ULTRAVIOLET RADIATION AND THE EYE - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 259

during summertime conditions. Although a higher global erythemal effective irradiance
is measured at high altitude 46 , the atmospheric scatter (diffuse component) is less, and
the horizon-sky UVR does not increase by climbing mountains 5 . All of these
observations point to the complex geometrical factors that challenge any outdoor UVR
measurements.
Since the eye receives most of its outdoor UVR exposure from ground reflectance,
the proper measurement of reflected sunlight adds a further challenge. The ground
reflection of solar UV-B radiation varies over 100-fold. Green grass reflects less than 1%
and most artificial surfaces and rock normally reflects less than 10%. Two exceptions are
ocean surf and white, gypsum sand, which reflect about 20-25%. Fresh snow reflects
about 85%, thus producing “snow blindness.” Open water reflects the entire sky (diffuse
plus direct component) and can be of the order of 20%, although the direct, specular
reflection from the sun's image is only about 2-3% of the incident, direct UV radiation.
Table 2 provides more detailed values for ground reflectance
Ocular exposure is far more affected than skin exposure by these geometrical
factors. For an industrial UV source such as a welding arc, the cornea is shown to be
more sensitive to UVR injury than the skin, but photokeratitis seldom accompanies
summer sunburn of the skin. This seeming paradox is explained by the fact that people
do not look directly overhead when the sun is very hazardous to view, whilst most
people may stare at the sun when it is comfortable to observe near the horizon.
Fortunately, at sunset, the filtering of UVR and blue light by the atmosphere allows us
to directly view the sun. When the solar elevation angle exceeds 10 degrees above the
horizon, strong squinting is observed which effectively shields the cornea and retina
from most direct exposure. These factors reduce the exposure of the cornea to a
maximum 5% of that falling on the exposed top of the head. However, if the ground
reflectance exceeds 15%, photokeratitis may be produced after a few hours exposure. If
one were to ignore the squinting factor and proper instrument field-of-view, the
photokeratitis threshold would be achieved in less than 15 minutes for midday summer
sunlight!
When wearing sunglasses, the pupil dilates proportionally to the darkness of the
sunglasses 47 . Coroneo et al 29-30 have shown that very oblique temporal rays can be
refracted into the critical nasal equatorial region of the lens and this could explain the
increased incidence of opacification originating in the nasal sector of the lens in cortical
cataract. The protective value of upper and lower lids, when they close down during
squinting, determines the ocular UVR exposure dose in different environments. A
brimmed hat or other headwear - associated with or without dark sunglasses - will
modify greatly the UVR exposure dose. The geometrical factors that should be modeled
by a radiometer now begin to appear as almost insurmountable. What can be done?
Even geometrical positioning of the body greatly affects the solar exposure of human
skin.
14. Biologically relevant measurements
With ocular UVR exposure so dependent upon geometry, the measurement
challenge is not only to employ detectors with a spectral response that matches the
action spectrum for the biological effect, but also provide a match of the geometry.