Figure 1.

Illustration of the mechanism underlying retromode scanning laser ophthalmoscopy [8,9,10,11]. Once the fundus has been illuminated (incidental light), there are two types of light returning back to the instrument detector: a direct reflex and scattered light. Varying the imaging aperture allows the scanning laser ophthalmoscope to assess the light returning from various parts of the eye. (A) Direct confocal mode: a central confocal aperture limits the passage to almost exclusively directly reflected light from the illuminated point on the retina, while other sources of light scatter are blocked. This increases image resolution and contrast. (B) Indirect mode, the so-called “dark-field” mode (RA aperture): a central circular stop blocks the directly reflected light, while more widely scattered light can pass through an annular aperture. This creates low-contrast transillumination images. (C) Retromode (DR or DL aperture): the opening of the ring aperture is restricted and deviates laterally from the confocal light path. The laterally deviated aperture is used to collect the backscattered light from just one direction, and block the directly reflected light and the light scattered from the other directions. This smaller aperture produces higher-contrast transillumination images with a narrower depth of focus compared to the dark-field images.