Transcript Laser Safety - Universiteit Twente

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Introduction
to
Laser Safety
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Examples of laser accidents
Overview of the eye
macula or macula lutea (yellow spot on the retina): allows color vision
fovea: central spot of macula allowing for sharp central vision (necessary for reading, TV,
driving, and any activity where visual detail is of primary importance
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413 Picosecond pulses cause bleeding/latent
viewing distortion
Description:
New frequency doubler didn't have AR coatings as
requested. As person left room, beam hit eye corner and
transmitted schlera and caused interocular bleeding.
Resided at 2 wks and eye normal at 2 months. Person still
complains at 8 yrs of floaters and vision that looks "like
looking through a dirty window".
Type:FD Nd:YAG
Wavelength:1064 nm
Class:IV
Exposure Time:60 ps
Divergence:Energy/Power:MW/cm2+
Pulse Rate:1 KHz
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165 Reflected beam
caused vision loss
Description:
Professor from China removed eyewear to "see better"
while doing an experiment with a crystal. Exposure
produced retinal burn and permanent vision loss. He
described seeing a white flash, central purple spot
surrounded by yellow halo. No pain reported.
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223 Retinal burn from beam off rear laser
mirror
Description:
Student WITH EYEWEAR ON (and witness to verify)
received exposure from the rear mirror of a "Continuium"
YAG laser. The student was wearing Glendale Broadband
(OD 4.0) eyewear; ANSI standard requires OD=6.0. Retinal
burn resulted with permanent damage.
Type:Nd:YAG
Wavelength:532 nm
Class:Exposure Time:7 ns
Divergence:Energy/Power:0.18/0.40
Pulse Rate:5 KHz
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356 Blurred vision from reflected exposure.
Description:
Student received reflected beam from plastic tool box lid
from Ti-Sapphire laser. No eye protection worn. Student
reported blurred vision and seeing black spots. He was
installing a laser transport tube (beam safety tube). The
student had not received laser safety training. At 1 month
student still had blurry vision.
Type: Ti-Sapphire
Wavelength:800 nm
Class:IV
Exposure Time:120 fs
Divergence:Energy/Power:15 mJ
Pulse Rate:10 KHz
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312 Off-axis beam causes macular burn in
left eye
Description:
Scientist bumped mirror mount in a complex optical array causing a stray beam to go off-axis. When leaning over the
table, he was struck in left eye by beam off lower array
mirror. Exam confirmed macular lesion which he states
disrupts vision. No eyewear worn and safety knowledge
was limited.
Type:Ti-Sapphire
Wavelength:800 nm
Class:IV
Exposure Time:50 ns
Divergence:Energy/Power:6 mJ
Pulse Rate:3.3K KHz
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307 Backscatter from mirror causes
hemorrhage and oveal blindspot
Description:
A 26 year old male Student aligning optics in a university
chemistry research lab using a "chirped pulse" TitaniumSapphire laser operating at 815 nm with 1.2 mJ pulse
energy at 1 KHz. Each pulse was about 200 picoseconds.
The laser beam backscattered off REAR SIDE of mirror
(about 1% of total) caused a foveal retinal lesion with
hemorrhage and blind spot in central vision.
A retinal eye exam was done and confirmed the laser
damage.
The available laser protective eyewear was not worn.
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283 Photophobia in right eye after beam
misalignment
Description:
Received "flash" into eye during alignment where he looked
back along the beam path to view reflection off laser face
plate. Result caused photophobia with burning sensation.
No retinal burns detected. Patient used sunglasses for
photophobia.
Type:HeNe
Wavelength:633 nm
Class:Exposure Time:~0.25 sec
Divergence:Energy/Power:6 mW
Pulse Rate:-
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Airway Fire
Description:
During laser surgery on a patient’s vocal cords, the surgeon
struck the endotracheal tube with a pulse from a CO2 laser.
