2
 

 

 

Figure 2 illustrates the modular design of the Electronic Shearo­gra­phy optical head mounded on an 18-inch square pallet.  It is composed of 2 modules: a beam expander module and a shearography module.  The beam expander module provides a steering mirror and a variable beam expander.  The heart of the system is an interferometer assembly, which is called the shearography module.  This is, in effect, a Michelson inter­fer­ometer through which an image of the object is relayed to the TV camera.  The objective lens, which is a 6x TV zoom lens, forms an image of the object on a field lens.  The interferometer consists of a cube beam-splitter and two mirrors, and two relay lenses are used for the image transfer, one at the input to the beamsplitter and one at its output.  These are specially designed lenses that form aberration free images when the aperture is located at the focal plane.  The lens at the input side of the beamsplitter collimates the rays coming from each image point and sends the collimated ray bundles though the interferometer.  At the output side of the beamsplitter, the other lens refocuses the rays into a sharp image on the TV camera.  Because the rays pass through the cube beamsplitter as collimated ray bundles, they do not acquire aberrations.  The initial field lens at the primary image plane is chosen to eliminate vignetting of the image.  This optical system provides images of much higher resolution than the pixels of the TV camera.  One of the two mirrors in the interferometer has a tilt adjustment and controls the image shear, which the user can set to any magnitude, horizontal, vertical, or at any angle.  On-screen calibration provided by the interferometry program allows direct measure­ment of the image shear with a set of calipers.  The other mirror is mounted on a piezo-electric translator and provides 90° phase shifts between TV frames.  The HoloFringe300 computer uses these frames to create an interferometric shearography image of the overlap between the two sheared images.  As with the Electronic Holography image, the image of the object does not disappear when the shearography processor is active.