function of speckle pattern.The advantage of this method is the capabilities of obtaining real time fringe pattern of vibration with high resolution and of performing frequency scanning at a wide range.Its application to the vibration pattern measurement of disk blade coupling in an aeroengine is a great success.