Lasers and Laser Safety
1. The laser
1a. General Information
 Lasers are classed in groups from I to IV. Class I are intrinsically safe due to
low emission levels or having an inaccessible beam. With Class IV not only
the direct beam is hazardous but also the reflections from rough surfaces.
 Most lasers currently in use for techniques such as PIV, LDA and LIF/Raman
spectroscopy are class IV
 Laser emissions vary in frequency from red to ultraviolet meaning they can be
visible and invisible. Most PIV, etc., techniques use lasers in blue/green
range.
 Lasers may be pulsed or continuous wave (CW).
 Pulsed lasers emit a short burst of energy, typically a few ns in duration, a set
number of times per second. PIV lasers typically vary from 10 - 15Hz for low
speed PIV to 2000Hz for time resolved. The output from a pulsed laser is
stated in Joules.
 Continuous wave lasers emit a continuous beam which is more hazardous
than pulsed lasers because it is constantly on. Their output is stated in Watts.
1b. External Laser anatomy
 Most lasers comprise a power supply with a head which are connected with
an umbilical to transfer the necessary electrical and other services from the
power supply to the head. Other services may include cooling water and a
means of operating a shutter and interlock.
 The power supply will have controls on it. These may be as simple as a key
switch and button to a panel which allows connection of external timing
controls to the trigger and, for Nd:Yag's, the Q switch.
 The head contains all the parts which generate the laser emission.
 All lasers have a key switch without which they are inoperable.
 Most lasers incorporate a voltless interlock - a pair of contacts which need to
be shorted before the laser will operate even when the key switch is on. This
is intended to be connected to an interlock system which should shut the
laser off if the interlock is tripped.
 The head will probably contain a shutter which may be manually operated
from the head or from the control panel on the power supply. If it is operated
from the control box it will probably close every time the emission is stopped
and have to be actively opened when the emission is required.
Internal laser anatomy is not required by most users although some techniques
do require going inside the laser. This information is beyond the scope of this
tutorial.
2. Hazards
 The beam, or its reflections, can cause serious tissue damage and may be a
fire hazard. This hazard is specific to lasers.
 There are significant hazards associated with lasers that are common to all
electrical equipment and many have very high voltages in their power supply
cabinets. Some lasers have chemical hazards.
3. Protection
The same rules apply to lasers as to any other hazardous equipment: reduce risk
by minimising likelihood of exposure to the hazard. Risk must be eliminated to
people not associated with the laser and reduced to a minimum for those who
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are. Those who will be exposed to some risk must be aware of them and able to
competently deal with them.
Protection is gained by a mixture of engineering and administrative controls.
Engineering controls are such items as physical barriers while administrative
control is a written set of procedures to be followed. Many of these are covered in
this document and should be incorporated and adapted for a specific case.
Personal protection is a last resort. This includes safety glasses.
3a. Zone system
Lasers should ideally be used within a three zone system.
Zone 1 contains the laser and what it operates on. It may have other items such
as optics and cameras. This area may be as small or as large as required enough to get hands in to work on the apparatus or to walk around in - but is
defined by a cordon beyond which the laser emissions cannot pass. This zone
can only be entered by those with experience in using and setting up the laser.
Zone 2 contains the laser controls and any readouts from the apparatus such as
computer loggers, and screens. Personnel with an interest in the experiment may
be in this zone which should be safe enough for them not to require personal
protective equipment. Zone 2 may be part of the same room as zone 1 as long as
there is a cordon separating them.
Zone 3 is the rest of the world which is separated from zones 1 and 2 by a
lockable door with warning notices and lamps preventing access from people with
no part in the experiment. It may have an interlock on it.
3b. Personal protective equipment
Laser safety glasses are the only specific form of personal protection available.
They should be purchased for, and kept with, a particular laser. Safety glasses
are available in different densities and to block particular wavelengths. Safety
glasses that are effective with one laser may be ineffective with another if it's
emission wavelength is different or if the power is higher.
Even safety glasses purchased for a particular laser will only reduce the risk of
injury by extending the time taken for damage to occur from a few milliseconds to
a couple of seconds. They should not be relied on for complete protection of the
eyes and never used as the only means of protection.
Lasers can also cause deep burning of the skin. Not only can the surface skin be
burned but also the deep epidermal layers can be permanently damaged.
3c. Interlocks
Interlocks are a means of shutting the power to a laser when someone from zone
3 breaches zone 2 or 1. They are only required if all other controls fail. They will
only fail in a poorly designed or poorly used system and will cause disruption to
the experiment. They should be employed and should not be defeated.
3d. Dark or light environment.
There is a long running debate over whether a laser room should have a dark or
light interior. A dark painted room without lights is supposed to show where the
beam is while a light painted room with illumination means the pupil of the eye
will be closed and reduce risk from damage if the beam strikes it.
