Light Hazards

Although eyesight requires light, it has long been evident that exposure to excessive amounts of light, particularly at certain wavelengths, is hazardous to various parts of the eye, as indicated in the following:

The cornea and lens are particularly susceptible to injury from UV light (180–400 nm), from which photokeratitis and cataract can result.

The retina is susceptible to photochemical injury from blue light in wavelengths of 400– 550 nm (310–550 nm for an aphakic eye). Such susceptibility to damage is the basis for incorporating UV-blocking and blue-blocking chromophores in some intraocular lenses (IOLs).

The retina is susceptible to thermal injury from optical radiation occurring from the visible to near-infrared wavelengths of 400–1400 nm.

The lens of the eye is susceptible to thermal injury from near-infrared radiation in the wavelengths from 800 nm to 3000 nm.

The cornea and lens of the eye are susceptible to thermal injury from radiation in the wavelengths from 400 nm to 1200 nm.

The cornea is susceptible to thermal injury from optical radiation in the wavelengths from 1400 nm to 1 mm.

Clinical Applications

Polarization

Several stereopsis tests incorporate the use of linear polarizers. The well-known “stereo fly test” displays 2 slightly displaced images that linearly polarize light in perpendicular meridians. The person wears glasses containing linear polarizers, also at right angles to each other; each eye sees just one of the images, thus creating a ​3-dimensional effect.

The cornea, retinal nerve fiber layer, and crystalline lens partially polarize light. Circularly polarized light entering the eye is reflected back and emerges elliptically polarized. The glaucoma diagnosis (GDx) test employs this effect to measure the thickness of the nerve fiber layer. However, because the cornea and crystalline lens also affect polarization, they may introduce artifacts into GDx nerve fiber layer measurements. A variable corneal compensator (VCC) may help mitigate such corneal polarization artifacts. However, even using the VCC, keratorefractive surgery can cause alterations in GDx measurements.

Reflected light is somewhat polarized parallel to the reflecting surface, and in most environments, reflecting objects tend to be horizontal. Accordingly, sunglasses incorporate vertically oriented linear polarizers to decrease glare from horizontal surfaces. To reduce glare from the corneal reflection, some direct ophthalmoscopes incorporate a circular polarizer.

Interference

Reflections from the front and back surfaces of a thin film interfere with each other, and depending on