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СКАЧАТЬ for each class, based upon the actual hazards posed by the laser beam. Generally, the scheme follows a grouping of four broad hazards classes: 1 through 4. Class 1 laser products cannot emit potentially hazardous laser radiation and poses no health hazard. Classes 2 through 4 pose an increasing hazard to the eye and skin. The classification system is useful, since safety measures are prescribed for each class of laser. More stringent safety measures are required for the highest classes.

      The laser safety classification system greatly facilitates the determination of appropriate safety measures. Laser safety standards and codes of practice routinely require the use of increasingly more restrictive control measures for each higher classification. Hence routine measurements are normally not required.

      4.1 Class 1

      Class 1 is considered an “eye‐safe,” no‐risk grouping. Most lasers that are totally enclosed (for example laser compact disc recorders) are Class 1. No safety measures are required for a Class 1 laser. Class 1 lasers are frequently referred to as “eye‐safe” lasers. In the past few years, a new, conditional class has been introduced: Class 1M, which is effectively “eye‐safe” unless the beam is directly viewed with optical instruments. Two conditions exist whereby optical instruments can collect more energy and increase the ocular hazard. Condition 1 is the most obvious and the most serious case: where a telescope or binocular is placed in the collimated laser beam and can introduce far more energy into the eye if there is a retinal hazard, or concentrate the energy on the cornea for laser wavelengths outside the retinal hazard region. The second condition is where a hand magnifier or jeweler's eye loupe is used to examine a highly diverging laser beam (generally emitted from an optical fiber) and recollimate the beam so that it can be concentrated on the cornea or focused on the retina. This is a rather unrealistic viewing condition except when service technicians examine the tip of an optical fiber. The safety criteria for this second viewing condition were re‐examined by standards committees, such as IEC TC76 in 2014, and it was concluded that the applicability should be limited to unfiltered microscope (eye‐loupe) optics used when servicing optical‐fiber communications systems.

      4.2 Class 2

      Class 2 refers to visible lasers that emit within the 400–700‐nm spectral band with a very low power (less than 1 mW) that would not be hazardous even if the entire beam power entered the human eye and was focused on the retina. The eye's natural aversion response to viewing very bright light sources protects the eye against retinal injury if the energy entering the eye is insufficient to damage the retina within the aversion response. The aversion response is composed of the blink reflex (approximately 0.16–0.18 second) and a rotation of the eye and movement of the head when exposed to such bright light. Current safety standards define the aversion response as lasting 0.25 seconds. At one time, in order to simplify teaching the meaning of the aversion response, some termed it the “blink reflex,” but while this helped some students, it later was called into question when it was pointed out that some individuals did not experience the blink reflex under this exposure. Thus, Class 2 lasers have an output power of 1 milliwatt (mW) or less that corresponds to the permissible EL for 0.25 seconds. Examples of Class 2 lasers are laser pointers and some alignment lasers. Another conditional sub‐class, Class 2M is analogous to Class 1M, but the optical hazard is applicable only for visible lasers that would be Class 2 if measured only with a 7‐mm collecting aperture.

      As a note, at one time, US safety standards also incorporated a sub‐category of Class 2, referred to as “Class 2A.” Class 2A lasers, which were not hazardous to stare into for up to 1000 seconds (16.7 minutes). Most laser scanners used in point‐of‐sales (super‐market checkout) and small inventory scanners are Class 2A, but this class was no longer required when the ELs were adjusted (ca. 2000) such that a point‐source laser was no more hazardous if viewed for any duration greater than 10–100 seconds.

      4.3 Class 3

      Class 3 lasers pose a hazard to the eye, since the aversion response is insufficiently fast to limit retinal exposure to a momentarily safe level, or if damage to other structures of the eye (e.g. cornea and lens) could take place. Skin hazards normally do not exist for incidental exposure. Such lasers would rarely pose a realistic fire hazard. Examples of Class 3 lasers are many research lasers and military laser rangefinders.

A special subcategory of Class 3 is termed “Class 3R” (once termed Class 3A in US standards). The remaining Class 3 lasers are termed “Class 3B.” Class 3R laser products are those with an output power between one and five times the “accessible emission limit” (AEL) for the Class 1 or Class 2 (if the emission is between 400 and 700 nm). Class 3R should be thought of as a transitional class, since the laser beam irradiance exceeds the applicable EL, referred to as the “MPE,” but injury is unlikely from a standpoint of probabilistic risk assessment, i.e. the likelihood is very small that the eye will be perfectly positioned, focused for worst‐case viewing (retinal hazards) and the person is particularly sensitive (19). The “R” in Class 3R refers to “reduced requirements” for product safety standards and very limited control measures are required for the user. Examples are many laser alignment and surveying instruments. In the 1.2–1.4 μm spectral band, the Class 3R may not exist for some CW lasers where the range between Class 1 and Class 4 becomes very small.

      4.4 Class 4

      Class 4 lasers may pose a potential fire hazard, a significant skin hazard, or a diffuse‐reflection hazard. Virtually all surgical lasers and material processing lasers used for welding and cutting are Class 4 if not enclosed. All lasers with an average power output exceeding 0.5 W are Class 4. If a higher power Class 3 or Class 4 is totally enclosed so that hazardous radiant energy is not accessible, the total laser system could be Class 1. The more hazardous laser inside the enclosure is termed an “embedded laser.”

      4.5 Newer Conditional Classes 1M and 2M

      In 2000, the world safety community began to recognize the need to make some minor revisions in the traditional laser classification systems, with the creation of a Class 1M and Class 2M and expand Class 3R for greater consistency in dealing with the potential for optically aided viewing. Telescopes can greatly increase the hazard of intra‐beam viewing of the beam at a great distance. In this case, a standardized 50‐mm aperture is assumed to collect a collimated beam at a distance (e.g. a 7 × 50 binocular), rather than the usual 7‐mm aperture to simulate the pupil size for a dark‐adapted eye. The IEC standard also assumed a Class 1M and Class 2M condition for viewing a highly diverging beam – if from a fiber‐optic – with an eye‐loupe or magnifier. The eye‐loupe viewing risk was never considered a realist risk for the ANSI Z136 standards, except in the application‐specific, or “vertical” standard, ANSI Z136.2, Safe Use of Lasers in Optical Communication Systems.

      The addition of those newer classes recognized the value of having conditional classes. Another proposal that met with some favor for dermatological lasers was the IEC Class 1C conditional class for skin‐treatment lasers that only fire when in firm contact with the skin. Technically the skin‐treatment lasers cannot be considered in Class 1, even if the eye is always protected, since the skin is exposed above skin ELs. There are industrial material‐processing lasers that are Class 4 but are considered technically Class 1 because they will only emit when in firm contact with the surface being treated. In this case, the treated surface is considered part of the protective housing.

      The subject of still further refinements sometimes arises in standards committees. From time to time, there have been efforts in laser standards committees to “update” or “improve” the classification of some Class 3 eye‐hazardous lasers and the most hazardous lasers (Class 4). Proposals are normally to insert additional classes based upon a concept that hazards increase gradually and the dynamic range of Class СКАЧАТЬ