Page 44 - Computational Retinal Image Analysis

34 CHAPTER 3 The physics, instruments and modalities of retinal imaging

Thermal damage is most important at longer wavelengths (>500 nm). The sensitivity
of the eye to damage depends on the intensity of the radiation and on the ability of the
tissue to diffuse the localized heat, and therefore it has different exposure limits based
on whether the illumination is a point-light or extended source and whether it is pulsed
or continuous. That is not the case for photochemical damage, which dominates for
shorter (blue, UV) wavelengths and longer exposures, as it depends only on the dose of
exposure. That is, the same damage can occur from intense radiation on short duration
than from less intense radiation but on longer durations (called the reciprocal principle).
Exposure hazards at cornea and lens are thermal and apply mainly to infrared wave-
lengths. The relevant recommendations of maximum exposure limits for ophthalmic
instruments can be: thermal or blue-light hazards, for continuous or pulsed sources, and
the limits vary with duration, extension and spectrum of the light source.

3.3 The fundus camera
The imaging principle of the fundus camera is similar to the indirect ophthalmoscope
and is essentially a low-magnification microscope, where the eye fulfills a similar
function to a microscope objective for imaging the retina. The short-focal length ob-
jective of the fundus camera forms an aerial image of the retina in its back focal plane.
Additional optics then re-image the aerial image onto a digital camera. The traditional
design invariably implements the Gullstrand principle to suppress reflections (separa-
tion of the illumination and imaging light paths near the cornea and pupil of the eye
as shown in Fig. 3 and also in Fig. 10). It includes an illumination system that projects
an annulus of light onto the eye pupil. The angular subtense of the cone of the illumi-
nation light at each point of the annulus corresponds to the field of the retina that is
illuminated and this is adjusted to ensure even illumination across the imaged field of
view. For cameras with an adjustable field of view (zoom) the illumination also ad-
justs in synchronism with the zoom. This variable illumination has clear implications
if flat fielding is used to compensate for variations in retinal illumination.
The coaxial illumination annulus and imaging is typically achieved using an an-
nular mirror for combining illumination and imaging beams as shown in Fig. 10.
A low-intensity inspection lamp is combined with high-intensity flash illumination
using some beam-splitting arrangement. The flash illumination has a duration of be-
tween about 1 and a few 10 s of ms, depending on energy setting, enabling the freez-
ing of eye motion that is desirable for a high-quality image to be recorded. Filters are
incorporated into the fundus camera to prevent harmful ultraviolet and infrared light
from illuminating the eye.
Fundus cameras that are designed for use with mydriasis enable an annulus with a
larger diameter in the region of 6–8 mm to be used, whereas a non-mydriatic fundus
camera requires that the annulus must fit within a smaller pupil of typically 4 mm
diameter. In consequence the central dark region is about twice the diameter for a
mydriatic camera allowing four-times greater light transmission to the camera. Some
non-mydriatic cameras employ infrared illumination for inspection and require a
dark room to enable natural dilation of the pupil.