Andrew R. Harvey

Phase pupil functions for reduction of defocus and spherical aberrations

Radially symmetric pupil plane phase retardation functions are derived that extend focal depth and alleviate third-order spherical aberration (SA) effects. The radial symmetry of these functions means that they can be more conveniently manufactured by use of traditional techniques such as diamond machining than previously reported filters with rectangular symmetry. The method employs minimization of the variation of Strehl ratio with defocus, W20, and SA, W40. The performance of the derived phase filters is illustrated by comparison with standard optical systems and with previously reported phase filters.

Birefringent Fourier-transform imaging spectrometer.

Fourier-transform imaging spectrometers offer important advantages over other spectral imaging modalities, such as, a wider free spectral range, higher spectral resolutions and, in low-photon-flux conditions, higher signal-to-noise ratios can be achieved. Unfortunately, for application in harsh environments, deployment of Fourier-transform instruments based on traditional moving-mirror interferometers is problematic due to their inherent sensitivity to vibration. We describe a new Fourier-transform imaging spectrometer, based on a scanning birefringent interferometer. This system retains the advantages of traditional Fourier transform instruments, but is inherently compact and insensitive to vibration. Furthermore, the precision requirements of the movement can be relaxed by typically two orders of magnitude in comparison to a traditional two-beam interferometer. The instrument promises to enable application of Fourier-transform imaging spectrometry to applications, such as airborne reconnaissance and industrial inspection, for the first time. Example spectral images are presented.

Generalization of the Lyot filter and its application to snapshot spectral imaging

A snapshot multi-spectral imaging technique is described which employs multiple cascaded birefringent interferometers to simultaneously spectrally filter and demultiplex multiple spectral images onto a single detector array. Spectral images are recorded directly without the need for inversion and without rejection of light and so the technique offers the potential for high signal-to-noise ratio. An example of an eight-band multi-spectral movie sequence is presented; we believe this is the first such demonstration of a technique able to record multi-spectral movie sequences without the need for computer reconstruction.

Spectral imaging of the retina

IntroductionThe work described here involved the use of a modified fundus camera to obtain sequential hyperspectral images of the retina in 14 normal volunteers and in 1 illustrative patient with a retinal vascular occlusion.MethodsThe paper describes analysis techniques, which allow oximetry within retinal vessels; these results are presented as retinal oximetry maps.ResultsUsing spectral images, with wavelengths between 556 and 650 nm, the mean oxygen saturation (OS) value in temporal retinal arterioles in normal volunteers was 104.3 (±16.7), and in normal temporal retinal venules was 34.8 (±17.8). These values are comparable to those quoted in the literature, although, the venular saturations are slightly lower than those values found by other authors; explanations are offered for these differences.DiscussionThe described imaging and analysis techniques produce a clinically useful map of retinal oximetric values. The results from normal volunteers and from one illustrative patient are presented. Further developments, including the recent development of a ‘snapshot’ spectral camera, promises enhanced non-invasive retinal vessel oximetry mapping.

A static Fourier‐transform spectrometer based on Wollaston prisms

A static Fourier‐transform spectrometer has been designed based on Wollaston prisms. It produces an interferogram in the spatial domain which is recorded using a 2D camera. A combination of two Wollaston prisms localizes the interference fringes coincident with the plane of the detector, thereby facilitating an extremely compact design. A personal computer is used to calculate the wavelength spectrum of the input light from the Fourier transform of the interferogram. In addition to its use as a compact and efficient spectrometer, the instrument’s use as a laser wavemeter has also been demonstrated.

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.

Decomposition of the optical transfer function: wavefront coding imaging systems

We describe the mapping of the optical transfer function (OTF) of an incoherent imaging system into a geometrical representation. We show that for defocused traditional and wavefront-coded systems the OTF can be represented as a generalized Cornu spiral. This representation provides a physical insight into the way in which wavefront coding can increase the depth of field of an imaging system and permits analytical quantification of salient OTF parameters, such as the depth of focus, the location of nulls, and amplitude and phase modulation of the wavefront-coding OTF.

Single-pulse, Fourier-transform spectrometer having no moving parts

A Wollaston prism is used in the design of a polarizing Fourier-transform spectrometer with no moving parts. The effective path difference between orthogonally polarized components varies across the aperture of the instrument, forming an interferogram in the spatial rather than temporal domain. The use of a charge-integrating linear detector array permits the entire interferogram to be sampled simultaneously so that a full spectrum is obtained for a single pulse of light.

Spectral Imaging in a Snapshot

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. Proof-of-concept demonstrations are reported for imaging of the ocular fundus and microscopy of fluorescently labelled living cells.

Circularly symmetric phase filters for control of primary third-order aberrations: coma and astigmatism

A quartic phase retardation function is described that reduces the variation of the intensity of the focal point of incoherent imaging systems suffering from primary third-order aberrations limited to coma and astigmatism. Corresponding modulation transfer functions are shown to remain practically invariant for moderate amounts of coma and astigmatism.