Shortwave infrared (SWIR) cameras are becoming increasingly attractive due to the improving size, resolution and decreasing prices of InGaAs focal plane arrays (FPAs). The rapid development of competitively priced HD performance SWIR cameras has not been matched in SWIR imaging lenses with the result that the lens is now more likely to be the limiting factor in imaging quality than the FPA. Adapting existing lens designs from the visible region by re-coating for SWIR will improve total transmission but diminished image quality metrics such as MTF, and in particular large field angle performance such as vignetting, field curvature and distortion are serious consequences.
To meet this challenge original SWIR solutions are presented including a wide field of view fixed focal length lens for commercial machine vision (CMV) and a wide angle, small, lightweight defence lens and their relevant design considerations discussed. Issues restricting suitable glass types will be examined. The index and dispersion properties at SWIR wavelengths can differ significantly from their visible values resulting in unusual glass combinations when matching doublet elements. Materials chosen simultaneously allow athermalization of the design as well as containing matched CTEs in the elements of doublets.
Recently, thinned backside-illuminated InGaAs devices have made Vis.SWIR cameras viable. The SWIR band is sufficiently close to the visible that the same constituent materials can be used for AR coatings covering both bands. Keeping the lens short and mass low can easily result in high incidence angles which in turn complicates coating design, especially when extended beyond SWIR into the visible band. This paper also explores the potential performance of wideband Vis.SWIR AR coatings.
Adaptive coded aperture imaging systems can resolve objects that are smaller than the pixel-limited resolution of the
detector focal plane array. This is done by combining multiple frames of data, where different frames are taken with
different coding patterns on the coded-aperture mask. In the mid-wave infrared the required signal to noise ratio
necessitates some form of light concentration. Optical design software has been used to model candidate optical systems
with the aim of achieving up to four times resolution enhancement along each linear dimension. As in some other
computational imaging systems, the requirements on the optical system are found to be different to those that are
normally used in more classical optical designs. The basic needs are a point-spread function of suitable extent that
changes gradually with angle and does not vary significantly with the expected changes in input spectra or system
temperature. Novel metrics have been derived and used to inform the optical design. The modeling and design trade-offs
and resulting performance are discussed.
Coded Aperture Imaging (CAI) is a new approach to system design whereby the optics are simplified in a controlled way
so that system performance can be recovered using appropriate computer based algorithms. Adopting Coded Aperture
approaches to sensor designs opens up possibilities of increasing the system design trade-space thereby giving the system
designer greater degrees of freedom to optimise the system. A comparison has been made between a system adopting CA
in its optical train with systems based on conventional optics approaches. These comparisons show that CA based
systems can provide significant benefits to the user in some applications.
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.
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.
A compact coaxial optical system is described that employs a Cassegrain telescope with switchable primary mirrors plus a catadioptric relay. The system can operate in the visible, near/thermal IR, and millimeter wavebands, is athermal, and provides multispectral compensation for a protective dome.
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.
This paper deals with diffractive surfaces used in optical designs: fabricated devices operating over the infrared wavebands are detailed, and the means of manufacture within the Thomson Optronic Group are described.
An arsenic trisulphide refractive plus diffractive hybrid element is both achromatic and athermal across the 3 - 5 micrometer waveband when set in an aluminum mount. We present two possible ways of manufacturing this element: using a photodarkening process to realize the diffractive structures, and by diamond turning, initially not thought to be viable. We demonstrate that the required depths in the diffractive surface are obtainable by both methods, and consider the advantages of each. Above all, however this lens is fabricated it offers, a very low cost, light weight solution to athermal singlet imaging in the 3 - 5 micrometer waveband.
The availability of small detector arrays that operate in either the lower or the upper thermal infrared atmospheric windows has created the need for relatively simple low-mass optics. Crucial to the success of such an optic is the correct choice of optical materials. Infrared optical materials can be ranked in terms of their relative `lightness', but material choice also depends on other factors such as the correction of primary chromatic aberration, secondary spectrum and thermal effects. This paper provides details of the relative mass of materials when used in single element or achromatic doublet form, the limit set by secondary spectrum on the diameters of specific low-mass combinations, and the possible elimination of thermal defocus by the choice of lightweight combinations of materials. The use of surface relief holograms on candidate materials is included as part of the investigation.
Hybrid optics -- diffractive structures on a refractive substrate -- have some specific applications in the lower of the two thermal infrared wavebands. These applications include optical systems where simplicity and/or low mass are major requisites. In many cases, however, the use of diffractive surfaces is limited to those substrate materials that can be diamond-machined successfully. The paper examines the optical merits of several substrate materials and gives design examples where hybrid optics have major advantages relative to conventional optics in terms of mass, thermal defocus, and complexity.
It is becoming widely recognized that there is a requirement for a head-up display (HUD) in the civil aircraft cockpit. Due to the variety of airframes used by commercial airlines there is the need for a universal HUD design which could be installed into a range of aircraft. Holographic components have been utilized to provide a display that is acceptable in terms of optical performance, and provide an optical relay that will fit the available space.
The requirement of not just survival but high performance operation in a hostile environment is a fundamental consideration in some optical systems. The paper details the effects of - and compensation for - deep-sea immersion, variable air-pressure and humidity, and a variety of thermal perturbations. The deleterious effects on optical materials resulting from a selection of hostile environments, including that of high radiation levels, are also considered and suggestions made concerning the optimum choice of materials.
The requirement of not just survival but high performance operation in a hostile environment is a fundamental consideration in some optical systems. The paper details the effects of -- and compensation for -- deep-sea immersion, variable air-pressure and humidity, and a variety of thermal perturbations. The deleterious effects on optical materials resulting from a selection of hostile environments, including that of high radiation levels, are also considered and suggestions made concerning the optimum choice of materials.
The difficult interface between optics and the human visual system is discussed, particularly in relation to the design of binocular optics. Many sections are also applicable to the simpler case of monocular optics.
Unconventional optical surfaces can now be readily manufactured with high precision. Good design practice requires that these surfaces be used to optimize the concept of an optical system rather than to provide a marginal performance improvement. The paper considers some of the possibilities.
Infrared optics defocus with temperature change due to the nature of the optical materials employed in their design. Methods of eliminating this defocus - mechanical, electro-mechanical and optical - are discussed and evaluated in detail.
Forward looking infrared optics defocus with temperature change due to the materials employed. Methods of elimination this defocus - mechanical, electro-mechanical and optical - are discussed and evaluated.
Conventional head-up display (HUD) optics are relatively limited in both instantaneous field of view (IFOV) and display brightness and may be inadequate some applications. Considerable improvements to both parameters may be made by the use of pupil relay optics employing powered holographic combiners but this type of system tends to be complex and relatively expensive. The usefulness of conventional HUD optics may be increased by the use of combiners that improve instantaneous elevation field of view or display brightness.