Laser Safety Seminar Marc Vrakking FOM Instituut for Atomic and Molecular Physics (AMOLF) LCVU, 10/12/2002

Laser Safety Seminar

Marc VrakkingFOM Instituut for Atomic and Molecular Physics (AMOLF)

LCVU, 10/12/2002

165: Frequency doubled Nd:YAG in a research lab.

A visiting professor from China lost part of the sight in his left eye after he removed his safety goggles during a test with a frequency doubled Nd:YAG laser. The professor was in he research lab of a university. He was working on an experiment with a crystal he had grown. He had removed his goggles so he could see better and the laser reflected into his eye, burning the retina.

http://www.rli.com/accident/case_studies/

255: Beam blinds scientist doing alignment of a Nd:YAG laser.

During optics alignment involving a 30 mJ pulsed Nd:YAG laser (10 Hz) on a target using a prism, the beam exceeded the prism's critical angle and struck the scientist in the eye resulting in a permanent retinal burn. Unfortunately, no protective eyewear was worn at the time. An ophthalmologist was consulted and confirmed retinal burns. Blurry vision resulted especially when reading.


307: Backscatter from mirror causes hemorrhage and foveal blindspot.

A 26 year old male Student aligning optics in a university chemistry research lab using a "chirped pulse" Titanium-Sapphire 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.


280: Graduate student receives macular lesion from picosecond laser.

A picosecond Nd:YAG pulsed laser operating at 1064 nm was on a laser optics table. The beam was directed from one table to another across an isle. The beam went onto the second table, where it was directed onto a liquid sample holder. Here, apparently, the beam was bigger than the liquid sample holder, so the edges of the beam went pass the sample bottle and then off that table into the room area where a Strip Chart Recorder (SCR) was located. A graduate student working on the experiment looked at the SCR and received about 10% of the beam into the eye. The student reportedly a "heard a popping sound" which was followed by a white spot in the vision center. The professor took the student to an eye doctor for a retinal exam which confirmed the burn exposure. The student did not experience shock. The beam caused a retinal burn. The student now complains that his "eyes get tired" while reading.

274: Technician receives retinal burn with a single Ti-Sapphire laser pulse.

A laser lab technician was working without laser protective eyewear. He was exposed to a single 7 ns pulse at a pulse energy of 10-50 µJ. In the setup, the beam was directed onto a metal "test slide" from the power meter manufacturer. This was used to test whether the beam would harm the power meter. The slide was accidentally tilted so-as-to reflect the beam into technician's eye (assume about ~4% reflection). At time of exposure the person perceived a bright flash that persisted (with eyes closed) as if he had looked at the sun. There was no pain nor did the person go into shock. There was eyewear was available but not for the 806 nm wavelength in use.


Ask yourself:

Could any of these accidents happen in my lab?

- Beam hazards- eye damage- skin damage

- Non-beam hazards- electrical hazards- toxic/carcinogenic laser dyes- hazardous gases (e.g. excimer lasers)- fire

Dangers associated with the use of lasers

In some of our labs at LCVU, where the emphasis is on Ti:Sapphire technology we have to be especially concerned with the beam hazards

http://www.adm.uwaterloo.ca/infohs/lasermanual/documents/section11.html

The majority of injuries involve the eye and, to a lesser extent, the skin

Summary of reported laser accidents in the United States and their causes from 1964 to 1992

http://www.adm.uwaterloo.ca/infohs/lasermanual/documents/section11.html

Summary of reported laser accidents in the United States and their causes from 1964 to 1992

The majority of injuries occur during alignment procedures, or because the protective eyewear was either inappropriate or not used

Laser eye damage

a short introduction

Basic layout of the human eye

www.yorku.ca

Eye transmission

http://www-training.llnl.gov/wbt/hc/LaserRefresh/EyeTransmission.html

The effects of the laser depends strongly on the wavelength

http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/8-50/INTRO.htm

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.

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 actually 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 eye damage depends strongly on whether it occurs by intrabeam exposure or scattered laser light

Potential eye damage

Skin HazardsSkin can suffer thermal burns and photochemical changes from laser exposure. These effects are almost entirely independent of the coherent nature of the laser light, but are aggravated by the high power density of lasers.

In the literature two types of skin damage are emphasized:

-prolonged exposure to UV laser light leads to erythema (sunburn)

-high intensity exposure to laser light burns the skin

In the literature there is very little mention of damage below the skin due to high intensity lasers.

http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/8-50/INTRO.htm

Example of eye damage

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.

Exposure Limits – Laser ClassificationClass 1 Lasers

Class 1 lasers do not emit harmful levels of radiation .

Class 2 Lasers (< 1mW, commonly found in alignment applications)

Capable of creating eye damage through chronic exposure. In general, the human eye will blink within 0.25 second when exposed to Class 2 laser light, providing adequate protection. It is possible to stare into a Class 2 laser long enough to cause damage to the eye.

