Laser
Fundamentals and Safety
NOTE:
The following is provided for
information purposes only and
is not intended to represent
a guideline for the safe use
of lasers or laser systems.
For
a more complete discussion of
laser safety principles and
regulations, please consult
the following organizations:
CDRH Center for Devices
and Radiological Health. An
agency within the U.S. FDA which
publishes and enforces legal
requirements on lasers.
To obtain copies of the CDRH
requirements, contact:
CDRH
(HFZ-312)
1390 Piccard Dr.
Rockville, MD 20850
Tele: (301)-427-1172
ANSI
A U.S. organization that publishes
standards for laser users. The
ANSI laser standard is not a
law, but it forms the basis
for state and OSHA requirements
for the use of lasers.
To obtain copies of the ANSI
Standards, contact:
Laser
Institute of America
12424 Research Pkwy. #130
Orlando, FL 32826
Tele: (407-380-1553
FAX: 407-380-5588
Laser
Fundamentals
What is a Laser?
The
word laser is an acronym which
stands for Light Amplification
by Stimulated Emission of Radiation.
As implied by its name, a laser
is a specialized light source
which is capable of producing
an intense beam of light (protons)
possessing several very special
properties. Every photon (particle
of light) produced by a laser
possesses identical scaler and
vector properties in terms of
frequency, phase, direction,
and polarization. These properties
are responsible for the real
world observations of a laser's
color, brightness, coherence,
and unidirectional.
The
principles of laser action were
first described by Townes and
Schalow in 1958 after first
being postulated by Einstein
in 1916. This initial technical
description of a laser and its
operation was followed by the
first demonstration of a working
laser in 1960 by Hughes Aircraft.
Properties of Laser Light
The
two primary properties of laser
light with respect to its use
in both laser shearography and
holography are interference
and the existence of laser speckle.
Interference
The term interference describes
the ability of multiple beams
of laser light to add together
either constructively or destructively.
Constructive and destructive
interference can be thought
of analogously in terms of waves
interacting in pool of water.
If the peaks of two identical
waves combine together, they
amplify each other and produce
a resultant peak which is essentially
twice as high. This is identical
to what happens when the peaks
of two light waves combine (in
phase) and is called constructive
interference. When the peak
of one wave combines with the
trough of another identical
wave, they will cancel each
other out. The corresponding
phenomenon is also demonstrated
with laser light and is called
destructive interference.
Laser
Speckle
Because of the ability of laser
light to interfere with itself,
multiple laser beams such as
those produced by the reflection
of laser light off of a physical
surface will combine to produce
what appears to be a random
pattern of bright and dark spots
when viewed with the human eye
or captured by a camera. This
seemingly random pattern referred
to as laser speckle, may be
used to define the relative
shape of the object being illuminated.
When
this information is combined
with a fixed wavefront (in this
case speckle pattern) having
the same properties as the laser
light illuminating the object
under investigation, an interference
pattern (also in the form of
a speckle pattern) is produced.
When this secondary speckle
pattern is compared to a similar
speckle pattern of the object
after being placed under or
subjected to some form of loading,
information about how the surface
of the object has been distorted
is revealed. This process, therefore,
provides the basis for both
electronic holography and electronic
shearography nondestructive
testing.
Laser
Safety
The most effective approach
to laser safety is one which
combines knowledge of the potential
hazards associated with a particular
laser system and common sense
usage. All lasers and laser
products are governed by a four
level classification guideline
set forth in the United States
by the Center for Devices and
Radiological Health (CDRH).
These classifications are based
on the relative dangers associated
with working with a given laser
system based upon the systems
operating wavelength (s), power
output (note: this term takes
on different meanings with regard
to continuous operation CW and
pulsed lasers), and beam expansion
characteristics.
For
laser systems, the overall classification
of the system is dependent upon
the aforementioned parameters
as governed by the design and
intended operation of that system.
If a laser system is modified
after manufacturing or is used
in a fashion other than what
it was designed for. The safety
classification of that system
must be considered as the highest
applicable to the laser associated
with that system.
Wavelengths
All lasers can be classified
into three broad wavelength
classifications: infrared, visible,
and ultraviolet. The wavelength
ranges for each of these classifications
is as follows:
Ultraviolet 180Nm-400Nm
Visible 400Nm-700Nm
Infrared 700Nm-1mm
As implied by its name, the
visible class of laser falls
fully within the range of human
vision. Although the upper extreme
of the ultraviolet region and
the lower extreme of the infrared
region are "visible"
these lasers are most often
referred to as invisible lasers.
