Wavelength-specific optical defocus can now be corrected for multiple colours simultaneously using a technique developed by researchers at the University of Washington. The team’s device, which works by varying the optical path of each wavelength by a different amount, was demonstrated in a system for testing subjects’ vision, by capturing aberration-corrected images of subjects’ retinas. As well as enabling high-resolution imaging of the retina, the approach could have applications in fluorescence imaging spectroscopy and the development of new corrective lenses (Optica 10.1364/OPTICA.6.000981).
Visual distortions introduced by irregularities in the eye act in both directions: they degrade the picture of the outside world as experienced by the subject, and they affect the clarity of retinal images captured using an ophthalmoscope. For the purposes of sight-testing or ophthalmoscopy, such higher-order aberrations (the kind not amenable to compensation by simple lenses) can be corrected using adaptive optics (AO).
But though this method is adaptable enough to handle distortions at the level of the individual eye, it does not differentiate between frequencies, applying the same correction across the full colour spectrum. As each frequency of light is refracted differently within the eye, once this correction has been applied there remains a residual wavelength-specific distortion – longitudinal chromatic aberration (LCA) – due to light of different colours being focused at different distances from the lens.
“For imaging and vision testing applications where multiple wavelengths need to be focused simultaneously on the retina, this wavelength-specific defocus needs to compensated precisely for every person,” explains Ramkumar Sabesan, who led the research group.
To achieve this in a vision-testing experiment, Sabesan and colleagues started with an AO-based system in which a subject’s contrast sensitivity is tested using patterns projected in red and green light. In this experiment, an infrared (monochromatic) reference beam is introduced into the subject’s eye, where it reflects off the retina. Distortions in the wavefront of the reflected infrared light carry information about the eye’s imperfections, and are corrected by a deformable mirror.
The deformable mirror configuration that restores the wavefront of the reference beam differs slightly from the configuration that would do the same for red or green light. The researchers therefore incorporated into the beam path a Badal optometer, in which a series of mirrors allow the optical distance between two lenses to be varied. The device was modified to include a long-pass filter that split the beam into two frequency bands. Green light was reflected onward to the next lens, while red light passed through to take an alternative, slightly longer path. This allowed a single deformable mirror to focus both wavelengths to the same degree.
The researchers used a similar setup for their retinal imaging experiment, with an infrared reference beam again informing the deformable mirror configuration required to correct monochromatic aberration. In this case, however, a supercontinuum laser was used to illuminate the subject’s retina in different visible frequency bands. These frequency bands entered a Badal optometer where they were separated by long-pass filters and directed along paths of different lengths.
Using this setup, the researchers were able to capture images of the retina at multiple wavelengths simultaneously, each with a resolution high enough to resolve individual cells in the fovea. The difference in focus required to achieve the best image for each frequency band varied between the four subjects in the experiment, demonstrating the benefit of being able to tailor the LCA correction to each individual. Until now, a population-average correction has been used.
“For many applications, the population-average estimate of LCA is sufficient,” says Sabesan. “However, in cases where cellular resolution across multiple wavelength bands is desired, small inter-individual variations need to be compensated.”
Next, the researchers intend to use this new ability to overcome chromatic imperfections in the eye to investigate how the retina perceives colour in the natural environment.
