Laser Safety in the Workplace

 

Lasers have a certain stigma attached to them: That they are a very dangerous form of light. I teach that a laser in the workplace is most like the intense light from a welding arc. The first safety precaution for everyone is to avoid staring into the laser light or any strong reflection of it. If the laser light is not collimated (expanding), which is typical of many NDT systems, it is safer the farther you are from the source. The light loses energy very rapidly (in accordance with the inverse square law). Uncollimated laser light (in which the rays are not parallel) is typically safe at distances greater than 3 m (10 ft). A collimated laser beam (consisting of parallel rays) can be more dangerous because it maintains its power density even at great distances; direct eye exposure should therefore be avoided (even with laser pointers).

Considering the physics, laser light is typically one color out of the rainbow of colors (blue, green, or red). So in that sense, it is normal light, but, like a welding arc, the light comes from a point source. Point source light is concentrated by the lens of the eye, which focuses the light to a point on the retina. If the eye views high intensity, point source light, it may burn the cell or cells that it is focused on, damaging vision.

Lasers come in many forms. The older versions were quite large gas ion lasers, such as the common helium neon (red), the industrial argon (blue-green), and the CO₂ (infrared), used for cutting. These are like high power fluorescent tubes with some precision optics. The newer laser technology is diode lasers, which can be as small as a laser pointer, or as powerful as an old gas ion laser, but in a smaller package. The attraction of laser light is that it is an absolutely pure color, unlike anything that is seen in the natural world. The beautiful red from a helium neon laser has a wavelength of precisely 632.7 nm. Any apparently similar red seen in the natural world actually consists of a range of of red wavelengths, among which the human eye cannot descriminate. Industry uses this feature of the laser (the single wavelength) as a measuring stick for high precision measurement through a technique called interferometry, for NDT methods such as holography, electronic holography (ESPI), and shearography. Other industrial applications use the pointing ability of the laser beam for measurement with the technique of laser profilometry.

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Lasers are powerful tools in the modern world, and have extensively infiltrated our lives since their development in the 1960s.

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Another feature of the pure laser light is that it creates a three dimensional speckle pattern in space. This is psychedelic to observe, affecting each person's eyes a little differently. It is absolutely harmless, because it consists of the pure laser light waves interfering with themselves, thereby creating bright and dark spots (constructive and destructive interference). This speckle pattern is used by the modern electronic forms of holography, electronic speckle pattern interferometry (ESPI), and shearography (shearing speckle interferometry).

Lasers are classified by the amount of power that they emit, which defines the amount of damage that they can do. If the laser is used in a system, then that system will have a system classification (which may be quite safer than the laser itself). The laser and the laser system will have specific caution or danger labels placed in a highly visible location on the enclosure, defining its classification. The classifications for lasers and laser systems are:

Class I - Very low power, or inherently safe
Class II - Caution: low power and safe (typically < 1 mW)
Class IIIa - Caution: safe for industry (typically < 5 mW)
Class IIIb - Warning: Danger for eyes (typically < 500 mW)
Class IV - Warning: Danger for eyes and skin (typically > 500 mW)

In addition to the power of a laser, its wavelength has a major impact on laser safety. If you cannot see the laser emission, that is, if the laser emits outside the visible range, it may be more dangerous, because your normal blink and aversion reactions do not function. You could be dangerously irradiated and not know it. The visible spectrum lies between 400-700 nm (violet to deep red). If the wavelength of the source is shorter it is called ultraviolet (UV), or if it is longer it is called infrared (IR).

All laser systems manufacturered are required to be reported to - and meet the requirements of - the Center for Devices and Radiological Health, a Division of the FDA (CDRH). These requirements are defined in Part 1040.10 of the Code of Federal Regulations, Title 21, Subchapter J (see below on how to reach CDRH). There must be a label on the product showing that it complies with the requirements of these federal regulations.

The laser safety reference for the user of laser systems is The Safe Use of Lasers, which is published by ANSI (Publication Z136.1); it is a detailed guide on how to use lasers safely. This can be obtained from the Laser Institute of America (see below). This document forms the basis of OSHA and local laws for the safe use of lasers.

Lasers are powerful tools in the modern world, and have extensively infiltrated our lives since their development in the 1960s. They are used in everything from CD players to fiberoptic communications to NDT with ESPI, shearography, and profilometry. Like any tool, they need to be handled with care and knowledge. The ASNT Laser Methods Group is spearheading the introduction of lasers into the NDT world, and has recently received approval for inspector level certification to NDT Level I and II under Recommended Practice No. SNT-TC-1A. If you need more information, please contact the author or the ASNT Laser Methods Group.

Contact Information
ASNT Laser Methods Group officers: Chair Jim Doyle; (888) 644-6972 X215; e-mail doyle@nwra.com; Vice Chair John Tyson; (610) 688-0887; fax (610) 687-9221; e-mail: tyson@trilion.com.

CDRH (Center for Devices and Radiological Health - FDA) CDRH (HFZ-312) 1390 Piccard Drive, Rockville, MD 20850; (301) 594-4654; fax: (301) 594-4672.

The Safe Use of Lasers, ANSI Publication Z136.1 is available from The Laser Institute of America, 12424 Research Parkway, #130, Orlando, FL 32826; (407) 380-1553; fax: (407) 380-5588.

* Trilion Quality Systems, 241 Atlee Rd., Wayne, PA 19087-3835; (610) 659-5363; fax: (610) 687-8221; e-mail tyson@trilion.

 

Copyright © 1999 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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