the tissue involved. Absorption is selective, as in the case of visible light; darker
material such as melanin or other pigmented tissue absorbs more energy.
c. Skin effects may vary from mild reddening (erythema) to blistering and
charring, depending upon the amount of energy transferred.
d. The effect upon the retina may be a temporary reaction without residual
pathologic changes or it may be more severe with permanent pathologic changes,
which may heal by fibrosis. The mildest observable reaction may be simple reddening--
as the energy is increased, lesions may occur which progress in severity from edema to
charring with hemorrhage and additional tissue reaction around the lesion. Very high
energies will cause gases to form, which may disrupt the retina and may alter the
physical structure of the eye. Portions of the eye other than the retina may be
selectively injured, depending upon the region where the greatest absorption of the
specific wavelength of the laser energy occurs and the relative sensitivity of tissue
affected.
e. Infrared light produces heat with its characteristic effect on tissue and the lens
of the eye. Some residual energy may reach the retina. Ultraviolet light can produce
symptoms similar to those observed in arc welders. It may cause severe acute
inflammation of conjunctiva and usually does not reach the retina. Light in the far
infrared, such as the 10 micron wavelength from the carbon dioxide lasers, is absorbed
by the cornea and conjunctiva and may cause severe pain and destructive effects (see
Figure 3-3).
3-13. CLASSES OF LASER
Because of the biological hazard, lasers are divided into classes. Hazard
controls vary depending on the type of laser being used and the manner of its use.
Most control measures depend upon the laser's classification.
a. In general, a Class I Exempt Laser Device is one that is considered to be
incapable of producing damaging radiation levels and is, therefore, exempt from any
control measures or other forms of surveillance.
b. A Class II Low Power Laser Device may be viewed directly, but must have a
cautionary label warning against continuous intrabeam viewing affixed to the device.
c. A Class III Medium Power Laser Device requires control measures that shall
prevent viewing of the direct beam.
d. A Class IV Laser or High Power Laser System requires the use of controls,
which shall prevent exposure of the eye and skin to the direct and diffusely reflected
beam.
MD0180
3-10
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