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In reality a darkened room is necessary for most flow visualisation techniques
using lasers and lasers used with such techniques are so powerful that with the
pupil closed or open damage will still occur. Good illumination is a good aid to
setting up the laser and optics.
Users should avoid wearing reflective or loose clothing and jewellery. Loose
clothing not only includes sleeves but ends of scarves and ties, etc. Jewellery is
not only reflective but can heat up under the influence of the beam.
5. University requirements for lasers and users
The University requires that all lasers above class II in use in the University are
registered with the University Laser Safety Officer (ULSO). The forms are quite
comprehensive and require a fair bit of information about the laser and its
location. They also require changing if the circumstances of the laser change.
Laser users must also be registered with the ULSO. In order for this to go ahead
the user must have an approved eye test, paid for by the user's contract or
account, and the details of which sent to the ULSO as proof.
Both of these are best achieved through the Departmental Laser Safety Officer
for Engineering.
4. Design of laser installations
Since a laser is hazardous whether used in a short term test or a long term facility
these notes should be applied to all situations where a laser is employed. These
are engineering controls of safety.
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All hazardous emissions should be kept in zone 1.
Zone 1 should be big enough for an operator to be able to get to all parts
without having to remove a lot of barriers or it will not be used and the laser
will be left unsafe.
Ideally personnel in zone 2 should not have to wear safety glasses but this
must be designed in.
Wherever possible the beam should be sited below eye level
Mirrors and lenses must be properly fixed as any movement in them will
cause unpredictable alterations to the beam's path.
Beams and sheets of light must be terminated in a suitable manner. A beam
must have a beam stop capable of absorbing the power of the emissions and
a sheet must have a suitably opaque sheet to prevent it travelling further than
required.
Reflected beams and sheets must also be blocked when they are no further
use to the experiment.
Beams should be contained in tubes as far as possible. Sheets may be
trammelled between two pieces of suitable material.
5. Setting up, adjusting, and using a laser apparatus
5a. General rules
There should be more than one person involved with all aspects of using a laser.
One of them should be designated as being in control of the installation and the
others act on this person's instructions. The controller is responsible for ensuring
everyone is aware when the laser is about to be switched on and that all others
are in suitable positions and have adequate protection when it is. The controller
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either operates the laser directly or instructs one other to do so when they are
told they can.
Written procedures should be produced and followed for all of the following.
These will usually be produced by the person who is in control of setting up the
apparatus and based on the methods they have developed for doing so safely. A
'scheme of work' form is available at www.le.ac.uk/eg/safety in the forms section.
5b. Setting up
This is the stage which offers most risk. There should be as few people
necessary to ensure it can be carried out efficiently. Perhaps one to operate a
computer and the controls in zone 2, and one or two in zone 1 to align optics.
One of those in zone 1 will be the controller.
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Work methodically with the optics starting from the beam exit point of the
laser to the point where it will be used.
Use the lowest setting of the laser intensity commensurate with being able to
see what is happening.
Apply a beam or sheet stop after the point the next optic is to be positioned
both where the beam is currently directed and where it will go after it is
inserted.
Apply the optic only with the beam off and adjust it as well as possible to its
correct position before switching the laser on. Always err on making the beam
go to a safe place if positioning is not correct. Do not allow it to reflect off lens
holders or edges. A laser pointer may be of use to determine this before the
main laser is switched on.
5c. Adjusting a setup.
Follow a similar method to that used in setting up. If adjustments can be
predicted design the system so that as little as possible needs to be moved. Aim
to move optics that are closer to the point of use rather than the beam exit point
as they will cause less variations further down the line.
Use a beam stop after the optic following the one being altered and make small
adjustments and check their effect down the line after each one.
5d. Using
Although work with a laser should require more than one person those with
experience in setting up and using a laser may work alone providing they set up a
'buddy' system. A buddy is a person who will be able to check on the user or be
checked in with by the user at mutually agreed intervals.
Work with lasers is not allowed out of hours
6. Managing laser equipment
6a. Safety glasses
Since safety glasses can be specific to a particular laser they are associated with
that laser and should not be used with other lasers.
They are not general purpose or disposable equipment and require taking care
of. They should be cleaned carefully and stored in their cases when not in use. If
the frame breaks it must be repaired before it can be used again. A scratched
lens will render the safety glasses useless and they should be discarded.
Report all damage to safety glasses to the key issuer.
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6b. Optics
Optics should be cleaned in an appropriate manner to keep reflections and
damage to a minimum. They require cleaning and careful storing to ensure they
perform well both from an experimental and safety point of view.
7. Access to Lasers
 Only registered users are allowed to operate a laser or be in zones 1 and 2 of
the enclosure.
 Keys are issued by the laboratory manager or deputy to the leader of the
team. The key issuer will require written evidence of the members of the team
or the buddy system. A declaration of compliance signed by the key holder's
supervisor will also be required.
 Keys are signed for and must be returned before 5pm the same day.
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