We demonstrate a multimode detection system in a scanning laser ophthalmoscope (SLO) that enables simultaneous operation in confocal, indirect, and direct modes to permit an agile trade between image contrast and optical sensitivity across the retinal field of view to optimize the overall imaging performance, enabling increased contrast in very wide-field operation. We demonstrate the method on a wide-field SLO employing a hybrid pinhole at its image plane, to yield a twofold increase in vasculature contrast in the central retina compared to its conventional direct mode while retaining high-quality imaging across a wide field of the retina, of up to 200 deg and 20 μm on-axis resolution.
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 Monte Carlo simulation of light propagation through the retina has been developed to understand the path-length distributions within the retinal vessel. For full-field illumination, the path-length distribution within the vessel comprises directly backscattered light and light that has passed once or twice through the vessel. The origins of these light path-length distributions can be better understood by investigating different combinations of single-point illumination and detection positions. Perhaps the most significant observation is that illumination at the edges of the vessel, rather than over the whole field of view, and detection directly above the vessel capture only the light that has taken a single pass through the vessel. This path-length distribution is tightly constrained around the diameter of the vessel and can potentially provide enhancements for oxygen saturation imaging. The method could be practically implemented using an offset-pinhole confocal imaging system or structured light illumination.
We report the design, manufacture and assessment of a phantom eye that can be used to measure the performance and accuracy of ophthalmic-OCT devices. We base our design on a wide-field schematic eye, R. Navarro, J. Opt. Soc. Am. A 2 (1985), to allow the assessment of device performance relative to ± 70° external field of view. We have fabricated the phantom eye and have verified the structural dimensions of the multi-material 3D-printed retinal targets using calibrated-OCT images.
We report the design of binary-amplitude masks that in conjunction with digital restoration enable mitigation of optical
aberrations. Essentially, the design process aims to maintain high modulation-transfer functions by reducing destructive
interference of optical-transfer-function phasors. Two optimization techniques are described: so-called contour masks
and the use of multiple pixelated masks. In general the resultant modulation-transfer function is 20% of a diffractionlimited
imaging system and due to the absence of nulls recorded images can be restored to a high-contrast diffractionlimited
image. Example applications are presented for correcting ocular aberrations and for conformal imaging.
The use of hybrid optical-digital techniques facilitates improved optimisation of imaging systems. It involves the
combined use of optical coding of an image using pupil-plane phase-modulation of the transmitted wavefront and postdetection
digital decoding. Previous research in hybrid imaging tends to emphasize constancy of the modulation transfer
function with aberrations and ignore the significant variations in the phase transfer function. We show that the
restoration artefacts introduced by phase mismatch effects can also be used to deduce the defocus PSF, and when this is
achieved, an overall improvement in image quality can be attained. Both numerical simulations and experimental
images of hybrid imaging systems are presented.
The design of modern imaging systems is intricately concerned with the control of optical aberrations. 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. We will discuss the trade offs involved in the application of this technique 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.
We present the results of the utilization of a Spatial Light Modulator Liquid Crystal Display for the implementation of
wavefront codification procedures in an imaging system. The light modulator works in transmission mode at the pupil of the instrument. The main disadvantage is that the procedure implies a calibration of the device as well as an inherent image processing. The more interesting feature we can obtain is the versatility related to the use of an electronic device at the pupil, as compared with conventional (fixed) manufactured ones.
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.
Hyperspectral imaging of the retina presents a unique opportunity for direct and quantitative mapping of retinal
biochemistry - particularly of the vasculature where blood oximetry is enabled by the strong variation of absorption
spectra with oxygenation. This is particularly pertinent both to research and to clinical investigation and diagnosis of
retinal diseases such as diabetes, glaucoma and age-related macular degeneration. The optimal exploitation of
hyperspectral imaging however, presents a set of challenging problems, including; the poorly characterised and
controlled optical environment of structures within the retina to be imaged; the erratic motion of the eye ball; and the
compounding effects of the optical sensitivity of the retina and the low numerical aperture of the eye. We have
developed two spectral imaging techniques to address these issues. We describe first a system in which a liquid crystal
tuneable filter is integrated into the illumination system of a conventional fundus camera to enable time-sequential,
random access recording of narrow-band spectral images. Image processing techniques are described to eradicate the
artefacts that may be introduced by time-sequential imaging. In addition we describe a unique snapshot spectral imaging
technique dubbed IRIS that employs polarising interferometry and Wollaston prism beam splitters to simultaneously
replicate and spectrally filter images of the retina into multiple spectral bands onto a single detector array. Results of
early clinical trials acquired with these two techniques together with a physical model which enables oximetry map are reported.
