Page 45 - Computational Retinal Image Analysis

3 Ophthalmic instruments 35

FIG. 10
Basic design of the traditional fundus camera.

Very wide fields of view require wide-angle illumination which compromises
the separation of illumination and imaging paths through the ocular media, and in
practice fundus cameras cannot provide ultrawide glare-free field-of-view due to
reflections and glare reducing image quality. See Fig. 3 and associated text for a
discussion. The considered “normal” field of view has traditionally been 30°, al-
though standard fundus cameras provide excellent quality images of 45° and even
higher [42].
The fundus camera can operate with different imaging modalities: in color, to re-
cord an image with similar color characteristics to the images that are observed with
the naked eye using an indirect ophthalmoscope; red-free, where the illumination
is filtered to remove red light and improve contrast of the vessels and other retinal
structures; and angiographic, where optical excitation of a fluorescent dye combined
with imaging of the fluorescence at a longer wavelength enables high-contrast imag-
ing of the retinal vasculature and sequential recording enables the dynamics of vessel
filling to be recorded.

3.4 Indirect ophthalmoscopes
To use an indirect ophthalmoscope a high-power condensing lens is held a few cm
from the eye to form an aerial image of the retina in its back focal plane. The aerial
image of the retina is viewed with a head mounted indirect ophthalmoscope, which
also provides illumination of the retina through the condensing lens. The field of
view is typically only a few degrees requiring the viewer to navigate around the
retina to view the whole retina in time sequence. A particular challenge is to view the
retina while reducing the impact of the reflections from the cornea. Choosing a lens
with a higher optical power will: decrease the magnification, decrease the working