Adverse effects of sunlight on the temperature of ocular tissues.

Sunlight radiation is one the most important environmental factors affecting the health of the human eye, motivating investigations on the thermal effects of external high-intensity radiation sources, and photochemical effects of sunlight itself. Here we examine the thermal effect of natural sunlight, especially its invisible infrared spectrum interacting with the human eye, and how this interaction is influenced by in-vivo natural reflexes of the eye-change in eyelid opening (squinting) and pupillary size-as well as by the presence of sunglasses. We show that the peak eye temperature occurs near the pupil and that both, natural, "built in" protection mechanisms of the eye, as well as proper use of sunglasses, markedly reduce imparted thermal stress. As a countermeasure to sunlight heating, we show that additional infrared-blocking coatings on sunglasses can reduce the thermal stress of the pupil by up to 67% under 1 sun solar irradiation, at an ambient temperature as low as 30 °C.