The pyramidal-mirror detector for scanning laser acoustic microscopy

Abstract

The pyramidal-mirror detector has important advantages over the knife-edge detector currently in use in scanning laser acoustic microscopy. A key element of this new detector is a four-sided pyramid with mirrored surfaces. In the operation of the microscope, the zero-order component of the light of the scanning laser beam that has been reflected from the microscope's coverslip is centered on the tip of the pyramid. Thus, each mirror face reflects one fourth of the light in this component. Making use of an arrangement of four photodetectors, one for each of the four faces of the pyramidal mirror, the light reflected from each face goes to a different photodetector and is processed separately from the light reflected from the other faces. This detector has an almost isotropic transfer function with no negative responses and fourfold symmetry around a null point in the center which coincides with the zero-frequency point in the spatial spectrum. This property makes it possible to detect spatial frequencies in all directions simultaneously with almost equal sensitivity. Oblique insonification, required for best operation with the knife-edge detector because of its region of negative response, is not preferred in the pyramidal-mirror detector and double side-band detection can be employed. Since there is no requirement that a negative response region be avoided, the spatial spectrum of this detector can be more than twice that of the knife-edge detector. The tighter the focusing of the scanning laser beam onto the coverslip, the higher the spatial frequencies detectable in the object. In fact, if the coverslip occupies a position in the very-near field of the object, evanescent-wave detection is possible. Low-frequency, highly penetrable ultrasound can be used and at the same time high spatial frequencies can be detected for obtaining high resolution. © 1999 John Wiley & Sons, Inc. Int J Imaging Syst Technol 10, 323–327, 1999