Safety concerns about laser pointers

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  • Safety concerns about laser pointersDavid H. Sliney and Jerome E. Dennis Citation: Journal of Laser Applications 6, 159 (1994); doi: 10.2351/1.4745352 View online: http://dx.doi.org/10.2351/1.4745352 View Table of Contents: http://scitation.aip.org/content/lia/journal/jla/6/3?ver=pdfcov Published by the Laser Institute of America Articles you may be interested in Green Diode Laser Pointers Have Now Dropped in Cost to About $10 Phys. Teach. 47, 479 (2009); 10.1119/1.3225522 Safety recommendations for laser pointers J. Laser Appl. 10, 174 (1998); 10.2351/1.521848 How to modulate a pointer laser Phys. Teach. 33, 58 (1995); 10.1119/1.2344137 Diffraction photographs with a Laser Pointer Phys. Teach. 32, 174 (1994); 10.1119/1.2343949 AAPT concerned about FDA lasersafety rules Phys. Today 27, 63 (1974); 10.1063/1.3128825

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  • JOURNAL OF LASER APPLICATIONS (1994) 6, 159-164

    LASER SAFETY

    Safety concerns about laser pointers

    DAVID H. SLINEYI AND JEROME E. DENNIS2

    1Laser Microwave Division, US Army Environmental Hygiene Agency, Aberdeen Proving Ground, MD 21010-5422, USA, and 2LightProducts Branch, Division of Standards Enforcement, Office of Compliance and Surveillance, Center for Devices and RadiologicalHealth, Food and Drug Administration, Rockville, MD 20857, USA

    Accepted for publication 7 Apt111994

    In the past two years considerable concerns have been expressed about the safety of Class 3A laserpointers. The concern has been that Class 3A diode-laser pointers have replaced the saferheliumneon (HeNe) Class 2 laser pointers. Hundreds of thousands of small HeNe visible-wave-length lasers have been traditionally used for alignment and pointing, laser demonstrations andlaser displays in science, education and industry, but can the diode laser be as safe and effective?Not infrequently, some people associate "lasers" with Buck Rogers and "Star Wars", and areconcerned whether their use in pubic is safe. This safety issue is raised and the risks of viewingsmall lasers are'compared with viewing the sun or bright spotlights. It is shown that HeNe lasersthrough Class 3a (up to 5 mW power) are not a significant eye hazard; however, Class 3A diodelasers may not elicit a strong "aversion response" in some individuals, and greater precautionsmay be necessary than with HeNe lasers of the same power.

    KEYWORDS: laser safety standards; diode-laser pointers

    INTRODUCTION 1

    With the advent of relatively low-cost visible laser diodesover the past few years, laser pointers for use in lecturesand demonstrations have become very popular. Sales inthe US alone are now of the order of many thousands ofunits annually. Recently, however, concern has been ex-pressed by some experts in the laser safety communityabout the safety of these products and the effectiveness ofexisting standards in addressing the hazards. The safetyconcerns stem from the facts that these products: (a) aretypically used by persons who cannot be expected to bereasonably familiar with the appropriate user precautionsnecessary to ensure the safe use of the products, and (b)are Class 3A, since they emit radiant powers of a fewmilliwatts (i.e. above the Accessible Emission Limit(AEL) of Class 2) at wavelengths of approximately 670 nm[1, 2].

    Comparable visibilities of laser spots

    At a diode wavelength of 670 nm the eye's visual sensitiv-ity is considerably reduced. At 670 nm the CIE photopicvisual sensitivity is 0.032 compared to 0.24 at 633.8 nm or1.0 at 550 nm. This is illustrated in Fig. 1. In other words,the apparent brightness of a pointer's spot from a 1 mW,

    Disclaimer: The opinions or assertions herein are those of the authorsand should not be construed as reflecting official governmental positions.

    0.8

    0.2

    0350 400 450 500 550 600 650 700 750 800

    Wavelength, (nm)

    FIGURE I. CIE photopic (daylight color vision) relativespectral sensitivity function V. Note the significant differencein visibility of the HeNe versus the 670 nm diode laser.