The tube, which carries oxygen to the patient and runs
through his mouth to his lungs, was not made of laserresistant material. Instead, it was made of polyvinyl
chloride (combustible to both Nd:YAG and CO2 lasers). It
caught fire and filled the man’s lungs with toxic smoke,
causing burns. The patient did not survive the procedure.
In general the anaesthesiologist has only six seconds to
recognize that a tube has ignited and remove it before the
fire peaks. Once ignited, the tubes are as hot as an oxygen
lance used in welding. The flame can reach a length of 5 to
10 inches. Laser beam interaction with secondary materials
is a known source of laser incidents. This sort of unplanned
interaction is a danger one needs to think of beforehand.
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Los Alamos Laser Accident
Description:
A postdoctoral employee received an eye exposure to
spectral radiation from an 800 nm Class 4 laser beam. The
extremely short pulse (100 fs) caused a 100-microndiameter burn in the employee's retina. The accident
occurred shortly after a mirror was removed from its mount
and replaced with a corner cube during a realignment
procedure. Although the beam had been blocked during
several previous steps in the alignment, it was not blocked
in this case. The employee was exposed to laser radiation
from the corner cube mount when he leaned down to
check the height of the mount. Neither of the two
employees performing the alignment was wearing the
appropriate laser eye protection. The system had two
modes of operation: 10 Hz and 1,000 Hz. In addition, the
researcher forgot that the part of the 800 nm beam he
could see represented only 1-2% of the beam.
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You have to ask yourself
can this happen in our laboratories?
Dangers associated with
the use of lasers
• Beam hazards
– eye damage
– skin damage
• Non-beam hazards
– electrical hazards
– toxic/carcinogenic laser dyes
– hazardous gases (e.g. excimer lasers)
– fire
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Majority of injuries involve the eye and to
lesser extend the skin
Summary of reported laser accidents in the United States and their
causes from 1964 to 1992
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Majority of injuries during alignment, or
no use or improper use of eyewear
Summary of reported laser accidents in the United States and their
causes from 1964 to 1992
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Sensitivity to damage: eye transmission
Effect of laser beam depends
strongly on wavelength
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Potential eye damage
In general terms, in supra-threshold exposures the
predominating mechanism is broadly related to the pulse
duration of the exposure.
Thus, in order of increasing pulse duration, the
predominant effects in the following time domains are:
– nanosecond and sub-nanosecond exposures, acoustic
transients and non-linear effects
– from 1 ms to several seconds, thermal effects
– in excess of 10 s, photochemical effects.
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Potential eye damage
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The biological damage caused by lasers is produced through
thermal, acoustical and photochemical processes.
Thermal effects are caused by a rise in temperature following
absorption of laser energy. The severity of the damage is
dependent upon several factors, including exposure duration,
wavelength of the beam, energy of the beam, and the area and
type of tissue exposed to the beam.
Normal focusing by the eye results in an irradiance amplification of roughly 100,000; therefore, a 1 mW/cm2 beam entering the eye will result in a 100 W/cm2 exposure at the retina.
The most likely effect of intercepting a laser beam with the eye
is a thermal burn which destroys the retinal tissue. Since retinal
tissue does not regenerate, the damage is permanent.
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Potential eye damage
Acoustical effects result when laser pulses with a duration less
than 10 microseconds induce a shock wave in the retinal tissue
which causes a rupture of the tissue. This damage is permanent, as with a retinal burn.
Acoustic damage is more destructive than a thermal burn.
Acoustic damage usually affects a greater area of the retina,
and the threshold energy for this effect is substantially lower.
Beam exposure may also cause Photochemical effects when
photons interact with tissue cells. A change in cell chemistry
may result in damage or change to tissue. Photochemical
effects depend strongly on wavelength.
N.B. the severity of the damage depends strongly on whether it
occurs by intrabeam exposure or scattered laser light
Skin hazards
• In general terms, the skin can tolerate a great deal more
exposure to laser beam energy than can the eye.
