Andrew Wood

Infrared imaging with a wavefront-coded singlet lens

We describe the use of wavefront coding for the mitigation of optical aberrations in a thermal imaging system. Diffraction-limited imaging is demonstrated with a simple singlet which enables an approximate halving in length and mass of the optical system compared to an equivalent two-element lens.

Hybrid optics in dual-waveband infrared systems

The diffraction-based performance limitations of dual waveband infrared systems which incorporate hybrid refractive-diffractive lenses are examined. These limitations must be understood in order to identify the key trade-offs and optimize the design of the diffractive element. The correction of chromatic aberration is considered and the range of conditions under which hybrid solutions offer an advantage is established. A dual waveband hybrid objective lens for an uncooled staring array camera has been designed, manufactured and evaluated.

Two-element lenses for military applications

To meet todays demanding requirements for increased performance, reduced size, lower mass, and cost, simple lenses containing multiple aspheric surfaces are required. It is now common for the number of aspheric surfaces used in an infrared lens to exceed the actual number of lens elements. Multiple aspheric and diffractive surfaces provide additional degrees of freedom in the lens design. This is required to achieve increased levels of imaging performance demanded by reduced pitch detectors. Aspheric surfaces also enable a greater diversity of materials to be used such that athermal solutions can be realized without the need for additional lens elements. More recent advances in detector technology will demand multispectral operation, but the requirements for simple, inexpensive optics will remain. Innovative use of aspheric components can also create very simple multispectral optics to fulfil this emerging need. This paper will review the range of applications that can be satisfied using no more than two optical components, highlighting the specific benefits that aspheric and diffractive surfaces provide. Consideration will also be given to future developments where enhanced functionality can be achieved using computational imaging techniques. Examples will be given for several military applications including weapon sights, drivers vision enhancement and remote weapon stations.

Infrared hybrid optics with high broadband efficiency

Hybrid refractive-diffractive optics are widely used in infrared systems, but their performance is limited by reduced diffraction efficiency away from the design wavelength. Two techniques are currently being investigated to improve broadband efficiency; dual-layer blaze structures and blazed-binary optics. This paper discusses the design of dual-layer blaze structures in detail, and presents some athermalised lenses which benefit from this approach. A brief summary of using blazed-binary structures to improve efficiency is presented.

Multi-aperture foveated imaging

Foveated imaging, such as that evolved by biological systems to provide high angular resolution with a reduced space-bandwidth product, also offers advantages for man-made task-specific imaging. Foveated imaging systems using exclusively optical distortion are complex, bulky, and high cost, however. We demonstrate foveated imaging using a planar array of identical cameras combined with a prism array and superresolution reconstruction of a mosaicked image with a foveal variation in angular resolution of 5.9:1 and a quadrupling of the field of view. The combination of low-cost, mass-produced cameras and optics with computational image recovery offers enhanced capability of achieving large foveal ratios from compact, low-cost imaging systems.

Diffractive optics in modern optical engineering

The influence of diffractive optics on modern optical engineering has been considerable. The paper discusses a wide variety of applications of diffractive surfaces in the infrared and visible wavebands. Successful developments in low mass and multi-waveband infrared optics are described and results presented from manufactured hardware. The use in the visible waveband for real-time 3-D imagery, high power magnification of full colour displays, and possible use in diffractive-only helmet displays are examined for their potential. The limitation in the use of conventional diffractive surfaces for wide spectral bandwidth applications is described along with a method for alleviating this problem.

Passively athermalized hybrid objective for a far-infrared uncooled thermal imager

The development of a Petzval objective lens which is passively athermalized over the temperature range minus 20 degrees Celsius to plus 50 degrees Celsius is described. The lens is compatible with a latest generation uncooled staring array imager operating in the far infrared band, currently under development at the UK Defence Research Agency (DRA). In order to minimize the number of lens components which are required, a diamond turned hybrid refractive-diffractive element is employed. Design options are presented and the manufacturing issues relating to the diffractive surface are described. The goal is to produce a cost effective solution rather than placing the emphasis on achieving the ultimate in performance. Results from optical performance tests are given, including interferometry at a range of temperatures and broadband MTF.

Compact multi-aperture imaging with high angular resolution

Previous reports have demonstrated that it is possible to emulate the imaging function of a single conventional lens with an N×N array of identical lenslets to provide an N-fold reduction in imaging-system track length. This approach limits the application to low-resolution imaging. We highlight how using an array of dissimilar lenslets, with an array width that can be much wider than the detector array, high-resolution super-resolved imaging is possible. We illustrate this approach with a ray-traced design and optimization of a long-wave infrared system employing a 3×3 array of freeform lenslets to provide a fourfold reduction in track length compared to a baseline system. Simulations of image recovery show that recovered image quality is comparable to that of the baseline system.

Optimality of pupil-phase profiles for increasing the defocus tolerance of hybrid digital-optical imaging systems

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.

Digital image processing as an integral component of optical design

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.