Our measurements confirm theoretical predictions of PLF in the human eye.
9 13 16 Using an anatomically based model eye, we found significant focusing in the nasal corneal region of temporal light incident at oblique angles beyond the frontal plane
(Figs. 3 4 5) . We also confirmed the directionality of PLF with a peak of intensity as foreshadowed by model eye computations.
13 16 We found a peak effect for UVA at an incidence angle of 118 ± 3° and at 122 ± 3° for UVB. This compares with intensity peaks predicted at 104° for UVA (λ = 365 nm) and 107° for visible light (λ = 555 nm).
16 To check the validity of the physical model eye, we compared our results with the theoretical intensification factor of peripherally focused UV light. The intensification factor for the physical measurements were of the same order of magnitude as the theoretical calculations. The intensification factor of peripheral light at the nasal limbus is calculated to be ×22.5 for visible light and ×8.5 for UVA (365 nm).
16 In the physical model eye without a contact lens, we found UVA intensification factors at the nasal limbus of ×15 to ×18.3.
Our measurements demonstrate that UV-blocking contact lenses are effective in significantly reducing PLF. This was found in tests with a light source at various oblique incidence angles as well as outdoors with the model eye exposed to sunlight. The attenuation of PLF was around an order of magnitude for UVA and UVB light for oblique angles beyond the frontal plane (incidence >90°). The oblique light path through a contact lens is calculated to increase absorption by 6% for UVA. The optical powers tested did not significantly affect the attenuating properties of the UV-blocking contact lens. The reduction of UV-induced PLF was confirmed when a UV-blocking soft contact lens extinguished fluorescence from the porcine anterior chamber, indicating substantial blocking of obliquely incident UVR.
In contrast, clear contact lenses (non-UV-blocking) had only a slight effect on PLF, mainly affecting the intensity at lower angles of incidence. The small, almost insignificant amount of protection provided by clear contact lenses may be attributable to the thickness and shape of the periphery of the contact lens. The present findings and calculations presented elsewhere
15 indicate that the lens periphery alters the nature of light-focusing but does not attenuate the PLF effect. Our measurements suggest that peripheral lens thickness and contact lens design differences may shift the incidence angle of peak focus intensity, depending on the different refractive powers. We point out that the focused light emergent from the nasal limbus was measured after a double passage through the cornea, rather than after a single passage followed by partial absorption.
13 23 As a consequence, our results would underestimate light concentration in the living human eye. However, this was assumed not to affect the comparison of the relative effect of contact lens wear, because the incident light traversed the cornea twice in both non-lens- and lens-wearing conditions.
Standard sunglasses (front-face only, non-wraparound type) provided no protection at all from PLF in sunlight
(Figs. 6 7) . These findings expose a deficiency in UVR protection by sunglasses and spectacles that is not widely recognized.
24 Even for front-on irradiance, the lens material may have inadequate absorption for UV protection,
15 25 26 and sunglasses must be correctly worn for optimum performance.
27 However, most sunglasses even when properly fitted are ineffective against obliquely incident irradiation.
3 24 Our outdoor measurements indicate that diffuse UVR can generate significant levels of PLF, and we conclude that traditional sunglass designs offer limited protection against PLF from this unsuspected source of UVR. We measured UV irradiance on cloudless days during the midday to midafternoon at a time of year when the solar zenith angle was small. In eastern Australia where our measurements were taken, these conditions tend to favor lower diffuse UV irradiation.
28 On cloudy days, Kimlin et al.
29 found that UV irradiation on the nose and face increases as a result of scattering of UV by cloud cover, although this finding may apply only in subtropical latitudes (27.5° south, 151.9° east) Therefore, our results may underestimate PLF under cloudy conditions.
It is worth noting that Sakamoto et al.
30 used a mannequin head with an anatomically average Japanese bone structure and exposed it to direct solar UVR. They found that the total UV surface irradiance in the temporal eyelid area was 12% higher than in the nasal portion. Between 12:10 and 3:02 PM, the temporal eyelid irradiance increased by 36%, whereas the nasal eyelid irradiance remained fairly constant. The imposition of clear glass spectacles reduced the temporal, central, and nasal surface intensities by only 1.5%, 6.4%, and 4.6%, respectively. In contrast, our mannequin head mimicked white facial anatomy
(Fig. 2) . Dosimetry studies of the white facial erythemal UV distribution show that, although the orbital area is relatively spared during outdoor exposure to direct UVR,
19 20 24 29 31 the temporal eyelid is relatively more exposed with an irradiance more than twice the nasal side level.
30 31 This explains why the temporal cornea has the highest UV irradiance when contact lens dosimetry is used.
32 In our system, the main contributor to PLF was diffuse UVR, which predominates above 30° solar elevation angle
33 (our γ
s was 64–77°) and the photodetector was aligned horizontally with half-angle of acceptance 1/2α < γ
s. To test the effectiveness of direct irradiance, we rotated the mannequin head by 90° to face the sun and confirmed a similar PLF effect for UVA and UVB (data not presented). This agrees with the measurements of Narayanan et al.,
34 who used a similar apparatus. Increased albedo from highly reflective ground, such as sand, can increase irradiance of the face and orbit
14 19 24 and possibly increase PLF. It appears that both direct and diffuse UVR can concentrate light at the nasal limbus.
Our findings indicate that UV-blocking contact lenses may provide a supplementary but important means of protecting eyes from chronic exposure to high levels of UV light. Consequently, the possibility arises that the risk of eye diseases such as pterygium and early cortical cataract may be reduced by wearing UV-blocking contact lenses. We confirm theoretical predictions that the UV-blocking contact lens can attenuate the amount of peripherally focused UV light by up to an order of magnitude, although our results are likely to be underestimates of the effect of PLF in vivo. Our data indicate that the UV-blocking lens was effective against UVA and UVB in different environments, even in the mountain area, where UV irradiance was highest, presumably from high levels of scattered UVR.
35 Contact lenses may have an additional advantage when the wearer is under tree shade where the diffuse UVA irradiation is still significant
36 37 and there is a higher proportion of UVB than in direct sunlight.
38
This study of peripheral light-focusing highlights the need to review current standards of UV protection by sunglasses and contact lenses. The inadequacy of most sunglass designs against PLF and the protection provided under different insolation conditions merit further study. The facial anatomy
30 and skin pigmentation
24 may predispose some wearers to a higher risk of PLF. Some groups of outdoor workers such as sailors
39 are at risk of chronic overexposure to UVR.
40 Spectacles are not feasible for some outdoor activities such as surfing,
41 and contact lenses may be the only effective protection against diseases such as pterygium.
42 Our results imply that diffuse UVR can initiate PLF in both the UVA and UVB wavebands and that corneoscleral optics is an effective modulator of light concentration. The preferential focusing found in the current study may be wavelength dependent, and although the action spectrum of PLF is unknown, protective contact lenses must have a specific formulation to block UVR, because simple tints are inadequate.
43 Sliney
24 35 has pointed out that dark sunglasses may undermine UV protection by disabling the eye’s natural reactions to sunlight such as squinting and may initiate counterproductive responses such as pupil dilation. Standard sunglasses reduce direct UV irradiance of the eye but do not eliminate the inferonasal concentration of light.
33 To counter UV-induced PLF, sunglasses should have adequate side protection to act as a horizon shade, as proposed by Urbach.
19 There is a need to revisit Sliney’s concept
33 of protective eyewear as “spatial filters” and to reevaluate sunglass design to incorporate protection against PLF. Further study of the effectiveness of protective visual devices against PLF due to direct and scattered UVR is indicated.
The authors thank Therese Pham for technical assistance.