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Lasers image the retina in color

17 Jun 2002

A laser-based ophthalmoscope that produces color images is helping scientists diagnose retinal disease.

A team of researchers based at the University of Aberdeen, UK, has developed a laser-based ophthalmoscope capable of producing color images of the back of the eye. The color images from the scanning laser ophthalmoscope (SLO) make it easier for clinicians to identify different retinal diseases (Physiol. Meas. 23 1).

Monochromatic flash cameras are traditionally used to image the retina. Such cameras use a high-intensity flash to dilate the pupil and illuminate the retina. Reflected light then passes back through the pupil but the image quality observed is reduced due to the high-intensity illumination.

As an alternative, the SLO uses a narrow laser beam to illuminate the retina such that a lower intensity light can be used. Ayyakkannu Manivannan, who leads the research, explained that "the input beam occupies only a very small part of the pupil, allowing more reflected light to pass through a large exit pupil. This permits lower intensity illumination, with resulting comfort for the patient."

Other advantages include improved contrast and real-time digital imaging allowing a diagnosis to be made at the time of observation.

The SLO uses three basic color lasers operating over the visible spectrum: an argon-ion laser operating at 488 nm (blue); a frequency-doubled YAG laser at 532 nm (green); and a diode laser at 670 nm (red). Each laser is pulsed in sequence and then combined into a singlemode fiber with a length of 2 m meaning the laser system can be remote from the patient.

This multicolor beam passes through the pupil and is scanned across the retina, each point receiving a red, green and blue pulse before the beam moves on. Because only one laser is on at a time, the researchers were able to use a single avalanche photodetector to gather the reflected light.

The researchers interfaced the photodetector to a computer where retinal images were displayed in real time. The instrument has been used in vivo to image the blood vessels at the back of the eye and the researchers plan to continue this study.

Jacqueline Hewett is news reporter on optics.org and Opto & Laser Europe

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