A novel methodology based on the scanning confocal microscope is presented which enables a general solution of the depth resolution problem in holography and other coherent imaging schemes. The method does not depend on a priori information about the object.
Background and historical perspective are provided, as well as some review of the Rayleigh-Sommerfeld diffraction formulation and other pertinent physical optics topics. Some signal processing and numerical methods specific to the simulation of the propagation and diffraction of light are presented and applied. Holograms were simulated, providing the initial test bed for these concepts.
Collimated confocal reconstruction of holograms, is discussed and demonstrated on a simulated hologram. The apertured scanning version is then discussed, and shown to have depth and lateral discrimination properties similar to those of the scanning confocal microscope.
The first application of confocal processing to real holographic data is presented, demonstrating the expected depth discrimination and contrast improvement. Frequency diverse microwave holograms were used as input, and therefore background and characterization of that system are provided. In addition, some improvements in computational reconstruction of spherical shell microwave holograms are presented.
As a demonstration for apertured scanning confocal hologram reconstruction, data from a typical microwave experiment was used as input. The experiment, using a 1/16 scale tank model, involved a scan of a full 360 degrees, and frequency diversity from 10 to 26 GHz. The results of confocal processing clearly demonstrate the desired effects of improved depth discrimination and contrast over conventional reconstruction. Details on the top of the tank become visible when strong returns from below the plane of interest, which are prominent in conventional reconstruction, are removed by the depth discrimination effect associated with the confocal arrangement.
By demonstrating computational implementation of the concepts associated with the confocal microscope, opportunities are provided for imaging in many regimes where lenses and/or mirrors of high quality are not available. Extension of confocal processing to these systems is briefly discussed. Also, many opportunities to apply recent advances in scanning confocal microscopy are recognized, which may be implemented computationally. These include edge traversal and detection, automatic refocusing, and super-resolving methods.