Light causes biological damage through both temperature effects due to absorbed energy and through photochemical reactions. The chief mode of damage depends on the wavelength of the light and on the tissue being exposed. For control of hazards from lasers, the damage is believed to be due principally to temperature effects, and the critical organs are the eye and the skin.
The structure of the eye is shown below. The optical components of the eye - those components that act together to focus an image of an object on the retina - are the cornea, aqueous humor, lens, and vitreous humor. The components of the eye most susceptible to laser damage are the cornea, retina, and lens. The active components of the eye are described in more detail below.
- Living tissue exposed directly to the environmental elements. It is protected by a thin tear film.
- The corneal epithelium has one of the highest metabolic rates in the entire body. The tear layer of 6-10 um thickness that protects this cell layer is fairly well balanced. The outer most surface of the tear layer is a superficial lipid mono-multilayer less than 0.5 um in thickness, then beneath this are mucin layers with gradually increasing concentrations of mucin. The result is the cornea has a mean index of refraction of 1.376. This provides approximately 70% of the refractive power of the eye.
- The cornea has a high metabolic rate - rejuvenating itself in 24 to 48 hours.
Pupil - Iris - Sclera
- Aperture of the eye.
- Normal range of 2 - 7 mm.
- Range decreases with age.
- 7 mm is used for hazard calculations.
- Adjusts the pupil of the eye.
- Circular, pigmented membrane.
- Lies behind the cornea.
- Dense fibrous shell.
- Maintains the roughly spherical shape of the eye along with the internal pressure of the eye.
- The retina is an extension of the brain and consists of several complex layers of nerve cells.
- Made up of rods and cones - rods for night and peripheral vision, cones for color and resolution.
- The macula is where the highest resolution takes place. The cones have a yellowish pigment to filter out blue light. Sharp vision is dependent on the formation of a real image on the macula.
- The fovea is in the center of the macula and is where the cones are concentrated.
- The crystalline lens is supported in place by fine ligaments which are connected to the ciliary body. The ciliary muscles control the eye's focusing ability.
- The lens is constructed of layers of cells, similar to the make-up of an onion.
- The lens provides fine tuning for the eye. It provides approximately 30% of the refractive power of the eye.
- The lens has a slow metabolism. Effects are delayed (cataracts). The lens hardens and yellows with age.
Light Induced Biological Damage
Laser irradiation of the eye may cause damage to the cornea, lens, or retina, depending on the wavelength of the light and the energy absorption characteristics of the ocular tissues.
The potential location of injury in the eye is directly related to the wavelength of the laser radiation. For laser radiation entering the eye:
- Near Ultraviolet Wavelengths (UVA) 315 - 400 nm
- Most of the radiation is absorbed in the lens of the eye.
- The effects are delayed and do not occur for many years (e.g.; cataracts).
- Far Ultraviolet (UVB) 280 - 315 nm and (UVC) 100 - 280 nm
- Most of the radiation is absorbed in the cornea.
- Keratocojunctivitis (snow blindness/welder's flash) will result if sufficiently high doses are absorbed.
- Visible (400 -760 nm) and Near Infrared (760 - 1400 nm)
- Most of the radiation is transmitted to the retina*.
- Overexposure may cause flash blindness or retinal burns and lesions.
- Far Infrared (1400 nm - 1 mm)
- Most of the radiation is transmitted to the cornea.
- Overexposure to these wavelengths will cause corneal burns.
NOTE: Laser retinal injury can be severe because of the focal magnification (optical gain) of the eye which is approximately 100,000 times. This means that an irradiance of 1 mW/cm2 entering the eye will be effectively increased to 100 W/cm2 when it reaches the retina.
More Notes on Ocular Laser Damage
- Thermal burns (lesions) in the eye are caused when the choroid layer blood flow cannot regulate the heat loading of the retina. Secondary bleeding into the vitreous humor may occur as a result of burns which damage blood vessels. This bleeding can obscure vision well beyond the area of the lesion.
- Although the retina can repair minor damage, major injury to the macular region of the retina may result in temporary or permanent loss of visual acuity or blindness. Photochemical injury to the cornea by ultraviolet exposure may result in photokeratoconjunctivitis (often called welders flash or snow blindness). This painful condition may last for several days and is very debilitating. Long term UV exposure can cause cataract formation in the lens.
- The duration of exposure also plays a role in eye injury. For example, if the laser is a visible wavelength (400 to 700 nm), the beam power is less than 1.0 mW and the exposure time is less than 0.25 second (the human aversion response time), no injury to the retina would be expected to result from an intrabeam exposure. Class 1, 2a and 2 lasers fall into this category and do not normally present a retinal hazard. Unfortunately, intrabeam or specular reflection viewing of Class 3a, 3b, or 4 lasers and diffuse reflections from Class 4 lasers may cause an injury before the aversion response can protect the eye.
0.25 seconds is considered the amount of time it takes a person to blink or avert their eyes.
- For pulsed lasers, the pulse duration also effects the potential for eye injury. Pulses less than 1 ms in duration focused on the retina can cause an acoustical transient, resulting in substantial damage and bleeding in addition to the expected thermal injury. Many pulsed lasers now have pulse duration less than 1 picosecond.
- The ANSI Z136.1 standard defines the Maximum Permissible Exposure (MPE) that the eye can receive without expecting an eye injury (under specific exposure conditions). If the MPE is exceeded, the probability that an eye injury can result increases dramatically.
The first rule of laser safety is: NEVER UNDER ANY CIRCUMSTANCES LOOK INTO ANY LASER BEAM! If you can prevent the laser beam and beam reflections from entering the eye, you can prevent a painful and possibly blinding injury.
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