A diagram of the human eye. The eye is the area of greatest concern with respect to laser safety.The various parts of the eye are described below, along with descriptions of how laser radiation can harm them.
The cornea is a transparent living membrane that covers the eye. It is very thin and composed of epithelial cells over a supporting matrix. The cornea has no blood supply; thus, its cells have no defense system and are easily damaged, both thermally and mechanically. If corneal injury is minor, then regrowth of epithelium can occur within a day or so without any permanent abnormality. If more extensive injury occurs, corneal scarring may ensue, with loss of the capacity of the cornea to conduct clear images. Damage to the outer cornea ranges from uncomfortable to painful, but will usually heal in one to two days.
Damage to the cornea and the conjunctiva tissue surrounding the eye usually occurs at greater power levels than damage to the retina; therefore, these issues only become a concern for those wavelengths that do not penetrate to the retina (UV and IR radiation). Since amplification by the lens is not involved, injuries can also be caused by diffuse and noncoherent light.
The lens focuses light to form images on the retina. The lens does not have an active cellular turnover and thus cannot repair itself. Over the course of repeated minor injury, or even one major injury, the lens may become progressively more opaque. This condition is known as cataract formation. While cataracts are a common side affect of aging, they are accelerated by exposure to certain light energies, including lasers.
When UV or IR laser light enters the eye, much of the light is absorbed in the lens. Depending on the level of exposure, this may cause immediate thermal burns or the development of cataracts over a period of years.
The retina is made up of a thin layer of light-sensitive cells called rods and cones, so named for their basic appearance. The rods make up the vast majority of the retina (over 95% of the total retinal area) and are sensitive to both movement and light and dark. They are more sensitive to light than are the cones, but they cannot see color. The cones make up less than 5% of the total retinal area, but that area is the critical central part of the retina, called the fovea centralis, where we see both color and fine detail. The retina is an actual extension of the human brain. Retinal cells detect only what we call "visible light."
Laser light in the visible and near-infrared (IR) region (400–1,400 nm) that enters the eye is focused on the retina, creating a hazardous concentration of laser energy out of a "minor" laser source. One mW of visible laser radiation entering the eye deposits 100 W/cm2 at the retina.