Benjamin Lochocki

Single pixel camera ophthalmoscope

Ophthalmoscopes to image the retina are widely used diagnostic tools in ophthalmology and are vital for the early detection of many eye diseases. Although there are various effective optical implementations of ophthalmoscopes, new, robust systems may have a future practical importance in cases where ocular media present significant opacities. Here, we present, as a proof of concept, a novel approach for imaging the retina in real time using a single pixel detector combined with spatially coded illumination. Examples of retinal images in both artificial and real human eyes are presented for the first time to our knowledge.

Spatial and spectral characterisation of the first and second Stiles–Crawford effects using tuneable liquid-crystal filters

A semi-automated bipartite Maxwellian illumination system was used to examine the Stiles–Crawford effects of the first and second kind for the fovea of the authors’ eyes. The illumination was realised with a tungsten–halogen source in combination with tuneable liquid-crystal spectral filters. Visibility and hueshift dependencies were examined spatially and spectrally. They have been found to be in fair agreement with former studies in terms of overall appearance but with a notable reduction in directionality for the blue spectral range. The observations are discussed in relation to recent theory that relates the Stiles–Crawford effect of the second kind to the Stiles–Crawford effect of the first kind via a pigment-specific colour visibility function. The results confirm a direct relationship between the two effects and a reduction in hueshift for narrowband illumination. Differences in the green spectral range remain, however, that may be a consequence of a different directionality of S-cones as compared to the M- and L-cones.

Performance evaluation of a two detector camera for real-time video

Single pixel imaging can be the preferred method over traditional 2D-array imaging in spectral ranges where conventional cameras are not available. However, when it comes to real-time video imaging, single pixel imaging cannot compete with the framerates of conventional cameras, especially when high-resolution images are desired. Here we evaluate the performance of an imaging approach using two detectors simultaneously. First, we present theoretical results on how low SNR affects final image quality followed by experimentally determined results. Obtained video framerates were doubled compared to state of the art systems, resulting in a framerate from 22 Hz for a 32×32 resolution to 0.75 Hz for a 128×128 resolution image. Additionally, the two detector imaging technique enables the acquisition of images with a resolution of 256×256 in less than 3 s.

Defocus-corrected analysis of the foveal Stiles–Crawford effect of the first kind across the visible spectrum

The Stiles–Crawford effect of the first kind describes a gradually diminished visibility of light that enters the eye towards the pupil rim. Although of retinal origin, it is commonly described by a Gaussian pupil apodization whose width is determined by a directionality parameter that depends on retinal eccentricity, wavelength and spatial coherence of the light. As the measurements are done psychophysically they are prone to subjective variations and difficult to obtain across the visible spectrum. In this work, requirements for accurate refractive correction when determining the directionality parameter at any given wavelength are discussed and we show that a current-controlled tunable liquid-polymer lens provides a convenient means to accomplish this without requiring mechanical readjustments. This may be the most convenient way to combat defocus across the visible spectrum in the analysis of the Stiles–Crawford effect as demonstrated through experiments and with a detailed Zemax eye-and-system analysis. The results obtained are discussed in relation to myopia and a reduced directionality for highly myopic eyes.

Uniaxial flicker analysis of the psychophysical Stiles-Crawford effects

Abstract Purpose: We report on a semi-automated system for frequency analysis of the Stiles–Crawford effect of the first kind (SCE-I) using flicker methodology designed to gain insight into the temporal dynamics of the perceived visibility for alternating pupil entrance points. We describe the system and its calibration in detail and discuss psychophysical measurement data obtained for the two authors. Methods: A uniaxial system is used for SCE-I characterization of two emmetropic subjects as a function of flicker frequency for narrow wavelength bands chosen in the range of 450–700 nm using a fibre-guided tungsten–halogen lamp as light source. The flicker is realized using two orthogonally mounted galvanometric scanning mirrors that allow linear trajectories at any angle across the pupil. A fast tuneable liquid-crystal neutral density filter is used for brightness adjustment and another liquid-crystal filter is used for wavelength adjustment at each pupil point allowing simultaneous hue-shift determination for the Stiles–Crawford effect of the second kind (SCE-II). Results: Validation of the system is realized with a CCD camera, a spectrometer and a powermeter, and the data obtained are used in the software to calibrate all subsequent human subject measurements. The psychophysical data obtained show a strong frequency dependence of the Gaussian SCE-I with a characteristic directionality parameter, ρ, that is found to increase from 0.03 to 0.06/mm2 with flicker in the range of 1–10 Hz. The simultaneously determined hue shift could not be determined beyond 1 Hz due to the longer time required for a subjective determination. Conclusion: We have reported on a fast uniaxial system for temporal characterization of the SCE-I. The psychophysical results obtained show that accurate specification of frequency in flicker analysis is mandatory when comparing SCE-I visibility and directionality curves obtained with those obtained using quasi-static bipartite fields. A uniaxial design offers unique advantages over that of common two-channel systems by completely eliminating spectral errors or brightness differences in the two branches that otherwise will impose on those of the visual system and degrade the psychophysical data. Future work with more subjects will be used to narrow the uncertainty and the causes of the effects observed.

A single pixel camera video ophthalmoscope

There are several ophthalmic devices to image the retina, from fundus cameras capable to image the whole fundus to scanning ophthalmoscopes with photoreceptor resolution. Unfortunately, these devices are prone to a variety of ocular conditions like defocus and media opacities, which usually degrade the quality of the image. Here, we demonstrate a novel approach to image the retina in real-time using a single pixel camera, which has the potential to circumvent those optical restrictions. The imaging procedure is as follows: a set of spatially coded patterns is projected rapidly onto the retina using a digital micro mirror device. At the same time, the inner product’s intensity is measured for each pattern with a photomultiplier module. Subsequently, an image of the retina is reconstructed computationally. Obtained image resolution is up to 128 x 128 px with a varying real-time video framerate up to 11 fps. Experimental results obtained in an artificial eye confirm the tolerance against defocus compared to a conventional multi-pixel array based system. Furthermore, the use of a multiplexed illumination offers a SNR improvement leading to a lower illumination of the eye and hence an increase in patient’s comfort. In addition, the proposed system could enable imaging in wavelength ranges where cameras are not available.

Non-Maxwellian characterization of the Stiles-Crawford effect by wavefront-projected tilt

The Stiles-Crawford effect is a result of cone photoreceptor waveguiding. Typically, a Maxwellian setup is used to obtain the characteristic visibility function. We analyse the impact of a non-Maxwellian system using a coherent light source.