Laser Fundamentals and Safety

NOTE: The following is provided for information purposes only and is not intended to represent a guideline for the safe use of lasers or laser systems.

For a more complete discussion of laser safety principles and regulations, please consult the following organizations:

CDRH Center for Devices and Radiological Health. An agency within the U.S. FDA which publishes and enforces legal requirements on lasers.
To obtain copies of the CDRH requirements, contact:

CDRH (HFZ-312)
1390 Piccard Dr.
Rockville, MD 20850
Tele: (301)-427-1172

ANSI A U.S. organization that publishes standards for laser users. The ANSI laser standard is not a law, but it forms the basis for state and OSHA requirements for the use of lasers.
To obtain copies of the ANSI Standards, contact:

Laser Institute of America
12424 Research Pkwy. #130
Orlando, FL 32826
Tele: (407-380-1553
FAX: 407-380-5588

Laser Fundamentals

What is a Laser?
The word laser is an acronym which stands for Light Amplification by Stimulated Emission of Radiation. As implied by its name, a laser is a specialized light source which is capable of producing an intense beam of light (protons) possessing several very special properties. Every photon (particle of light) produced by a laser possesses identical scaler and vector properties in terms of frequency, phase, direction, and polarization. These properties are responsible for the real world observations of a laser's color, brightness, coherence, and unidirectional.

The principles of laser action were first described by Townes and Schalow in 1958 after first being postulated by Einstein in 1916. This initial technical description of a laser and its operation was followed by the first demonstration of a working laser in 1960 by Hughes Aircraft.

Properties of Laser Light

The two primary properties of laser light with respect to its use in both laser shearography and holography are interference and the existence of laser speckle.

Interference
The term interference describes the ability of multiple beams of laser light to add together either constructively or destructively. Constructive and destructive interference can be thought of analogously in terms of waves interacting in pool of water. If the peaks of two identical waves combine together, they amplify each other and produce a resultant peak which is essentially twice as high. This is identical to what happens when the peaks of two light waves combine (in phase) and is called constructive interference. When the peak of one wave combines with the trough of another identical wave, they will cancel each other out. The corresponding phenomenon is also demonstrated with laser light and is called destructive interference.

Laser Speckle
Because of the ability of laser light to interfere with itself, multiple laser beams such as those produced by the reflection of laser light off of a physical surface will combine to produce what appears to be a random pattern of bright and dark spots when viewed with the human eye or captured by a camera. This seemingly random pattern referred to as laser speckle, may be used to define the relative shape of the object being illuminated.

When this information is combined with a fixed wavefront (in this case speckle pattern) having the same properties as the laser light illuminating the object under investigation, an interference pattern (also in the form of a speckle pattern) is produced. When this secondary speckle pattern is compared to a similar speckle pattern of the object after being placed under or subjected to some form of loading, information about how the surface of the object has been distorted is revealed. This process, therefore, provides the basis for both electronic holography and electronic shearography nondestructive testing.

Laser Safety
The most effective approach to laser safety is one which combines knowledge of the potential hazards associated with a particular laser system and common sense usage. All lasers and laser products are governed by a four level classification guideline set forth in the United States by the Center for Devices and Radiological Health (CDRH). These classifications are based on the relative dangers associated with working with a given laser system based upon the systems operating wavelength (s), power output (note: this term takes on different meanings with regard to continuous operation CW and pulsed lasers), and beam expansion characteristics.

For laser systems, the overall classification of the system is dependent upon the aforementioned parameters as governed by the design and intended operation of that system. If a laser system is modified after manufacturing or is used in a fashion other than what it was designed for. The safety classification of that system must be considered as the highest applicable to the laser associated with that system.

Wavelengths
All lasers can be classified into three broad wavelength classifications: infrared, visible, and ultraviolet. The wavelength ranges for each of these classifications is as follows:

Ultraviolet 180Nm-400Nm

Visible 400Nm-700Nm

Infrared 700Nm-1mm

As implied by its name, the visible class of laser falls fully within the range of human vision. Although the upper extreme of the ultraviolet region and the lower extreme of the infrared region are "visible" these lasers are most often referred to as invisible lasers.