    670 nm beam is only 3% of a 1 mW, 555 nm beam of thesame spot size or 13% of the brightness of a 632.8 nmHeNe laser of the same size. The yellow-green wave-length of 555 nm is the peak of the photopic (daylight)sensitivity function. To achieve the same brightness of a0.8 mW Class 2, HeNe laser pointer, the manufacturer of

    1042-346X 1994 LASER INSTITUTE OF AMERICA

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  • 160 SLINEY AND DENNIS

    a 670 nm diode laser pointer must achieve an outputpower of 6 mW, which would place the laser product inClass 3B! In fact, most 670 nm laser pointers measured bythe authors do not exceed 4 mW, which corresponds invisibility to a 0.53 mW HeNe laser.

    Pointers used with firearms

    In addition to their use as pointers, similar products aremarketed as aiming aids for firearms. However, becausefirearms have lethal potential, it is hoped that a similarlevel of caution would be practised to that taken with thefirearms to which these lasers are attached.

    HE NE AND DIODE LASER USE

    Until recently, the most common visible-wavelength laserhas been the HeNe laser which emits a beam of red lightat 632.8 nm. Because the HeNe laser was consideredrelatively efficient and can be readily constructed to emita low power of 1-5 mW, it has been by far the mostcommon visible-wavelength continuous-wave (CW) laserused in teaching, displays and alignment. However, duringthe past years, a family of GaAlAs visible, red-light emit-ting diode lasers have been rapidly replacing the Class 3AHeNe laser in many applications. The diode lasers areless expensive and more compact than their more visibleHeNe gas laser counterparts, but the beam is less visible.Laser pointers are generally either Class 2 or Class 3A.

    VISIBLE -BEAM LASER HAZARD

    Class 2 and Class 3 visible-wavelength lasers are a safetyconcern only if one looks directly into the collimated laserbeam. Since the beam of some of these lasers may be only1-3 mm in diameter, most, if not all, of the beam canenter the pupil of the eye which itself normally has adiameter of 2-7 mm. However, for most laser applicationsit is easy to see that it is highly unlikely for such a smalllight beam to be so perfectly oriented as to be directedunintentionally into someone's eye. But if the small beamshould enter the eye, the viewer will be surprised by such abright light and immediately blink and turn his or herhead. This is the normal physiological response to brightlight, and occurs if one suddenly turns one's head intothe sun. It is referred to in laser safety standards as theaversion response [3]. The blink reflex is normally con-sidered to take place in less than 0.2 s, and certainly lessthan 0.25 s. Therefore, if one unexpectedly looks directlyinto the sun when it is not near the horizon, the longestexposure duration should be 0.25 s. The same aversionduration applies to looking at a bright spotlight at a veryclose distance. It is not normal to continue to look (i.e. tostare) at the sun or other very bright lights. If one forcesoneself to stare at the sun for more than 2 min, the result

    can be a permanent injury to the retina, with loss of vision.When people force themselves to look at the sun during aneclipse, the result is referred to as "eclipse blindness" [3].

    APPLICABLE LASER SAFETY STANDARDS

    There are two general types of laser standard in the USA.One is designed for the user [2]. It was developed byconsensus and is strictly voluntary but, because of itswidespread acceptance, many companies and organiza-tions consider it regulatory. Thus when a dispute arises, itis considered the accepted standard of safety in manycourts of law. The second type of standard is a regulationof the Federal Government, but applies only to lasermanufacturers and commercial laser products [1]. Thestandards themselves are complicated, and a detailed ex-planation cannot be presented here. However, some gen-eral explanation of how these standards relate to low-power visible lasers can be provided. Both of thesestandards are relevant to the concerns raised here, andmake use of hazard classifications in order to specify safetycontrol measures.

    Lasers are grouped into several hazard classes based upontheir potential for injury. Class 1 is considered safe underany viewing conditions, whereas Class 4 at the other endof the scale includes very powerful lasers used in weldingand cutting, for which stringent safety measures apply.Class 2 lasers are those visible lasers which have an outputpower of 1 mW or less. Class 2 lasers do not pose a realhazard under conditions of prudent use. These lasers, likethe sun or a slide-projector lamp, are not a realistic hazardwhen used sensibly, but they represent a theoretical poten-tial hazard. If one were to force oneself to stare directlyinto the laser beam ("intrabeam viewing"), one couldexceed the appropriate Maximum Permissible Exposure(MPE) limit A critical assumption is that any Class 2 laseris of a visible wavelength sufficient to be seen as toodazzling to view. Class 3 is divided into two groups: a andb. Class 3b lasers require safety measures to precludehazardous intrabeam viewing, since the direct beam ishazardous to the eye. For visible-wavelength lasers, Class3a is currently defined in the US (but not internationally)[4] as those with an output power greater than 1 mW butless than 5.

