To extend the depth-of-focus of incoherent imaging systems an aspherical pupil plane element is employed to encode the incident wavefront in such a way that the image recorded by the detector can be accurately restored over a large range of defocus. This approach alleviates the defocus and related aberrations whilst maintaining a diffraction-limited resolution for incoherent imaging systems. This offers the potential to implement diffraction-limited imaging systems using simple and low-cost one- or two-element achromatic athermal lenses. However these performance are associated with reductions in signal-to-noise ratio of the displayed image. This effect sets a serious limit to future applications, particularly in thermal imaging where the optical systems involve fast optics. Fast optics means a small depth of field andsignal-to-noise ratio of the deconvolved image can be reduced significantly for small amounts of defocus. Fortunately, recent and future improvements in detector sensitivity offer some scope for allowing modest amounts of noise amplification whilst maintaining current detection performance levels. The system performance of the present hybrid optical/digital technique, called wavefront coding, depends substantially on the proper design of the phase mask. Merit functions are derived to explore the optimum design.
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