Amplitude and phase filters for mitigation of defocus and third-order aberrations

This paper gives a review on the design and use of both amplitude filters and phase filters to achieve a large focal depth in incoherent imaging systems. Traditional optical system design enhances the resolution of incoherent imaging systems by optical-only manipulations or some type of post-processing of an image that has been already recorded. A brief introduction to recent techniques to increase the depth of field by use of hybrid optical/digital imaging system is reported and its performance is compared with a conventional optical system. This technique, commonly named wavefront coding, employs an aspherical pupil plane element 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. As reported in earlier work, this approach alleviates the effects of defocus and its related aberrations whilst maintaining diffraction-limited resolution. We explore the control of third order aberrations (spherical aberration, coma, astigmatism, and Petzval field curvature) through wavefront coding. This method offers the potential to implement diffraction-limited imaging systems using simple and low-cost lenses. Although these performances are associated with reductions in signal-to-noise ratio of the displayed image, the jointly optimised optical/digital hybrid imaging system can meet some specific requirements that are impossible to achieve with a traditional approach.