Class 2a Lasers (special purpose < 1mW, e.g. barcode readers)

Class 3a Lasers (1-5 mW)

Not hazardous when viewed momentarily with the naked eye, but they pose severe eye hazards when viewed through optical instruments (e.g., microscopes and binoculars).

Class 3b Lasers (5-500 mW or less than 10 J/cm2 for a ¼-s pulsed system)

Injury upon direct viewing of the beam and specular reflections. Specific control measures must be implemented.

Class 4 Lasers (> 500 mW or greater than 10 J/cm2 for a ¼-s pulsed system)

They pose eye hazards, skin hazards, and fire hazards. Viewing of the beam and of specular reflections or exposure to diffuse reflections can cause eye and skin injuries. All control measures to be outlined must be implemented.At LCVU we use almost exclusively Class 3 and Class 4 lasers!

Exposure Limits – Retinal Injury Thresholds - I

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 Thresholds - II

Numerical example:

A 4% reflection from a 2.5 mJ laser in a 2 mm beam, gives an exposure of

(10-4 J)/(0.78 x (0.2)2 cm2) = 3.2 10-3 J /cm2 ,

exceeding the threshold value on the cornea of appr. 10-7 J/cm2 by a factor 3.2 104.

To be adequately protected against this exposure, protection with an optical density (OD) of log(3.2 104) = 4.5 is required

Prevention

• 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 nonbeam hazards

• Failure to follow Activity Hazard Document

• 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

Some common unsafe practices that are causes of preventable laser accidents

http://ehs.lbl.gov/ehsdiv/pub3000/CH16.html

• 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 jewelry 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.

• A solid stray beam shield must be securely mounted above the laser use area to prevent accidental exposure to the laser beam.

• All laser users must receive an orientation to the laser use area by an authorized laseruser of that area.

• The lowest possible/practical power must be used during alignments.

• When possible, a course alignment should be performed with a HeNe alignment laser.

• Have beam paths at a safe height, below eye level when standing or sitting. Not at a level that tempts one to bend down and look at the beam. If necessary, place a stepplatform around optical table.


Guidelines to help prevent accidents during alignment

Control measures for Class 3 or 4 laser systems, an example -

I

All commercial lasers require a protective housing and interlock systems that prevent emission of laser radiation when the housing is opened.

Removable master key switch.

All lasers are controlled by an area interlock system and remote shut-off device. When the terminals are open-circuited, the laser must not emit any radiation in excess of the Maximum Exposure Limit.

It is recommended that the plane of the laser beam be above or below the level of a seated or standing person.

Each Class 4 laser or laser system must have a shutter that prevents the emission of laser light in excess of the MPE when the beam is not required.

Class 3b and Class 4 lasers may only be operated in laser control areas.

All personnel who require routine entry into a Class 3B laser controlled area shall undergo an appropriate training program.


Control measures for Class 3 or 4 laser systems, an example -

IIhttp://ehs.lbl.gov/ehsdiv/pub3000/CH16.html

The area must be posted with appropriate warning signs that indicate the nature of the hazard.

Only personnel who have been authorized may operate the laser.

All laser beams must be terminated within the control area. Beam stops provide protection from misaligned beams, and should be placed in all appropriate and practical locations.

Appropriate eye protection must be provided for all personnel within the laser control area. The eye protection must have an appropriate optical density and/or reflective properties based on the wavelengths of the beams encountered, the beam intensity, and the expected exposure conditions. The need for laser eye protection must be balanced by the need for adequate visible light transmission.

Access to the area by spectators or visitors must be limited and controlled by the laser user.

Control measures for Class 3 or 4 laser systems, an example - III


Light levels in excess of the MPE must not pass the boundaries of the control area.

Class 4 laser control areas must be equipped with interlocks or alternate controls to preclude the entry of unprotected personnel while Class 4 laser radiation is present in the control area.

There must be provisions for rapid escape from a laser control area under all normal and emergency conditions.

Wherever possible, lasers should be monitored and fired from remote locations.

A visible or audible signal must be provided at the entrance to the control area to indicate when the laser is energized and operating.

Special Requirements for Invisible Lasers

Since IR and UV lasers produce no visible light, this can contribute to their hazard potential

At LCVU near-IR laser radiation generated in Ti:Sapphire lasers is present in several laser rooms

Therefore anybody entering the area of those experiments should wear goggles providing protection around 800 nm (available near the entrance). These goggles can only be taken off in the room when it’s explicitly stated by the users inside that all IR beams are completely blocked at the output of the laser.

Documents

Laser Safety Seminar Marc Vrakking FOM Instituut for Atomic and Molecular Physics (AMOLF) LCVU, 10/12/2002