ULTRAVIOLET RADIATION AND THE EYE - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 262

fraction of the global ultraviolet irradiance (diffuse plus direct radiation incident upon a
horizontal surface). Therefore, the acceptance angle (field-of-view) should mimic those
of the human eye. However, this acceptance angle varies with sky brightness. Thus, the
instrument field-of-view must be adjustable in an instrument designed to really measure
the UVR exposure dose to the cornea and lens. The UV exposure to the anterior
segment of the eye can be measured in a mannequin fitted with UV detectors at the
ocular positions, and measurements can be made with and without UV-absorbing
spectacles, or as would occur when a person wears different types of sunglasses. The
exposure is greatly affected by the type of sunglass frame and partially to the UV
transmittance of the sunglass lenses. In some instances, UV exposures of some specific
ocular tissues can actually equal or exceed those when not wearing sunglasses.
The retina is exposed to visible light and some IR-A radiant energy within an
imaged scene. Although the lid opening varies with ambient scene luminance
(brightness), it is possible to mathematically predict the opening of the lids and the
angular field-of-view from studies of lid opening. The light exposure to the retina is not
at all uniform in outdoor daylight conditions. The central and superior regions of the
retina receive much more light than the inferior retina (Figure 6). Thus, instrumentation
should simulate these geometrical factors.
Figure 6. Retinal illumination pattern. During most outdoor conditions, much of the inferior retina is
shaded, and only under conditions of bright sand or snow do both lids close to allow only the
illumination of the macula and a horizontal band on either side.