    Class 3a represents a narrow transition range betweenClass 3b and Class 2. Initially it was to include only those1-5 mW lasers which had an expanded beam to be saferby emitting less than 1 mW within a 7 mm aperture (i.e.within a dilated pupil). This definition is still used in theinternational standard [4]. A Class 3a warning label similarto that for Class 2 cautioned one not to "stare into beamor view with optical instruments." The Center for Devicesand Radiological Health (CDRH), which is responsible for

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  • LASER SAFETY 161

    the Federal laser performance standard for manufacturers,later revised the definition of Class IIIa (the Federalstandards use Roman numeral notation to designate theclasses; the IEC standards use upper-case letters to desig-nate classes 3A and 3B). The revised CDRH definitionincluded all 1-5 mW lasers since there had never beena report of eye injury in years of use of hundreds ofthousands of 1-5 mW HeNe lasers (Federal Register,Vol. 48, No. 231, pp. 54164-54182, Nov. 30, 1983, andVol. 50, No. 161, pp. 33682-33702, August 20, 1985). Theuser standard (ANSI Z136.1-1993) soon followed suit, butthe old distinction stijl exists in the international lasersafety standard [4]. To the user of a HeNe laser [4] theimportant fact is that there is no reason to expect thatactual eye injury could occur until the laser power enteringthe eye exceeded 5 mW [5]. A 5 mW laser might becapable of injuring a small part of the retina and cause ablind spot if an individual forced himself or herself to stareinto the bright light for more than 10-20 s. One shouldremember that the 1-5 mW HeNe laser has been usedwidely in the construction industry for alignment of pipe,etc., for more than 15 years without any known eyeinjuries.

    The American National Standard [2] requires that Class 3aand Class 2 lasers used for display and alignment be usedso as not to expose people to the direct beam unless thebeam irradiance has dropped below the MPE. The beamshould not be directed at the eye, and in an unsupervisedlocation steps should be taken to prevent access of thepublic to the direct beam. Of course diffuse reflectionsfrom a screen or scattered light from smoke are not thedirect beam and are considered completely safe (and com-fortable to view). It should be remembered that if visiblelaser light from a HeNe laser is comfortable to view, it isfar below safe exposure limits (MPEs) [3].

    Federal Laser Product Performance Standard

    Laser pointers and aiming devices, as well as all otherproducts containing lasers that are sold or imported intothe US, are subject to the Federal Performance Standardfor Laser Products that is administered by the Center forDevices and Radiological Health (CDRH) of the UnitedStates Food and Drug Administration [1]. This standardrequires that all lasers be put into a hazard class deter-mined by the output radiant power level. This class, inturn, determines the requirements for controls, indicatorsand warnings on the product and in its supportingliterature such as promotional brochures and instructionsfor use. However, this standard impacts on the manufac-turers and importers of these products and on the productsthemselves, but can do nothing to ensure (by regulation)that the products will be used safely once the laser hasbeen purchased.

    ANSI Standard for the Safe Use of Lasers

    Safety during use is the scope of-the American NationalStandard for the Safe Use of Lasers, published by theLaser Institute of America (LIA) and the AmericanNational Standards Institute (ANSI Z136.1-1993) [2].However, because these products are sold without restric-tion to the general public, it would be unrealistic toassume that the purchasers would be aware of the exist-ence of this standard, much less of its requirements. Someof the states and the Federal Occupational Safety andHealth Administration (OSHA) of the US Department ofLabor base their laser safety requirement on the ANSIstandard, but the authority is generally restricted to safetyin the workplace. Although the ANSI standard does con-tamn requirements for laser product safety, it defers to theCDRH standard for products sold commercially and thatare certified by their manufacturers under the CDRHstandard.

    International Standard

    In the international arena, the most important standard isthe Radiation Safety of Laser Products, Equipment Classi-fication, Requirements and User's Guide (Publication 825)[3, 4] of the International Electrotechnical Commission(IEC). This standard addresses both product safety aspectsand user safety.

    Harmonization of Standards

    Despite recent efforts to harmonize the requirements un-der these three important standards, there are differencesthat confuse the requirements that are applicable to theseproducts [5]. Pointers are considered to be in the categoryof "surveying, leveling or alignment laser products" underthe CDRH standard or "alignment laser products" underthe IEC standard because they are intended to ". . .deter-mine or delineate the . . . position of a point, or defining. . . a straight line." This definition puts these products inthe same category as construction lasers.

