As with any therapeutic light application, nontherapeutic damage to the eye caused by light is primarily dependent on the wavelength as well as the irradiance and time of light exposure.
For example, the sun itself produces a power density (irradiance) of about 10 W/cm2, which will rise to 170 W/cm2 if that sunlight is focused and falls well within the range of thermal interaction (see Fig 2-14); in fact, retinal photocoagulation was first performed by focusing sunlight onto the retina. Similarly, prolonged irradiation or cumulative exposure from the operating microscope or indirect ophthalmoscope through a focusing lens, in particular on the eye with a dilated pupil, may be harmful to the eye. For instance, there is some evidence that cases of post-cataract-extraction cystoid macular edema are related to microscope illumination.
The anterior segment of the eye is essentially transparent to wavelengths from about 400 nm to 1400 nm and opaque to light outside that wavelength range, thereby reducing and protecting the subsequent ocular media from direct exposure to UV as well as infrared (IR) light. This is thanks to the crystalline lens essentially blocking UV-A light (315–400 nm) and the cornea essentially blocking UV-B (280–315 nm), UV-C (280 nm and below), as well as IR-B and IR-C (1400 nm to 1 mm) radiation. The filtering properties of anterior-segment components is inherently related to their absorptive capabilities, in case of UV radiation causing breakdown of the absorptive molecules in the cornea and lens, and in case of IR radiation causing a temperature rise and subsequent denaturation of protein in the cornea. Therefore, the anterior segment is not only susceptible to injury from UV irradiation, from which photokeratitis (from short-term exposure to UV-B and UV-C) and cataract (from long-term cumulative exposure) may result, but also to thermal injury from IR radiation. Damage from thermal injury may be caused from any light above 400 nm.
In the retina, natural chromophores, including melanin, hemoglobin, and xanthophyll, as previously discussed, strongly absorb wavelengths from about 400 nm to 580 nm. This makes the retina susceptible to photochemical injury in that region, especially from blue light. Note, that in an aphakic eye, this susceptibility to damage extends to below 400 (to about 315 nm) without the UV-A absorption capability of the natural lens, and is the basis for incorporating UV-blocking and blue-blocking chromophores in some intraocular lenses. The retina is also susceptible to thermal injury from optical radiation occurring from the visible to near-infrared wavelengths of 400–1400 nm.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.