Lasers
The word LASER is an acronym
for Light Amplification by Simulated Emission
of Radiation. A laser is a source
of intense, coherent and directional optical radiation. Lasers and laser
systems are becoming quite common in today's high technology society.
Lasers are used for a variety of applications. For example:
Research
- study of mechanisms at
interfaces
- detection of single
molecule
Medical
- eye surgery
- therapy for Carpel Tunnel
Syndrome
- photocoagulation
Commercial
- supermarket checkout
scanners
- determining site
boundaries for construction
Industrial
- cutting
- welding
Properties
of laser light
Laser light has three properties that differentiate it from ordinary
light. Laser
light is:
monochromatic
directional
coherent
Monochromatic
The light emitted from a laser is monochromatic, that is, it is of one
color/wavelength. In contrast, ordinary white light is a combination of many
colors (or wavelengths) of light.
Directional
Lasers emit light that is highly directional, that is, laser light is emitted
as a relatively narrow beam in a specific direction. Ordinary light, such as
from a lightbulb, is emitted in many directions away
from the source.
Coherent
The light from a laser is said to be coherent, which means that the
wavelengths of the laser light are in phase in space and time. Ordinary light
can be a mixture of many wavelengths.
Operating
principles
Lasers consist of a:
lasing
medium
excitation
mechanism
feedback
mechanism
output
coupler
Lasing medium
The lasing medium of a laser is a substance that emits light in all
directions and can be a gas, liquid, solid, or semiconducting
material.
Excitation mechanism
The excitation mechanism of a laser is the source of energy used to
excite the lasing medium. Excitation mechanisms typically used are electricity
from a power supply, a flashtube, lamp, or the energy from another laser.
Feedback mechanism
A laser's feedback mechanism is used to reflect light from the lasing
medium back into itself and typically consists of two mirrors at each end of
the lasing medium. As the light is bounced between the mirrors, it increases in
strength, resulting in amplification of the energy from the excitation
mechanism in the form of light.
Output coupler
The output coupler of a laser is usually a partially transparent mirror
on one end of the lasing medium that allows some of the light to leave the
lasing medium in order that the light be used for the production of the laser
beam. The output coupler is usually part of the feedback mechanism.
Types
of lasers
There are many types of lasers available for research, medical,
industrial, and commercial uses. Lasers are often described by the kind of
lasing medium they use: gas, liquid, solid, semiconductor, or dye.
Lasers are also often characterized by duration of laser light emission.
A continuous wave (CW) laser is a laser which emits a steady beam of light,
whereas a pulsed laser emits laser light in an off-and-on or pulsed manner. A
Q-switched laser is a pulsed laser which contains a shutter-like device that
does not allow emission of laser light until opened. Energy is built up in a
Q-switched laser and released by opening the shutter-like device to produce a
single very intense laser pulse.
Bioeffects
Mechanisms of tissue damage
There are three mechanisms by which tissue can be damaged by laser light:
Thermal
Acoustic
Photochemical.
Thermal Effects
Thermal effects are the major cause of tissue damage by lasers. Energy
from the laser is absorbed by the tissue in the form of heat, which can cause
localized, intense heating of sensitive tissues. The amount of thermal damage
that can be caused to tissue varies depending on the thermal sensitivity of the
type of tissue. Thermal effects can range from erythema
(reddening of the skin) to burning of the tissue. Factors that affect thermal
damage to tissue are:
- amount of tissue affected
- wavelength of light
- energy of the beam
- length
of time that the tissue is irradiated.
Acoustic Effect
Laser beams are capable of causing a localized vaporization of tissue
which in turn can create a mechanical shockwave to be propagated through the
tissue. Shockwaves can cause tearing of tissue.
Photochemical
Laser light can also cause changes to the chemistry of cells, which can
result in changes to tissue.
Wavelengths
of concern
The portion of the electromagnetic (EM) spectrum concerned with lasers is
called the optical portion of the spectrum, which consists of the infrared
(IR), ultraviolet (UV) and visible portions of the EM spectrum:
|
Infrared (780 nm - 1 mm) |
Ultraviolet (200-400 nm) |
Visible (400-780 nm) |
|
Far-IR (IR-B and IR-C) (1400 nm - 1mm) |
Far-UV (UV-B and UV-C) (200-315 nm) |
|
|
Near-IR (IR-A) (780-1400 nm) |
Near-UV (UV-A) (315-400 nm) |
|
Effects
on eye tissue
The parts of the eye which are of concern in a discussion of laser
hazards to the eye are the cornea, lens, pupil/iris, and retina.
