We describe a new filter that simultaneously achieves spectral filtering and image replication to yield a two-dimensional,
snapshot spectral imager. Filtering is achieved by spectral demultiplexing; that is without rejection of light; so optical throughput efficiency is, in principle, unity. The principle of operation can be considered as a generalisation of the Lyot filter to achieve multiple bandpasses. We report on the design and experimental implementation of an eight-band system for use in the visible. Proof-of-concept demonstrations are reported for imaging of the ocular fundus and microscopy of fluorescently labelled living cells.
We describe a new filter that simultaneously achieves spectral filtering and image replication to yield a two-dimensional, snapshot spectral imager. Filtering is achieved by spectral demultiplexing; that is without rejection of light; so optical throughput efficiency is, in principle, unity. The principle of operation can be considered as a generalisation of the Lyot filter to achieve multiple bandpasses. We report on the design and experimental implementation of an eight-band system for use in the visible and the design of an eight-band long-wave infrared system.
We describe demonstrations of two new techniques for snapshot pectral imaging in two dimensions. The first, based on a generalisation of the Lyot filter, we believe to be the first technique able to spectrally image in snapshot mode with modest resolution and without the need for data inversion. The second demonstration is of a biologically inspired foveal hyperspectral imager, which mitigates the data acquisition and processing bottleneck encountered in traditional hyperspectral imaging approaches.
We present a new approach to hyperspectral imaging that is inspired by biological imaging systems, such as human vision, which employ high spectral and spatial discrimination only in a small central patch. This foveal technique addresses several problems of conventional approaches to HSI: they cannot provide snapshot, high spectral-resolution imagery in a two dimensional format. The ability to provide the data in a single snapshot removes temporal mis-registration issues. High signal to noise ratios naturally result from the absence of any multiplexing technique and the corresponding loss of light. Other reported snapshot techniques are either low spectral resolution or provide only a one-dimensional field of view. A high-spectral-resolution imager with a wide field of view could produce giga-sample data rates, which would make real-time data processing problematic. By gathering hyperspectral data from only a selected portion of the scene, we reduce the data processing rates to manageable levels. For many applications only a small field of view is required, but needs to be cued for situational awareness. In our system, this is provided for by a wide field of view, panchromatic imager, which fills a similar role to peripheral vision in the biological systems mentioned above. Our technique images the selected region onto a coherent fibre bundle, which reformats the input into a line array constituting the input to a dispersive hyperspectral imager. Computer processing reformats the dispersed one-dimensional output into a rectangular image and applies calibration routines to produce a high spectral resolution, small hyperspectral image. This is combined with a high-spatial-resolution panchromatic image. Experimental results will be presented.
We have previously shown those circumstances for which the multiplex advantage of temporally scanned Fourier transform imaging spectrometers enables higher signal-to-noise ratios than other techniques. Unfortunately, for many real-life applications, such as aerial reconnaissance, deployment of FT instruments based on traditional moving-mirror interferometers is problematic due to their inherent sensitivity to vibration.
We will describe a new type of Fourier transform imaging spectrometer, employing moving birefringent prisms to create the necessary path difference modulations. This new system retains the accepted sensitivity advantages of traditional Fourier transform devices, but because it employs common-path interferometry and because path differences are introduced within a single optical element, the system is inherently very robust. Furthermore, the precision of the movement can be typically two orders of magnitude lower than for a traditional two-beam interferometer, resulting in a simpler instrument. Experimental results will be presented.
Hyperspectral imaging (HSI) shows great promise for the detection and classification of several diseases, particularly in the fields of "optical biopsy" as applied to oncology, and functional retinal imaging in ophthalmology. In this paper, we discuss the application of HSI to the detection of retinal diseases and technological solutions that address some of the fundamental difficulties of spectral imaging within the eye.
HSI of the retina offers a route to non-invasively deduce biochemical and metabolic processes within the retina. For example it shows promise for the mapping of retinal blood perfusion using spectral signatures of oxygenated and deoxygenated hemoglobin. Compared with other techniques using just a few spectral measurements, it offers improved classification in the presence of spectral cross-contamination by pigments and other components within the retina. There are potential applications for this imaging technique in the investigation and treatment of the eye complications of diabetes, and other diseases involving disturbances to the retinal, or optic-nerve-head circulation.
It is well known that high-performance HSI requires high signal-to-noise ratios (SNR) whereas the application of any imaging technique within the eye must cope with the twin limitations of the small numerical aperture provided by the entrance pupil to the eye and the limit on the radiant power at the retina. We advocate the use of spectrally-multiplexed spectral imaging techniques (the traditional filter wheel is a traditional example). These approaches enable a flexible approach to spectral imaging, with wider spectral range, higher SNRs and lower light intensity at the retina than could be achieved using a Fourier-transform (FT) approach. We report the use of spectral imaging to provide calibrated spectral albedo images of healthy and diseased retinas and the use of this data for screening purposes. These images clearly demonstrate the ability to distinguish between oxygenated and deoxygenated hemoglobin using spectral imaging and this shows promise for the early detection of various retinopathies.