Background and Overview of Hazards

The light emitted by a laser can range from the UV (100 to 400 nm), through the visible (400 to 700 nm) to the infrared (700 nm to 1mm). Laser light can be coherent (in phase), and collimated (directional), meaning that it does not spread out as does the light of a flashlight. For those reasons, laser light maintains its high intensity over a long distance.

Laser light can be a continuous wave (CW) or pulsed with varying pulse durations and frequencies.

Eye Hazards

The eye is composed of structures that absorb light of different wavelengths. The cornea and lens absorb wavelengths in the UV region between 100 and 400 nm and in the infrared between 1400 nm to 1 mm. Exposure to laser light of these wavelengths can cause injury to the cornea or lens.

Light in the visible (400 to 700 nm) and near-infrared (700 to 1400 nm) is of greatest concern because it is focused in the eyeball 100,000 fold to a very small spot on the retina. For example, a collimated beam with an irradiance of 1 mW/cm² entering the eye will expose the retina to 100 W/cm². Exposure to a laser beam in this wavelength region can cause permanent damage to the retina. Depending on the location and extent of injury, it can be barely noticeable or lead to a severe loss of vision.

Skin Hazards

Skin injuries can be caused by powerful lasers at any wavelength. Focused beams can evaporate tissue and result in third degree burns. In the UV region (< 400 nm) even low level exposures from scattered radiation can cause erythema (sunburn), skin cancer, or accelerated skin aging.  The most damaging region of ultraviolet is 280 - 315 nm, also known as UV-B.

Fire Hazards

High powered laser beams of class 4, in particular infrared lasers can ignite combustible material (e.g. card board, curtains).

Non-beam Hazards

Lasers can pose hazards not related to the beam. Those are:

• Electrical shock hazards from high-voltage components of powerful lasers; 
• Fumes and particles released from laser cutting or similar applications;
• Chemical hazards from gases and dyes used as lasing medium;
• Intense process radiation from laser welding or other metal processing applications.

Engineering Controls and Work Practices

Beam Control

For class 3b and 4 lasers, measures must be implemented to control the beam and prevent unwanted reflections and scattering. In general, the more the beam is enclosed, the safer the set-up. Use the following measures to control the beam:

• Mount the laser on a solid surface so it cannot accidentally move. Keep the beam below or above eye-level, unless the application requires else.
• Enclose the beam as much as the application allows using beam tubes, laser barriers, or filter windows appropriate for the wavelength.
• Use beam blocks and dumps to terminate any unwanted beams and reflections.
• If the beam cannot be fully enclosed, confine it to the experimental area using laser barriers and curtains and separate the laser area from other parts of the room.
• Keep any radiation from reaching the room entrance.
• Avoid unwanted reflections by removing any reflective items, such as tools and unused optical components from the laser area.
• Remove jewelry and watches from your arms and hands.

Warning Mechanisms

Rooms in which class 3b and 4 lasers are operated must have a warning sign on the door. DRS provides such signs for registered lasers. It is therefore important to keep the laser inventory current.

To keep people from unknowingly entering a laser area while the laser is on, a warning mechanism must be implemented. If the room is not equipped with a light on the outside, make a flip-sign or get a battery operated light and place it at the entrance to the laser area. Make sure to activate the warning mechanism before turning on the laser and to deactivate it when it is safe to enter. A permanent sign saying that the laser is on is NOT an adequate warning mechanism because it will be ignored.

Unless the beam is fully enclosed and the system is interlocked, an authorized user must be present or the room kept locked during laser operations.

Interlocks

Many laser systems have interlocked protective housings which prevent access to high-voltage components or laser radiation levels higher than those accessible through the aperture.  These interlocks must not be bypassed before obtaining adequate training and authorization of the Principal Investigator (PI).  Additional control measures must be implemented that take into account the higher radiation levels, the potential of additional wavelengths, and high voltage while the interlock is bypassed.

Maintenance and Repair

Only a knowledgeable person who has been specifically authorized by the PI to perform such work should perform maintenance, servicing, or repair of a laser.  When such work involves accessing an embedded laser of a higher class, the controls appropriate to the higher class must be applied.

Personal Protective Equipment

Laser eyewear must be worn within the Nominal Hazard Zone (NHZ). The NHZ is the space where the laser radiation exceeds the maximum permissible exposure. Unless the beam is fully enclosed, the NHZ for class 3b and 4 lasers encompasses usually the entire laser area, i.e. the area behind the laser barriers and curtains.

Laser eyewear must be designed for the specific wavelength of the laser and have sufficient optical density (OD) for the power of the laser. Optical density is given in a whole number that describes the factor of attenuation in powers of ten. For example, an OD of 3 attenuates the intensity by a factor of 1000. Wavelength and OD must be printed on the eyewear.

The table below shows required OD values for continuous wave lasers in the visible range (400 to 700 nm).

Pulsed lasers may require higher OD values depending on the energy per pulse and pulse duration.

If unsure about the required OD, ask the laser manufacturer or contact DRS.

Eyewear Maintenance and Inspection

Proper care of laser safety eyewear will prolong the life of the eyewear and keep personnel protected.  Use a lens cleaning solution and cloth that are safe to use with the filters to remove dirt, oil, and dust without scratching, delaminating or damaging dielectric coatings.  Store eyewear in a hard or soft case to prevent scratches.

Inspect the eyewear before using it to ensure there are no scratches, holes, cracks, pits, discolorations or other damage that would decrease the intended safety level.  Illinois regulations require that eyewear be checked every six month and that the inspection be documented. If eyewear is found to be unacceptable remove it from service and obtain a new pair. 

Laser Alignments

Most laser accidents occur during alignment procedures because the beam needs to be visible for this process. If you have never aligned a laser, you should practice the process with a low power laser until you feel confident that you can perform the process safely. Below are tips to make the alignment procedure safer:

• For new set-ups, perform the initial alignment with a lower power laser or reduce the power to a safe level using an optical attenuator.
• Always use laser eyewear.
• Use laser viewing cards to visualize the beam. Those are available for various wavelengths and emit light of a different wavelength than that of the laser. With the right combination of cards and safety eyewear the beam emitted from the card becomes visible through the laser safety eyewear.
• Use a shutter or beam block to block the beam at the source except when it is actually needed.
• Use a beam block to terminate the beam down range of the optics being aligned.
• Place beam blocks behind optics to terminate beams that might miss the optical component during alignment.
• Start the alignment procedure with the laser viewing card blocking the laser aperture before turning on the beam. Follow the beam with the card and align the optical components. Only release the beam once you are sure the component is adjusted correctly.
• Keep the unused hand behind your back to avoid accidental blocking of the beam, skin injuries and reflections.

Emergency Procedures

In case of a laser accident, make sure the shutter is closed or the laser is de-energized. If an eye injury is suspected, have the injured person keep their head upright and still to restrict bleeding in the eye. Help the victim seek medical attention. Notify your supervisor and DRS of the accident.

References

• ANSI Z136.1 (2007)
• Laser Safety Information Bulletin, Laser Institute of America