Wave optics modelling of image formation in non-confocal adaptive optics scanning light ophthalmoscopy

Non-confocal adaptive optics scanning light ophthalmoscopy (AOSLO) provides non-invasive imaging of cellular-scale phase objects in the living eye. However, there is not yet agreement on the mechanism of contrast generation and hence how images should be interpreted. Recently, we described a ray model of optical lensing by compartmentalized tissue structures. To probe the limitations of this simple model, we applied numerical methods to simulate double-pass electromagnetic wave propagation through idealized phase objects in the human retina. The results show that (i) with the detector plane conjugate to the illumination plane, intensity at the detector plane is very smooth with a wide quasi-linear zone (even for a perfectly coherent source); (ii) the center of mass of the light distribution is deflected in proportion to the local phase gradient at the focal plane, even at object edges where the light distribution becomes significantly skewed; (iii) the signal of integrated light intensity falling upon off-axis detectors is robustly proportional to beam displacement and hence to local phase gradient. Skew arising from the illumination of object edges is equal and opposite across the midline, so it is well handled by comparison of symmetrically placed off-axis detectors; (iv) orthogonal non-confocal off-axis detector pairs may be combined to render a single image without directional bias, expressed by the mean spherical lensing effect which corresponds to the 2D Laplacian of the illumination-plane phase profile. Thus, non-confocal AOSLO demonstrably provides a quantitative phase-contrast imaging modality enabled by measurements of angular deflection of the scanned beam.