While the performance of optical imaging systems is fundamentally limited by diffraction, the design and manufacture of practical systems is intricately associated with the control of optical aberrations. The fundamental Shannon limit for the number of resolvable pixels by an optical aperture is generally therefore not achieved due to the presence of off-axis aberrations or large detector pixels. We report how co-called computational-imaging (CI) techniques can enable an increase in imaging performance using more compact optical systems than are achievable with traditional optical design. We report how discontinuous lens elements, either near the pupil or close to the detector, yield complex and spatially variant PSFs that nevertheless provide enhanced transmission of information via the detector to enable imaging systems that are many times shorter and lighter than equivalent traditional imaging systems. Computational imaging has been made possible and attractive with the trend for advanced manufacturing of aspheric, asymmetric lens shapes at lower cost and by the exploitation of low-cost, high-performance digital computation. The continuation of these trends will continue to increase the importance of computational imaging.
A thermal imaging system has been demonstrated, which uses wavefront coding to provide an extended depth of field.
Aberrations are introduced into the optical system, which are optimised to be insensitive to defocus; a single image
processing algorithm can be successfully applied to images with a range of focus errors. This requires both the design of
the wavefront coding surface and the implementation of efficient and effective image processing electronics. The design
of the freeform wavefront coding surface goes hand-in-hand with that of the electronics architecture. An optimised
decoding algorithm is implemented in the image processing electronics to achieve real-time imaging performance.
A significantly increased defocus tolerance can be obtained by combining pupil phase-modulation with digital demodulation
in a hybrid imaging system. Designing the optimal pupil phase-modulation is however not a trivial task. We
show how hybrid imaging fidelity can be predicted and used to compare arbitrary phase-modulations. The evaluations of
two anti-symmetric and a symmetric phase-modulation yield initial design values that can be used for the optimization of
specific hybrid designs.
A phase mask at the aperture stop of a hybrid digital-optical imaging system can improve its tolerance to aberrations.
The choice of the introduced phase modulation is crucial in the design of such systems. Several successful phase masks
have been described in the literature. These masks are typically derived by searching for optical-transfer-functions that
retain restorability under aberrations such as defocus. Instead of optimizing the optical-transfer-function for some desired
characteristics, we calculate the expected imaging error of the joint design directly. This was used to compare thirddegree
polynomial phase masks, including the cubic phase profile and a commonly used generalization. The analysis
shows how the optimal phase profile depth is always limited by noise and more importantly, numerical simulations show
that only a finite range of the third-degree polynomial profiles yield optimal performance.
The design of modern imaging systems is intricately concerned with the control of optical aberrations in systems that can
be manufactured at acceptable cost and with acceptable manufacturing tolerances. Traditionally this involves a multi-parameter
optimisation of the lens optics to achieve acceptable image quality at the detector. There is increasing interest
in a more generalised approach whereby digital image processing is incorporated into the design process and the
performance metric to be optimised is quality of the image at the output of the image processor. This introduces the
possibility of manipulating the optical transfer function of the optics such that the overall sensitivity of the imaging
system to optical aberrations is reduced. Although these hybrid optical/digital techniques, sometimes referred as
wavefront coding, have on occasion been presented as a panacea, it is more realistic to consider them as an additional
parameter in the optimisation process. We will discuss the trade-offs involved in the application of wavefront coding to
low-cost imaging systems for use in the thermal infrared and visible imaging systems, showing how very useful
performance enhancements can be achieved in practical systems.
A biocular magnifier is an optic that is sufficiently large to be used by both eyes together, and which presents a magnified virtual image at a finite distance from an observer. The design of such an optic is one of the most difficult tasks in optical design due to the extreme optical parameters, the relatively high level of residual aberrations, and the interface between the image and two-eye vision. As such, biocular magnifiers of reasonable magnifying power (>x4.5) cannot easily be analysed in a meaningful way using conventional optical design software. A number of years ago, a unique computer program was written that enabled the analysis of biocular magnifiers in the way in which they were used, that is with the spatial image being viewed by two small mobile apertures of nominally fixed separation. Utilising the power of modern PC computers, this program has been extended considerably so that it provides detailed graphical analysis of the visual aberrations appropriate to an extreme optical system usable by both eyes together. The reasoning behind the software and examples of the graphical analysis of biocular designs is given in the paper.