Objective measurement of transverse chromatic aberration with the adaptive optics scanning laser ophthalmoscope

Transverse chromatic aberration (TCA) is one manifestation of chromatic dispersion of the ocular media, in which the images formed by light of different wavelengths are displaced angularly on the retina. The amount of displacement is a function of two factors, the wavelength difference and the displacement of the chief ray from the achromatic axis. While the impact of TCA on foveal vision is perceptually small, high-resolution retinal imaging, as exemplified by the adaptive optics scanning laser ophthalmoscope, demands correction of chromatic aberrations if light of different wavelengths is wished to be targeted to specific retinal locations. Here, an image-based measurement technique is presented that bears the possibility to record and correct TCA in real-time. By a spatially interleaved light delivery configuration, the images of two (or more) illumination channels can be recorded simultaneously. Using a fast cross-correlation algorithm, the spatial offset between correspondent video frames is computed and output as a vector field of offsets across each frame. Inversion of each vectors' direction serves to correct for two-dimensional TCA across the imaging field with sub-pixel precision, to make possible the acute delivery of cone targeted stimuli. Preliminary results are presented that were validated with a psychophysical hyperacute five-dot alignment task. Future applications for multi-wavelength retinal imaging and stimulation are discussed. Transverse chromatic aberration (TCA) is one manifestation of chromatic dispersion of the ocular media, in which the images formed by light of different wavelengths are displaced angularly on the retina. The amount of displacement is a function of wavelength difference and the displacement of the chief ray from the achromatic axis, along which - by definition - TCA is zero. While the impact of TCA on foveal vision is perceptually small, high-resolution retinal imaging, as exemplified by the adaptive optics scanning laser ophthalmoscope, demands correction of chromatic aberrations if light of different wavelengths is wished to be targeted to specific retinal locations. Here, an image-based measurement technique is presented that bears the possibility to record and correct TCA in real-time. By a spatially interleaved light delivery configuration, the images of two (or more) illumination channels can be recorded simultaneously. Using a fast cross-correlation algorithm, the spatial offset between correspondent video frames is computed and output as a vector field of offsets across each frame. Inversion of each vectors' direction serves to correct for two-dimensional TCA across the imaging field with sub-pixel precision, to make possible the acute delivery of cone targeted stimuli. Preliminary results are presented that were validated with a psychophysical hyperacute five-dot alignment task. Future applications for multi-wavelength retinal imaging and stimulation are discussed.