Cornea
The cornea is the transparent layer of tissue covering the surface of the
eye. The cells on the surface of the cornea have a lifetime of only about 48 hours, therefore cell turnover is quite fast. Injury to
cells on the surface of the cornea is generally repaired quickly, but injury to
deeper layers of the cornea can result in permanent change to the cornea.
Lens
The lens of the eye focuses light to form images in the eye. Damage to
the lens can cause the destructive interference of light within the lens,
resulting in a "milky" area or cataract.
Pupil/iris
The pupil of the eye is the aperture through which light is directed into
the eye. The iris is a layer of muscle tissue which can contract and expand
around the circumference of the pupil in order to determine pupil size.
Retina
The retina is made up of layers of nerve cells and is used for reception
of the light in the eye. Damage to cells in the retina can result in loss of
vision.
Sensitivity to specific wavelengths
Visible and IR-A
Visible and IR-A wavelengths of light are transmitted through the cornea
and lens of the eye, and are absorbed mostly by the retina. The visible and
IR-A portions of the spectrum (400-1200 nm) are often referred to as the
"Retinal Hazard Region" because these wavelengths of light can damage
the retina. The amount of hazard to the retina from viewing
of a laser beam in the Retinal Hazard Region increases with increased pupil
size and increased duration of the laser beam.
UV-A
UV-A wavelengths of light are mostly absorbed in the lens of the eye and
can cause photochemical damage to the lens.
UV-B, UV-C, IR-B, IR-C
UV-B, UV-C, IR-B and IR-C are absorbed by the cornea of the eye. Exposure
to these wavelengths can result in conjunctivitis, "milky" cornea,
and inflammation.
Effects
on skin
The layers of the skin which are of concern in a discussion of laser
hazards to the skin are the stratum corneum, epidermis,
and the dermis.
Stratum corneum
The stratum corneum is the outermost layer and
consists of dead epithelial cells. The stratum corneum
is capable of filtering some ultraviolet and far-IR wavelengths of light and
prevents the light from penetrating to the deeper layers of the skin.
Epidermis
The epidermis layer lies beneath the stratum corneum
and is the outermost living layer of the skin.
Dermis
The dermis mostly consists of connective tissue and lies beneath the
epidermis.
Sensitivity
of specific wavelengths
IR-A
IR-A wavelengths of light are absorbed by the dermis and can cause deep
heating of skin tissue.
UV-B, UV-C
UV-B and UV-C, often collectively referred to as "actinic UV,"
can cause erythema and blistering as they are
absorbed in the epidermis. UV-B is a component of sunlight that is thought to
have carcinogenic effects on the skin.
Laser
Exposure Levels:
MPE
NHZ
NOHD
MPE
Definition: MPE, or Maximum Permissible Exposure, is the the maximum level of laser radiation to which a human can
be exposed without adverse biological effects to the eye or skin.
Factors important in determination of MPE
There are three factors involved in the determination of the MPE:
The wavelength of the laser light
the
energy involved in the exposure
the
duration of the exposure
MPE values for eyes and skin are listed for various combinations of
wavelength and exposure duration in tables 5 and 7 of ANSI Standard
Z136.1-1993, which is available from the Laser Institute of America.
To go to the site of Laser Institute
of America click here.
NHZ
Definition: NHZ stands for Nominal Hazard Zone, and is the zone inside
which laser radiation that is direct, reflected, or scattered exceeds the MPE for
the laser. Control measures are not needed outside the NHZ.
NOHD
Definition: NOHD is an acronym for Nominal Ocular Hazard Distance. The
NOHD is the distance along the axis of the direct laser beam to the human eye
beyond which the MPE of the laser is not exceeded.
Non-beam
Hazards
Electrical
Chemical
Non-beam Optical
Explosion/Fire
Electrical
Hazards
Sources of electrical hazards:
The most widely encountered non-beam hazard from
lasers is electric shock, or even death, from sources of electricity. Sources
of electrical hazard from lasers come primarily from the power supply of CW
lasers and the capacitor banks of pulsed lasers.
Injury prevention
The following precautions should be followed to
help prevent electrical injury when working around laser equipment:
Use one hand when working around
power supplies, capacitors, or other electrical equipment.
Avoid wearing metallic items.
Never handle electrical equipment
when hands are wet or when standing on wet ground.
Personnel should be trained in CPR
in case an electrical accident occurs.
Chemical Hazards
Sources of chemical hazards
One of the major sources of chemical hazards from lasers is from the
organic dyes used in dye lasers. Most dyes used in dye lasers are fluorescent
organic compounds. Some dyes (the rhodamines) are
considered to be mutagenic or carcinogenic, while other dyes (polymethine compounds) are toxic.