Pupil plane encoding has shown to be a useful technique to extend the depth of field of optical systems. Recently, further studies have demonstrated its potential in reducing the impact of other common focus-related aberrations (such as thermally induced defocus, field curvature, etc) which enables to employ simple and low-cost optical systems while maintaining good optical performance. In this paper, we present for the first time an experimental application where pupil plane encoding alleviates aberrations across the field of view of an uncooled LWIR optical system formed by F/1, 75mm focal length germanium singlet and a 320x240 detector array with 38-micron pixel. The singlet was corrected from coma and spherical aberration but exhibited large amounts of astigmatism and field curvature even for small fields of view. A manufactured asymmetrical germanium phase mask was placed at the front of the singlet, which in combination with digital image processing enabled to increase significantly the performance across the entire field of view. This improvement is subject to the exceptionally challenging manufacturing of the asymmetrical phase mask and noise amplification in the digitally restored image. Future research will consider manufacturing of the phase mask in the front surface of the singlet and a real-time implementation of the image processing algorithms.
Proc. SPIE. 5987, Electro-Optical and Infrared Systems: Technology and Applications II
KEYWORDS: Signal to noise ratio, Infrared imaging, Monochromatic aberrations, Point spread functions, Imaging systems, Image restoration, Wavefronts, Computer programming, Modulation transfer functions, Personal protective equipment
Pupil plane encoding enables extended depth of field and greatly reduced sensitivity to aberrations in an imaging system (field curvature, thermally induced defocus, astigmatism, etc.). The application of pupil plane encoding has potential in thermal imaging where it can enable the use of simple, low-cost, light-weight lens systems. We present numerical and modelling studies of the application of this technique to an uncooled LWIR imaging system, F/1, 75mm focal length, germanium singlet with a detector array size of 240x320 with 50 micron pixel. The initial singlet is corrected from coma and spherical aberration, but its performance across the field of view is greatly limited by astigmatism. The introduction
of an encoding asymmetrical germanium phase mask at the aperture stop of the system, combined with digital image processing, allows the removal of astigmatism and improved imaging performance across the field of view. This improvement is subject to a noise amplification in the digitally restore image. There is as a tradeoff between the maximum correction to astigmatism and reduced signal-to-noise ratio in the recovered image.
Proc. SPIE. 5612, Electro-Optical and Infrared Systems: Technology and Applications
KEYWORDS: Thermography, Signal to noise ratio, Infrared imaging, Point spread functions, Imaging systems, Image processing, Image restoration, Wavefronts, Modulation transfer functions, Temperature metrology
Wavefront coding involves the insertion of an asymmetric refractive mask close to the pupil plane of an imaging system so as to encode the image with a specific point spread function that, when combined with decoding of the recorded image, can enable greatly reduced sensitivity to imaging aberrations. The application of wavefront coding has potential in the fields of microscopy, where increased instantaneous depth of field is advantageous and in thermal imaging where it can enable the use of simple, low-cost, light-weight lens systems. It has been previously shown that wavefront coding can alleviate optical aberrations and extend the depth of field of incoherent imaging systems whilst maintaining diffraction-limited resolution. It is particularly useful in controlling thermally induced defocus aberrations in infrared imaging systems. These improvements in performance are subject to a range of constraints including the difficulty in manufacturing an asymmetrical phase mask and significant noise amplification in the digitally restored image. We describe the relation between the optical path difference (OPD) introduced by the phase mask and the magnitude of noise amplification in the restored image. In particular there is a trade between the increased tolerance to optical aberrations and reduced signal-to-noise ratio in the recovered image. We present numerical and experimental studies based of noise amplification with the specific consideration of a simple refractive infrared imaging system operated in an ambient temperature varying from 0°C to +50 C. These results are used to delineate the design and application envelope for which infrared imaging can benefit from wavefront coding.
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 optimized optical/digital hybrid imaging system can meet some specific requirements that are impossible to achieve with a traditional approach.