Laser
Power Pulsed vs. Continuous
Operation (CW)
The measure of a laser's power
is based significantly upon
whether the laser in question
is designed to operate continuously
(CW) or in a pulsed mode. According
to the CDRH Standard, a pulsed
laser is any laser which delivers
its energy over a period of
0.25 seconds or less. Because
of significant differences in
laser operation, the power of
CW lasers is measured in terms
of the average power output
of the laser or laser system
during the designed operating
time of the system. For pulsed
lasers, the energy rating of
the laser or laser system is
given in terms of the total
energy per pulse, pulse duration,
repetition rate and emergent
beam radiant exposure.
NOTE:
At this time (4/94), all Laser
Technolgy, Inc. laser systems
are designed to operate in the
CW mode.
CDRH
Classifications
Class
1
Class 1 laser designations represent
the safest form of laser or
laser system. These systems
are either interlocked such
that the operator and bystanders
do not have access to the laser
emissions or are at a power
level low enough to where there
is no danger of damage to eyes
or exposed skin. No medical
surveillance of operators is
required for the operation of
such a laser or laser system.
Laser light may be visible or
non-visible.
Class
2
Class 2 laser designations apply
to visible lasers and laser
systems only. These lasers or
laser systems have an accessible
emissions level greater than
Class 1 but less than 1 mW.
The use of Class 2 laser systems
does not require a medical surveillance
program although general knowledge
of laser safety is required.
Recommended safety precautions
for the use of Class 2 laser
systems include:
1.
Never stare directly into the
laser beam, its source, or at
bright, specular surface requirements.
2. Limit access to the laser
system to those familiar with
its operation and safety requirements.
3. Ensure that bystanders are
briefed as to the presence of
the operating laser system and
that they should avoid staring
into the beam or at nay bright
reflection emanating from the
test area.
Class
3a
Class 3a laser designations
apply to visible lasers and
some non-visible lasers whose
power output exceeds that for
Class 2 (1mW) but is less than
5mW. Medical surveillance of
appropriate personnel is no
required for the use of Class
3a laser systems. Safety requirements
associated with the use of Class
3a laser systems are essentially
the same as those associated
with Class 2 lasers. The primary
safety concern for Class 3a
lasers is to avoid staring into
the laser beam or at bright
specular reflections emanating
from the test area. Applicable
hazard signs must be posted
in the immediate vicinity of
the work area as required.
Class
3b
Class 3b laser system designations
refer to both visible and non-visible
laser whose outputs exceed those
set forth for Class 3a lasers
but is less than 500mW to 500mW.
Hazards from Class 3b laser
systems include potential eye
damage as well as the potential
for burns to skin exposed to
the direct emissions of high
powered Class 3b lasers (i.e.
approaching 500mW).
Medical
surveillance for appropriate
personnel is required for the
use of Class 3b laser systems
as is a formalized introduction
to lasers and laser safety.
In addition to the safety requirements
mandated for Class 3a lasers,
the following must also be addressed.
1.
A control area around the work
area where Class 3b emissions
will be present must be established
and posted with appropriate
warning signs.
2.
A visual and audible warning
indicator must precede any laser
emissions.
3. The laser system must include
a keyed safety interlock to
prevent unauthorized use of
the laser system.
4. Laser safety glasses must
be made available to those working
in the immediate area of the
Class 3b emissions.
NOTE:
All Laser Technology, Inc. laser
systems incorporate an integral
beam expansion system which
causes the power density of
the laser emissions to drop
off rapidly with regard to the
distance from the source (1/r2
relationship). Because of this,
the restricted control area
for the laser systems tends
to be small (e.g. 5-10' beyond
test area).
Class
4
Class 4 laser systems represent
both visible and non-visible
laser system which exceed the
500mW restriction placed on
Class 3b laser systems. As in
the case of Class3b lasers,
medical surveillance of appropriate
personnel is mandatory. In addition
to the hazards associated with
Class 3b lasers, fire and airborne
pollutants (e.g. ozone) also
need to be considered. Class
4 laser systems require strict
supervision and restricted access
to virtually all personnel.
Because of these restrictions,
Laser Technology, Inc., incorporates
Class 4 lasers only as integral
components of laser systems
designed to be maintained and
operated as Class 3b or lower
laser systems.