Additionally, some of the solvents used during dye preparation can be
irritants, highly toxic, and/or highly reactive.
Other hazards include gases from gas lasers, as well as gases that are
formed by the interaction of the laser with target materials, and coolants such
as liquid nitrogen.
Prevention
To prevent chemical accidents, the following are suggested:
preparation
of dye solutions should be carried out in fume hoods and/or glove boxes
personal
protective equipment such as lab coats, gloves and goggles, should be worn
during dye preparation
dye
solutions and reagents should be stored properly
adequate
ventilation shall be provided if gases are produced
Non-beam Hazards:
Sources of non-beam optical hazards
There are hazards from light generated by lasers
which do not originate
from the beam
itself. These optical hazards include:
UV light as a product of laser
welding
UV light from discharge tubes and
pumping lamps
visible
and IR-A light from pumping systems
Prevention
Hazardous levels of non-beam optical emissions
shall be shielded.
Explosion/Fire
Sources of explosion and fire hazards
The following are potential sources for
explosion hazards in lasers:
lamps
(arc lamps, filament lamps)
capacitor
banks
Potential sources of fire hazards include:
electrical
circuits
improper
beam enclosures
ignition
of gases or fumes from the laser
flammable
laser dyes
Prevention
There are several suggestions in ANSI
Z136.1-1993 for prevention of explosion and fire while using lasers:
components
that are capable of exploding shall be enclosed in housings that can withstand
a potential explosion.
beam
enclosures should be constructed of flame resistant materials .
electrical
circuitry shall be evaluated for the potential to cause fire .
Hazard Classifications
(ANSI Z136.1-1993
Laser Hazard Classification)
Class 1 (lowest powered
lasers and considered "harmless" unless tampering with the device has
occurred. An example of a Class 1 laser product is a CD-ROM player. )
Class 2 (low-powered lasers and only
considered a hazard if one intentionally stares into the beam. An example of a
Class 2 laser is a supermarket checkout scanner.)
Class 3a (medium-powered lasers that
pose an ocular hazard only if the
laser light is
collected and focused into the eye, such as through collecting optics. Many
laser pointers used during lectures are Class 3a lasers.)
Class 3b
(medium-powered lasers and pose an ocular hazard when the beam is viewed
directly.)
Class 4 (high-powered lasers
(>0.5 W in power or >125 kJ per pulse) and pose an ocular hazard not only
via the direct laser beam, but also from diffuse and specular
reflections. Class 4 lasers also pose a hazard to skin and can cause fires.)
Control Measures
Engineering
Adminstrative
PPE
Engineering Controls
Engineering controls are design features or
devices that are applied to a laser or its environment for the purpose of reducing
laser hazards. Engineering controls are considered to be the most effective
types of control.
Examples of engineering controls are beam
housings, beam shutters, attenuators, and remote firing controls.
Administrative Controls
Administrative controls consist of procedures
and information provided to personnel for the purpose of reducing laser hazards.
Examples of administrative controls include warning signs and labels, standard
operating procedures, and safety training.
PPE
PPE stands for Personal Protective Equipment.
Examples of PPE used to reduce laser hazards are eyewear, gloves and special
clothing. Of particular importance in prevention of laser hazards is eyewear.
Laser protective eyewear is usually made of filters which absorb and/or reflect
specific wavelengths of laser light.
There are three general types of laser
protective eyewear:
neutral
density - equally absorb and reflect visible wavelengths of light
cut-off
- transmits light at one end of spectrum, but not other
bandpass
- transmits light in a narrow range of wavelengths
An important factor to look
for in choosing laser protective eyewear is the optical density (OD) of the
lenses. The OD of the eyewear is a measure of its capacity to
filter light; OD is the opposite of transmission. The higher
the OD, the less the light that is transmitted to the eye. The OD must
be chosen so as not to impair vision significantly, yet at the same time, must
be chosen so as to be capable of reducing the laser light to the MPE.
When choosing laser protective eyewear it is
also important to select eyewear that is designed to filter the wavelengths of
the laser light that will be used. Eyewear designed to filter shorter
wavelengths of light are not appropriate for use with lasers that emit longer
wavelengths of light.
It should be noted that wearing of laser
protective eyewear that is reflective may pose a hazard to other personnel in
the vicinity, as laser light can be reflected into the eyes of others present
who may not be wearing protective eyewear.
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Last Update:
By: Serdar Z. Elgun