• The biological effect of irradiation of skin by lasers
operating in the visible and infra-red spectral regions
may vary from a mild erythema to severe blisters.
• An ashen charring is prevalent in tissues of high surface
absorption following exposure to very short-pulsed, highpeak power lasers.
• The pigmentation, ulceration, and scarring of the skin
and damage of underlying organs may occur from
extremely high irradiance.
• In the wavelength range 1500 nm to 2600 nm, biological
threshold studies indicate that the risk of skin injury
follows a similar pattern to that of the eye.
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Spectral region
Ultra-violet C
(180 nm to 280 nm)
Ultra-violet B
(280 nm to 315 nm)
Ultra-violet A
(315 nm to 400 nm)
Visible
(400 nm to 780 nm)
Infra-red A
(780 nm to 1400 nm)
Eye
Photokeratitis
Skin
Erythema (sunburn)
Accelerated skin
ageing
Photochemical cataract Pigment darkening
Photosensitive
Photochemical &
reactions
thermal retinal injury
Cataract, retinal burn
Infra-red B
(1,4 μm to 3,0 μm)
Aqueous flare, cataract,
corneal burn
Infra-red C
(3,0 μm to 1 mm)
Corneal burn only
Skin burn
Example of eye injury
Experience has demonstrated that most laser injuries go
unreported for 24–48 hours by the injured person. This is a
critical time for treatment of the injury.
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Retinal Burn
A range of injuries induced with a Nd:YAG laser on a monkey retina.
The white spots in the centre are thermal burns, i.e. coagulation of retinal
layers. With larger energies, holes in the retina are produced which result
either in bleeding into the vitreous (the gel-like substance which fills the centre
of the eye ball), or the bleeding is contained in the layers of the retina, which
results in functional loss in the affected area.
Photograph courtesy of J. Zuclich, TASC Litton, TX, USA.
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Eye Damage: Focusing
Remember: Your Eyes Are Designed to Focus
With safety rule
Cornea Damage
BAD
Retina Damage
WORSE
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Laser classification
Class 1 Lasers
Lasers that are safe under reasonably foreseeable
conditions of operation, including the use of optical
instruments for intrabeam viewing.
Class 1M Lasers
Lasers emitting in the wavelength range from 302,5 nm to
4000 nm which are safe under reasonably foreseeable
conditions of operation, but may be hazardous if the user
employs optics within the beam.
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Laser classification
Class 2 Lasers
Lasers that emit visible radiation in the wavelength range
from 400 nm to 700 nm where eye protection is normally
afforded by aversion responses, including the blink reflex.
This reaction may be expected to provide adequate
protection under reasonably foreseeable conditions of
operation including the use of optical instruments for
intrabeam viewing. Outside this wavelength range AEL ≤
AEL of a class 1 laser.
Class 2M Lasers
Like class 2 lasers, however, viewing of the output may be
more hazardous if the user employs optics within the
beam. Outside visible range AEL ≤ AEL of a class 1M laser.
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Laser classification
Class 3R Lasers
Lasers that emit in the wavelength range from 302,5 nm to
106 nm where direct intrabeam viewing is potentially
hazardous but the risk is lower than for Class 3B lasers.
The accessible emission limit is within five times the AEL of
Class 2 in the wavelength range from 400 nm to 700 nm
and within five times the AEL of Class 1 for other
wavelengths.
Class 3B Lasers
Lasers that are normally hazardous when direct intrabeam
exposure occurs. Viewing diffuse reflections is normally
safe.
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Laser classification
Class 4 Lasers
Lasers that are also capable of producing hazardous diffuse
reflections.
They may cause skin injuries and could also constitute a
fire hazard.
Their use requires extreme caution
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Retinal injury thresholds
Health Physics
October 2000,
Volume 79,
Number 4
At 10-12 seconds the threshold for a retinal injury is appr. 10-7 J/cm2 (i.e.
