Keeping safe around lasers

“When the beam struck my eye, I heard a distinct popping sound caused by a laser-induced explosion at the back of my eyeball. My vision was obscured almost immediately by streams of blood floating in the vitreous humour. It was like viewing the world through a round fish bowl full of glycerol into which a quart of blood and a handful of black pepper have been partially mixed.” Dr. C.D. Decker.

Lasers are used in a variety of industrial applications, including test and measurement equipment discussed elsewhere in this issue of Machinery & Equipment MRO. They can be dangerous, so to better understand lasers, let’s begin by classifying them.

The term laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Light can be produced by atomic processes that generate laser light. A laser consists of an optical cavity, a pumping system and an appropriate lasing medium.

Lasers are used in a variety of applications in today’s industries (see Table 1).

In recognition that all lasers and laser systems do not pose the same degree of risk, a classification system was developed to categorize these systems, depending on their potential to cause biological damage. ANSI Z136.1, American National Standard for Safe Use of Lasers, and Canadian Standards Association CAN/CSA-Z3, Laser Safety in Health Care Facilities, are generally accepted standards.

Four classes of lasers define the risk of damage associated with exposure to the laser beam, and range from Class 1 (no potential damage) to Class 4 (high degree of potential damage). Each will be briefly described below.

Class 1: Not known to be able to cause biological damage. Examples: A) A very low-power laser which emits a low energy beam which is unable to cause biological damage (i.e., a visible spectrum emission less than 0.4 microwatts). B) A high-power laser enclosed in such a fashion that direct access to the beam is not possible. Note that during maintenance or servicing, access to a high-power laser beam may occur (only qualified individuals are authorized to conduct such activities.)

Class 2: Ocular hazard exists when chronic viewing occurs. Limited to visible lasers (either continuous wave or repetitive pulsed), normal aversion of the eye is sufficient to prevent injury. This class emits above the Class 1 level but is less than 1 milliwatt of radiant power.

Class 2A: This designation is applied to lasers which are A) not intended for viewing (i.e., supermarket laser scanners) based upon 1,000 second exposure. The upper power limit is 4.0 microwatts.

Class 3A: An intra-beam hazard exists for ocular injury but not a serious skin hazard. These intermediate-power lasers (1-5 milliwatts, continuous wave) are permitted to exceed the power limit of Class 2 by five times. They often have an expanded beam such that no greater than 1 milliwatt can enter a fully dilated pupil (7 mm). Depending upon the reference standard, these may include both invisible and visible lasers.

Class 3B: Both ocular and skin damage is possible when direct exposure occurs (including mirror reflections). In this class, scattered reflection is not usually considered dangerous, unless the laser is operating at the upper power limit and the beam is viewed at a close distance.. This class may be visible or invisible but the laser cannot produce greater than 500 milliwatts of continuous-wave power.

Class 4: Ocular or skin damage is possible through direct or indirect exposure. Class 4 also indicates a risk of fire from these high-power lasers (greater than 500 milliwatts continuous wave).

What are the major hazards associated with lasers? The hazards associated with laser use fall into two broad categories: beam and non-beam hazards.

1. Beam Laser

A. Eye injury. Because of the high degree of beam collimation, a laser serves as an almost ideal point-source of intense light. A laser beam of sufficient power can theoretically produce retinal intensities at magnitudes that are greater than conventional light sources, and even larger than those produced when directly viewing the sun. Permanent blindness can be the result.

B. Thermal injury. The most common cause of laser-induced tissue damage is thermal in nature, where the tissue proteins are denatured due to the temperature rise following absorption of laser energy.

C. Other. Other damage mechanisms have also been demonstrated for other specific wavelength ranges and/or exposure times. For example, skin can be affected.

2. Non-Beam Laser

A. Industrial hygiene. Potential hazards associated with compressed gases, cryogenic materials, toxic and carcinogenic materials and noise should be considered. Adequate ventilation shall be installed to reduce noxious or potentially hazardous fumes and vapours produced by laser welding, cutting and other target interactions.

B. Explosion hazards. High-pressure arc lamps and filament lamps or laser welding equipment must be enclosed in housings that can withstand the maximum pressures resulting from lamp explosion or disintegration. The laser target and elements of the optical train that may shatter during laser operation must also be enclosed.

C. Non-beam optical radiation hazards. This relates to optical beam hazards other than laser beam hazards.

D. Collateral radiation. Radiation, other than laser radiation, associated with the operation of a laser or laser system, e.g., radio frequency (RF) energy associated with some plasma tubes, x-ray emission associated with the high-voltage power supplies used with excimer lasers, and so on.

E. Flammability of laser beam enclosures. Enclosure of Class 4 laser beams and terminations of some focused Class 3B lasers can result in potential fire hazards if the enclosure materials are exposed to irradiances exceeding 10 W/cm2.

All individuals who operate Class 3b and Class 4 lasers or laser systems must obtain training on the hazards associated with the equipment and proper safety control measures.

Safety control measures

Class 3b and Class 4 lasers or laser systems must be operated under administrative and/or engineering control measures approved by a Laser Safety Officer prior to operation. These measures might include written procedures for operation and/or laser servicing.

Eye protection

Eye protection is required and its use is enforced by the supervisor when engineering controls may fail to eliminate potential exposure in excess of the applicable Maximum Permissible Exposure (MPE). Laser radiation is generated both by systems producing discrete wavelengths and by tunable laser systems producing a variety of wavelengths. For this reason it is impractical to select a single eye protection filter which will provide sufficient protection from all hazardous laser radiation. Therefore it is important to pick eye protection specific for the wavelength and power of the particular laser.

Laser protective eyewear requirements

1. Laser protective eyewear must be available and worn in by all personnel within the Nominal Hazard Zone (NHZ) of Class 3b and Class 4 lasers, where exposures above the MPE can occur.

2. All laser protective eyewear must be clearly labelled with the optical density and the wavelength for which protection is afforded.

3. Laser protective eyewear must be inspected for damage prior to use.

Understanding the requirements for the lasers and following the appropriate recommendations will reduce laser-related accidents in the workplace.

Simon Fridlyand, P.Eng., is president of S.A.F.E. Engineering, a Toronto-based company specializing in industrial health and safety issues and PSR compliance. He can be reached 416-447-9757 or simonf@safeengineering.ca. For more information, visit www.safeengineering.ca.

Table 1: Major categories of laser use

Alignment

Annealing

Balancing

Biomedical

Cellular research

Dental

Diagnostics

Dermatology

Ophthalmology

Surgery

Communications

Construction

Alignment

Ranging

Surveying

Cutting

Displays

Drilling

Entertainment

Heat treating

Holography

Information handling

Copying

Displays

Plate making

Printing

Reading

Scanning

Typesetting

Videodisk

Marking

Laboratory instruments

Interferometry

Metrology

Plasma diagnostics

Spectroscopy

Velocimetry

Lidar

Special photography

Scanning microscopy

Military

Distance ranging

Rifle simulation

Weaponry

Nondestructive training

Scanning

Sealing

Scribing

Soldering

Welding