Patty's Industrial Hygiene, Physical and Biological Agents. Группа авторов

a high‐power, Class 4 laser is partially enclosed such that the risk of occupational exposure is very low, the level of control can be greatly reduced, but it is not necessary to invent another “conditionally safe” class. It is best to try to educate people that Class 4 does not necessarily imply a serious risk and that the controls adopted should be appropriate to the circumstances, rather than defining a new class for “conditionally safe” products. The current system of classification is based on the level of hazard with only an implication for the degree of risk, and this is the basis of the recommendations given in user guidelines. The class indicates the potential hazard of the product, but the actual risk is not fully defined by the class, since it depends on the environment in which it is located and the people potentially exposed. For example, a high‐power, Class 4 industrial, material‐processing laser that is Class 4 because the enclosure does not have a fully light‐tight enclosure, and perhaps does not have a roof could be quite acceptable in locations where the ceiling is close to the enclosure, but not in another location where there are walkways at a higher level that provide a direct view into the enclosure. In a user safety standard, the laser safety expert might classify the enclosure adequate for Class 1, but the manufacturer might have to retain the Class 4 label, since the system could be installed in any location. ANSI Z136.1 incorporates the concept of greatly relaxed control measures when Class 1 conditions exist and the hazardous area is very small or inaccessible.

      4.6 The Nominal Hazard Zone

For lasers where the direct beam and potential reflections are very limited, the concept of a nominal hazard zone (NHZ) is particularly useful. Control measures are only required within the defined NHZ for the laser operation. This may occur where the emitted beam is close to the AEL for Class 1, or as is more likely, when the beam is highly diverging within the work zone. A beam can rapidly diverge if beyond the focal point of the laser beam (e.g. for a laser welder or cutting beam), or the beam diverges rapidly from an optical fiber tip.

5 LASER SYSTEM CONTROL

      In commercial laser products, the laser operation is controlled by the operator through the use of a series of electronic and optical controls on the laser and its delivery system. First, there is the main power switch that energizes the laser system, a control that varies the output power, and a firing or operation switch that triggers the laser output. This may be in the form of a foot or hand switch. For many systems, the laser output may be under microprocessor control for some or all of its exposure conditions.

      Stand‐by or ready switches are frequently encountered to enable the user to place the equipment in a stand‐by status to avoid accidental or unintentional firing of the laser. The standby function maintains power to the laser during workpiece preparation and allows rapid start‐up of the laser as application is needed.

      A shutter to provide predetermined exposure durations may be installed to interrupt a CW laser emission. Further controls that are frequently present are a timer and a time‐interval or pulse‐duration control switch.

      5.1 Delivery Systems

      All industrial and medical laser systems require a beam delivery system that is responsible for directing the power output of the laser to its target site of action. The application requirements and design of a laser's delivery system will, to a great extent, determine its hazards. It is quite apparent that a fixed delivery system that is incorporated into another instrument or machine will be safer than delivery systems that can be freely moved in space. A CO₂ laser beam directed downward by a focusing lens is fixed in space and even if directed by a joystick or similar control generally allows very limited movement of the beam. In many material‐processing systems, the laser beam is fixed (or with very limited movement) and is directed downward, and the work‐piece is moved under it. Fixed delivery systems are also typically found in many medical lasers (e.g. in ophthalmology) and may be connected to a microscope that permits the surgeon to both view the operative site and deliver the laser energy to the desired treatment site. Examples of more hazardous systems are hand‐held laser rangefinders, high‐power Class 3B laser pointers, and hand‐held surgical lasers which employ a freely moveable handpiece.

      Delivery systems that are available to carry the laser beam's output from the laser cavity to the point of application may use either fiber optics or a conventional optical path, which is either fixed or articulated. The fiber optic delivery system is generally favored because of ease of use and its great flexibility for directing the beam to the worksite.

      There are very real and practical limits to the use of fiber optics. Suitable fiber optic materials capable of transmitting the laser beam are not available for all wavelengths. For example, the 10.6 μm output wavelength of the CO₂ laser does not pass through conventional quartz or glass fiber optics. Because of this, CO₂ lasers, which are in widespread use in surgical and industrial laser systems, are limited to situations for which an articulated arm can be used. There have been frequent efforts to extend the applications of the CO₂ laser by developing fiber optic materials that transmit its output. These efforts have been only partly successful.

      An articulated arm contains a series of mirrors that are mounted on pivots to allow the laser beam to be guided from its source in the laser cavity to its point of application. Typical delivery systems available today for the carbon‐dioxide laser employ hollow tubes to form an articulated arm. The laser energy may be transmitted through a fixed optical system as is commonly done in laboratory instruments or in excimer laser material processing equipment.

6 LASER BEAM MEASUREMENTS

      To those in radiation protection, it is at first somewhat surprising to learn that instrumentation and measurements are not the focus of laser safety. Laser measurements often require sophisticated equipment and fortunately are seldom essential for laser hazard evaluation, since the manufacturer must classify the laser product, and as noted above, actual use of the MPEs (ELs) is infrequent (8, 9, 20).

      Unlike some workplace hazards, there is generally no need to perform measurements for workplace monitoring of hazardous levels of laser radiation. Because of the highly confined beam dimensions of most laser beams, the likelihood of changing beam paths and the difficulty and expense of laser radiometers, current safety standards emphasize control measures based upon hazard class and not workplace measurement (monitoring). Measurements must be performed by the manufacturer to assure compliance with laser safety standards to assure proper hazard classification. Indeed, one of the original justifications for laser hazard classification related to the great difficulty of performing proper measurements for hazard evaluation.

In this regard, MPEs are exposure limits measured at the points in space where individuals are potentially exposed and are used for occupational safety and health assessments. ACGIH has published and revised laser threshold limit values™ (TLVs) since the late 1960s, and ANSI Z136 has incorporated MPEs since 1973. On the international scene, guidelines for human exposure originate from ICNIRP. The IEC product‐safety committee, IEC TC76, has gone on record on a number of occasions over the last 20–30 years that it recognizes WHO and ICNIRP as the source of the MPEs. IEC develops product safety standards that regulate emission and employ AELs, which are derived by IEC from the MPEs and other considerations.

      When considering potential health hazards, it is always crucial to establish the output wavelength or wavelengths (9). If for some reason there is a doubt about whether more than one wavelength is being produced in a laser operation, one can simply place a prism at the output aperture of the laser and look for indications of secondary beams on a white target card if the power is sufficiently low (e.g. below 5 mW) to visualize without eye protection. UV and blue laser lines would be visible by fluorescence from the white paper even through eye protection, but IR lines would be visible only with very special phosphor cards or an IR image converter.

7 LASER EYE PROTECTION

      Laser eye protection becomes of great importance when engineering controls such as enclosures, baffles,