Confocal retinal imaging using a digital light projector with a near infrared VCSEL source

Abstract

A custom near infrared VCSEL source has been implemented in a confocal non-mydriatic retinal camera, the Digital Light Ophthalmoscope (DLO). The use of near infrared light improves patient comfort, avoids pupil constriction, penetrates the deeper retina, and does not mask visual stimuli. The DLO performs confocal imaging by synchronizing a sequence of lines displayed with a digital micromirror device to the rolling shutter exposure of a 2D CMOS camera. Real-time software adjustments enable multiply scattered light imaging, which rapidly and cost-effectively emphasizes drusen and other scattering disruptions in the deeper retina. A separate 5.1″ LCD display provides customizable visible stimuli for vision experiments with simultaneous near infrared imaging.

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“…Detection is performed using a complemenrtary metal‐oxide‐semiconductor (CMOS) rolling shutter to read sequentially only those in synchronisation with the illumination lines, providing an electronic rolling shutter as described previously . When sampling at 1024 × 1024 pixels for 38 and 28 deg, the digital resolution was nominally 11.1 and 8.2 μm/pixel, respectively, for an emmetropic eye.…”

“…Contrast in images is improved compared with fundus photography by using imaging devices that incorporate (1) scanning of the illumination and confocal detection apertures to limit unwanted light from other sources and/or (2) illumination and detection with coherent light using methods such as OCT to boost the signal and allow excellent axial resolution. In scanning techniques, a well‐focused and concentrated light source, such as a laser, is swept across the retina and the detection is synchronised with the illumination, and simultaneously constrained to either one point at a time, for example as in a scanning laser ophthalmoscope or one line at a time, such as in the line‐scanning laser ophthalmoscope, the laser scanning digital camera, or the digital light ophthalmoscope (DLO) . The latter two are wholly digital, in that a two‐dimensional detector array is used with one dimensional (line) illumination.…”

“…It does no good to scan light across the retina and then to use a sample‐and‐hold detector without a method to unmix the light from different retinal areas. The line scan technique can be used on a two dimensional detector, as long as there is a method for separating in space and/or time the light returning from two or more parts of the retina …”

“…In scanning techniques, a well-focused and concentrated light source, such as a laser, is swept across the retina and the detection is synchronised with the illumination, and simultaneously constrained to either one point at a time, for example as in a scanning laser ophthalmoscope 19,27,33 or one line at a time, such as in the linescanning laser ophthalmoscope, 34 the laser scanning digital camera, 35 or the digital light ophthalmoscope (DLO). 36,37 The latter two are wholly digital, in that a two-dimensional detector array is used with one dimensional (line) illumination. Scanning boosts contrast because it eliminates optical cross talk, i.e.…”

“…Adapted from Elsner et al 17 space and/or time the light returning from two or more parts of the retina. [35][36][37] Confocal and multiply scattered light imaging Contrast and image crispness are improved by the use of confocal apertures, often shaped like pinholes of varying diameters, in a plane conjugate to the retina. The confocal aperture blocks the detection of light from any structure that is not in focus, with light returning directly through the aperture typically providing a stronger signal than the small fraction of light that has bounced off structures many times and by chance passes through the aperture.…”