105 W/cm2). Because of the x 105 enhancement in the eye this value is
elevated to 10-2 J/cm2 (i.e. 1010 W/cm2) on the retina. These exposure
levels are further enhanced by self-focussing.
Exposure limits, Retinal injury
example
• A 4 % reflection from a 2.5 mJ laser pulse in a 2
mm beam, gives an exposure of
(10-4 J)/(p x 0.12 cm2) = 3.2 10-3 J/cm2.
• This exceeds the threshold value of the cornea
of about 10-7 J/cm2 by a factor of 3.2 104.
• To be adequately protected against this
exposure, protective eyewear must have an
optical density (OD) of at least log10(3.2 104) =
4.5
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Some common unsafe practices:
preventable laser accidents
• Not wearing protective eyewear during
alignment procedures
• Not wearing protective eyewear in the laser
control area
• Misaligned optics and upwardly directed beams
• Equipment malfunction
• Improper methods of handling high voltage
• Available eye protection not used
• Intentional exposure of unprotected personnel
• Lack of protection from non-beam hazards
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Some common unsafe practices or
preventable laser accidents
•
•
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•
•
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•
Failure to follow (Laser) Safety Instructions
Bypassing of interlocks, door and laser housing
Insertion of reflective materials into beam paths
Lack of pre-planning
Turning on power supply accidentally
Operating unfamiliar equipment
Wearing the wrong eyewear
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Guidelines to help prevent
accidents during alignment
• No unauthorized personnel will be in the room or area.
• Laser protective eyewear will be worn.
• The individual who moves or places an optical
component on an optical table is responsible for
identifying and terminating each and every stray beam
coming from that component.
• To reduce accidental reflections, watches and reflective
jewellery should be taken off before any alignment
activities begin.
• Beam blocks must be used and must be secured.
• When the beam is directed out of the horizontal plane, it
must be clearly marked.
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Guidelines to help prevent
accidents during alignment
• The lowest possible/practical power must be used during
alignments.
• Have beam paths that differ from the eye level when
standing or sitting. Do not use paths that tempts one to
bend down and look into the beam.
• All laser users must receive an introduction to the laser
area by an authorised laser user of that area
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Responsibilities
The department is responsible for the safety
of its employees and exercises its
responsibility by providing guidelines and
periodic control by designated safety
personnel.
We all have a personal responsibility to
make sure that our working conditions and
working habits are safe and in accordance
with the guidelines.
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Acknowledgement / References
• This presentation is inspired by a similar presentation on
Laser Safety by the Molecular & Laser Physics Group of
the Radboud University, Nijmegen NL.
• Laser incidents taken from laser accident database of
Rockwell Laser Industries, Inc.
(http://www.rli.com/resources/accident.asp)
• Certain descriptions and numbers are taken from the
international standard: IEC 60825-1 Edition 1.2.
• The model of the eye is provided by the National Eye
Institute, USA.
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Aqueous flare = the presence of floating particles in the fluid of the
anterior chamber of the eye
Cataract = opacity or reduction in clarity of the lens of the eye
Erythema = redness of the skin caused by dilatation and congestion of
the capillaries, often a sign of inflammation or infection
Fovea centralis = a small depression near the center of the retina,
constituting the area of most acute vision
Inflammation = A localized protective reaction of tissue to irritation,
injury, or infection, characterized by pain, redness, swelling, and
sometimes loss of function
Lesion = a localized pathological change in a bodily organ or tissue
Macula letua (yellow spot) = A minute yellowish area containing the forea
centralis located near the center of the retina of the eye at which
visual perception is most acute.
Macular lesion = Loss of central vision
Photokeratitis = Inflammation of the cornea produced by ultraviolet
radiation
Photophpobia = an abnormal sensitivity to or intolerance of light,
especially by the eyes, as may be caused by eye inflammation, lack of
pigmentation in the iris, or various diseases
Schlera = outer hard white coat (cover) of the eyeball