Laser Power Pulsed vs. Continuous Operation (CW)
The measure of a laser's power is based significantly upon whether the laser in question is designed to operate continuously (CW) or in a pulsed mode. According to the CDRH Standard, a pulsed laser is any laser which delivers its energy over a period of 0.25 seconds or less. Because of significant differences in laser operation, the power of CW lasers is measured in terms of the average power output of the laser or laser system during the designed operating time of the system. For pulsed lasers, the energy rating of the laser or laser system is given in terms of the total energy per pulse, pulse duration, repetition rate and emergent beam radiant exposure.

NOTE: At this time (4/94), all Laser Technolgy, Inc. laser systems are designed to operate in the CW mode.

CDRH Classifications

Class 1
Class 1 laser designations represent the safest form of laser or laser system. These systems are either interlocked such that the operator and bystanders do not have access to the laser emissions or are at a power level low enough to where there is no danger of damage to eyes or exposed skin. No medical surveillance of operators is required for the operation of such a laser or laser system. Laser light may be visible or non-visible.

Class 2
Class 2 laser designations apply to visible lasers and laser systems only. These lasers or laser systems have an accessible emissions level greater than Class 1 but less than 1 mW. The use of Class 2 laser systems does not require a medical surveillance program although general knowledge of laser safety is required.
Recommended safety precautions for the use of Class 2 laser systems include:

1. Never stare directly into the laser beam, its source, or at bright, specular surface requirements.
2. Limit access to the laser system to those familiar with its operation and safety requirements.
3. Ensure that bystanders are briefed as to the presence of the operating laser system and that they should avoid staring into the beam or at nay bright reflection emanating from the test area.

Class 3a
Class 3a laser designations apply to visible lasers and some non-visible lasers whose power output exceeds that for Class 2 (1mW) but is less than 5mW. Medical surveillance of appropriate personnel is no required for the use of Class 3a laser systems. Safety requirements associated with the use of Class 3a laser systems are essentially the same as those associated with Class 2 lasers. The primary safety concern for Class 3a lasers is to avoid staring into the laser beam or at bright specular reflections emanating from the test area. Applicable hazard signs must be posted in the immediate vicinity of the work area as required.

Class 3b
Class 3b laser system designations refer to both visible and non-visible laser whose outputs exceed those set forth for Class 3a lasers but is less than 500mW to 500mW. Hazards from Class 3b laser systems include potential eye damage as well as the potential for burns to skin exposed to the direct emissions of high powered Class 3b lasers (i.e. approaching 500mW).

Medical surveillance for appropriate personnel is required for the use of Class 3b laser systems as is a formalized introduction to lasers and laser safety. In addition to the safety requirements mandated for Class 3a lasers, the following must also be addressed.

1. A control area around the work area where Class 3b emissions will be present must be established and posted with appropriate warning signs.

2. A visual and audible warning indicator must precede any laser emissions.

3. The laser system must include a keyed safety interlock to prevent unauthorized use of the laser system.

4. Laser safety glasses must be made available to those working in the immediate area of the Class 3b emissions.

NOTE: All Laser Technology, Inc. laser systems incorporate an integral beam expansion system which causes the power density of the laser emissions to drop off rapidly with regard to the distance from the source (1/r2 relationship). Because of this, the restricted control area for the laser systems tends to be small (e.g. 5-10' beyond test area).

Class 4
Class 4 laser systems represent both visible and non-visible laser system which exceed the 500mW restriction placed on Class 3b laser systems. As in the case of Class3b lasers, medical surveillance of appropriate personnel is mandatory. In addition to the hazards associated with Class 3b lasers, fire and airborne pollutants (e.g. ozone) also need to be considered. Class 4 laser systems require strict supervision and restricted access to virtually all personnel. Because of these restrictions, Laser Technology, Inc., incorporates Class 4 lasers only as integral components of laser systems designed to be maintained and operated as Class 3b or lower laser systems.