Superposition of such backprojected signals from an array of surface detectors can locate objects and provides a means for image reconstruction. The advantage of PAI is the ability to detect the edges of deep objects with roughly mm resolution rather than the cm resolution of photon migration measurements alone. The two polarization components of a linearly polarized HeNe laser beam are split by a Wallaston prism into two angles to yield two beams which are collimated by a lens and turned by a mirror down onto a liquid phantom. The two beams interfere at the photodiode yielding a signal which is sensitive to the differential total pathlength of photons after reflection off the phantom surface. The two beams are reflected down onto the phantom surface.
Pressure waves arriving at the surface reach one beam site before arriving at the second beam site.
The differential height of the surface at the two beams affects the round-trip path of the interferometer and generates a change in the interferometric signal at the detector. Laser irradiation was delivered via a mirror down through the clear water. The adjustable mirror varied the site of laser irradiation of the absorbing gel. Point spread function for optical transducer. The response signal from a point source of acoustic energy generated by the pulsed laser.