can perforate the Bruch membrane, and choroidal neovascularization can result.
The cornea does not absorb enough visible light, even highly concentrated light, to produce a significant temperature increase. However, the cornea is opaque to some infrared wavelengths, so an infrared laser is used in laser thermokeratoplasty (LTK) to photocoagulate and shrink corneal collagen in the treatment of hyperopia.
Photoablation
Photoablation uses high-energy ultraviolet photons to break covalent chemical bonds. An excimer laser, for example, generates photons at a wavelength of 193 nm; these photons are absorbed by and break the covalent bonds in corneal collagen, thereby vaporizing the collagen molecules. Because the energy of photoablation is used only to break bonds, no heat is produced and the technique does not scar adjacent tissue. Presently, photoablation is used only for keratorefractive procedures.
Photodisruption
The posterior capsule is transparent to visible and near-infrared light, including the 1.06-µm wavelength produced by the Nd-YAG laser. This type of laser is pulsed, so the energy it produces is released in a very short time, producing a large momentary power. Also, the laser beam is focused, concentrating the power into a small area. In the vicinity of the focus, electrons are stripped from their atoms by ionization, but they quickly recombine, which produces a spark and an acoustic wave that mechanically disrupts the posterior capsule. (The spark is essentially a miniature lightning bolt, and the acoustic wave is analogous to thunder.) During a photodisruption procedure, it is the mechanical (acoustic) wave and not the laser light itself that breaks the capsule.
Photoactivation
Photoactivation is the conversion of a chemical from one form to another by light. Vision itself depends on the photoactivation (cis–trans isomerization) of rhodopsin in photoreceptor outer segments. A clinical application of photoactivation includes the use of verteporfin, a drug that remains chemically inert until activated by light, after which it destroys neovascular tissue.
Light Scattering
Molecules or small particles suspended in a medium disperse or scatter light. Light scattering by the ocular media increases with age; results include decreasing contrast of the retinal image, diminishing contrast sensitivity, and increasing glare sensitivity. Three mechanisms of light scattering—Rayleigh scattering, Mie scattering, and the Tyndall effect —influence the perception of light as well as the appearance of the eye.
Rayleigh Scattering
Rayleigh scattering is produced by particles that range in size from as small as molecules to as large as about one-tenth the wavelength of incident light. The degree of this form of light scattering varies