Taken from Quanta medical:
Exposure to the laser beam is not limited to direct beam exposure. Particularly for high powered lasers, exposure to beam reflections may be just as damaging as exposure to the primary beam.
Intrabeam exposure means that the eye or skin is exposed directly to all or part of the laser beam. The eye or skin is exposed to the full irradiance or radiant exposure possible.
Specular reflections from mirror surfaces can be nearly as harmful as exposure to the direct beam, particularly if the surface is flat. Curved mirror-like surfaces will widen the beam such that while the exposed eye or skin does not absorb the full impact of the beam, there is a larger area for possible exposure.
A diffuse surface is a surface that will reflect the laser beam in many directions. Mirror-like surfaces that are not completely flat, such as jewelry or metal tools, may cause
diffuse reflections of the beam. These reflections do not carry the full power or energy of the primary beam, but may still be harmful, particularly for high powered lasers. Diffuse reflections from Class 4 lasers are capable of initiating fires.
Whether a surface is a diffuse reflector or a specular reflector will depend upon the wavelength of the beam. A surface that would be a diffuse reflector for a visible laser may be a specular reflector for an infrared laser beam.
Eye Hazzards
The major danger of laser light is hazards from beams entering the eye. The eye is the organ most sensitive to light. Just as a magnifying glass can be used to focus the sun and burn wood, the lens in the human eye focuses the laser beam into a tiny spot than can burn the retina. A laser beam with low divergence entering the eye can be focused down to an area 10 to 20 microns in diameter.
The laws of thermodynamics do not limit the power of lasers. The second law states that the temperature of a surface heated by a beam from a thermal source of radiation cannot exceed the temperature of the source beam. The laser is a non-thermal source and is able to generate temperatures far greater than it's own. A 30 mW laser operating at room temperature is capable of producing enough energy (when focused) to instantly burn through paper.
Per the law of the conservation of energy, the energy density (measure of energy per unit of area) of the laser beam increases as the spot size decreases. This means that the energy of a laser beam can be intensified up to 100,000 times by the focusing action of the eye. If the irradiance entering the eye is 1 mW/cm
2, the irradiance at the retina will be 100 W/cm
2. Thus, even a low power laser in the milliwatt range can cause a burn if focused directly onto the retina.
NEVER point a laser at someone's eyes no matter how low the power of the laser.
Structure Of The Eye
Damage to the eye is dependent upon the wavelength of the beam. In order to understand the possible health effects, it is important to understand the functions of the major parts of the human eye.
cornea is the transparent layer of tissue covering the eye. Damage to the outer cornea may be uncomfortable (like a gritty feeling) or painful, but will usually heal quickly. Damage to deeper layers of the cornea may cause permanent injury.
Figure 9. Cross section of the human eye.
The
lens focuses light to form images onto the retina. Over time, the lens becomes less pliable, making it more difficult to focus on near objects. With age, the lens also becomes cloudy and eventually opacifies. This is known as a cataract. Every lens develops cataract eventually.
The part of the eye that provides the most acute vision is the
fovea centralis (also called the
macula lutea). This is a relatively small area of the
retina (3 to 4%) that provides the most detailed and acute vision as well as color perception. This is why eyes move when you read or when you look as something; the image has to be focused on the fovea for detailed perception. The balance of the retina can perceive light and movement, but not detailed images (peripheral vision).
If a laser burn occurs on the fovea, most fine (reading and working) vision may be lost in an instant. If a laser burn occurs in the peripheral vision it may produce little or no effect on fine vision. Repeated retinal burns can lead to blindness.
Fortunately the eye has a self-defense mechanism -- the blink or aversion response. When a bright light hits the eye, the eye tends to blink or turn away from the light source (aversion) within a quarter of a second. This may defend the eye from damage where lower power lasers are involved, but cannot help where higher power lasers are concerned. With high power lasers, the damage can occur in less time than a quarter of a second.
Symptoms of a laser burn in the eye include a headache shortly after exposure, excessive watering of the eyes, and sudden appearance of
floaters in your vision. Floaters are those swirling distortions that occur randomly in normal vision most often after a blink or when eyes have been closed for a couple of seconds. Floaters are caused by dead cell tissues that detach from the retina and choroid and float in the vitreous humor. Ophthalmologists often dismiss minor laser injuries as floaters due to the very difficult task of detecting minor retinal injuries. Minor corneal burns cause a gritty feeling, like sand in the eye.
Several factors determine the degree of injury to the eye from laser light:
- pupil size
- The shrinking of pupil diameter reduces the amount of total energy delivered to the retinal surface. Pupil size ranges from a 2 mm diameter in bright sun to an 8 mm diameter in darkness (night vision).
- degree of pigmentation
- More pigment (melanin) results in more heat absorption.
- size of retinal image
- The larger the size, the greater the damage because temperature equilibrium must be achieved to do damage. The rate of equilibrium formation is determined by the size of the image.
- pulse duration
- The shorter the time (ns versus ms), the greater the chance of injury.
- pulse repetition rate
- The faster the rate, the less chance for heat dissipation and recovery.
- wavelength
- determines where the energy deposits and how much gets through the ocular media.
Eye Absorption Site vs. Wavelength
The wavelength determines where the laser energy is absorbed in the eye.
Lasers in the visible and near infrared range of the spectrum have the greatest potential for retinal injury, as the cornea and the lens are transparent to those wavelengths and the lens can focus the laser energy onto the retina. The maximum absorption of laser energy onto the retina occurs in the range from 400 - 550 nm. Argon and YAG lasers operate in this range, making them the most hazardous lasers with respect to eye injuries. Wavelengths of less than 550 nm can cause a photochemical injury similar to sunburn. Photochemical effects are cumulative and result from long exposures (over 10 seconds) to diffuse or scattered light.
Table 3 summarizes the most likely effects of overexposure to various commonly used lasers.
Skin Hazzards
Lasers can harm the skin via photochemical or thermal burns. Depending on the wavelength, the beam may penetrate both the epidermis and the dermis. The epidermis is the outermost living layer of skin. Far and Mid-ultraviolet (the actinic UV) are absorbed by the epidermis. A sunburn (reddening and blistering) may result from short-term exposure to the beam. UV exposure is also associated with an increased risk of developing skin cancer and premature aging (wrinkles, etc) of the skin. Thermal burns to the skin are rare. They usually require exposure to high energy beams for an extended period of time. Carbon dioxide and other infrared lasers are most commonly associated with thermal burns, since this wavelength range may penetrate deeply into skin tissue. The resulting burn may be first degree (reddening), second degree (blistering) or third degree (charring).
Some individuals are photosensitive or may be taking prescription drugs that induce photosensitivity. Particular attention must be given to the effect of these (prescribed) drugs, including some antibiotics and fungicides, on the individual taking the medication and working with or around lasers.
KME
string went out with yankee's