    Since first promulgated in 1976 the CDRH standard hasimposed a radiant power limit of 5 mW within the visiblewavelength range (400-700 nm in the ANSI Standard,400-710 nm in the CDRH Standard) on these products.Since the most recent amendments to the standard in 1985,these products have been in Class Ina and would normallybe required to have a beam shutter, emission indicator andDANGER logotype label warning against direct eye ex-posure. Similar warnings are to be provided in the instruc-tions for use. However, under authorities provided by itsregulations, the CDRH has granted approvals for momen-tary power switches in lieu of beam shutters and emissionindicators for specific products of this type upon writtenrequest by the manufacturers or importers.

    JOURNAL OF LASER APPLICATIONS 6(3) (1994)

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  • 162 SLINEY AND DENNIS

    The ANSI standard does not include a specific definitionunder which such lasers could be limited. However, visiblelaser pointers with output levels between 1 and 5 mWare placed in Class 3A. As such, a beam shutter is re-commended but not required and an emission indicator isneither recommended nor required.

    The IEC standard, although it contains a definition ofalignment laser products, does not contain a specific out-put limit for such products. The User's Guide section ofthis standard contains guidance recommending that onlyClass 2 lasers be used in outdoor and construction applica-tions, and recommends additional safety precautions whenClass 3A lasers are used. Under this standard, visible ClassIIIA laser products are limited to a radiant power of 5 mWand also an irradiance of 25 W m-2, i.e. 2.5 mW cm-2.

    The above discussion illustrates that the requirementsunder the safety standards are not consistent in theirtreatment of these laser products as far as classificationand safety features are concerned. The standards areconsistent in requiring that direct eye exposure is to beavoided, i.e. the equivalent warning statement is required.

    MAXIMUM PERMISSIBLE EXPOSURE (MPE) LIMIT

    The ANSI standard probably provides the superior ap-proach in guiding a discussion of the exposure hazardsassociated with these products. This standard builds uponthe principle of avoidance of human exposure to levels oflaser radiation that exceed defined levels of MPE. TheMPEs are derived from photobiological and thermal tissueresponse data. An MPE can be stated either in terms ofthe maximum energy that can be permitted during aspecified duration of exposure, or, as in this case, in termsof the maximum duration of exposure than can be per-mitted at a specified wavelength and radiant power. Sincethese products are limited by the CDRH standard to5 mW and at visible wavelengths, the MPE for the eye isthe appropriate limit for discussion.

    From the expression for the MPE for Ocular Exposure(Intrabeam Viewing) to a Laser Beam (Table 5,ANSI Z136.1-1993), we obtain a radiant exposureof 0.636 mJ cm-2, corresponding to an irradiance of2.5 mW cm-2. The total power entering a 7 mm (0.4 cm2)pupillary area is then 1 mW. This calculation is the basis ofthe AEL for Class 2. However, it is well recognized that aslight excursion over the 1 mW AEL will place a laser inClass 3A; however, this power cannot suddenly be danger-ous; the risk of eye injury clearly increases with increasingpower. There is no fine line between "safe" and "hazard-ous." The actual 0.25 s retinal injury threshold for aminimal retinal image at 632.8 nm was shown in carefullaboratory studies by Ham et al. to be approximately

    12.9 mW ( 4.1 mW as 1 S.D.) [5]. The lowest individualthreshold obtained in that study was at 6 mW. Thethreshold gradually decreases with increasing exposureduration. The MPE at 10 s is 1 mW cm-2, or 0.4 mWentering the eye; hence, if any individuals were to forcethemselves to stare into a less dazzling 670 nm diode laser,the risk of an actual injury might become significant afterseveral seconds. Hopefully, individuals with greatly de-pressed color sensitivity would still limit their exposure toa fraction of a second or, in any case, certainly less than10 s.

    Most alignment lasers used in the construction industryhave been Class 3A, and have a maximum beam irra-diance of approximately 2-5 mW cm-2 because the beamis typically expanded to a diameter greater than 7 mm, andtherefore below the MPE for a 0.25 s exposure or virtuallynever more than twice the MPE. If the 5 mW beam powerwere averaged over a 7 mm diameter aperture, the beamirradiance would be 12.5 mW cm-2. With the constrictedpupil in real-life outdoor situations, a beam power exceed-ing 1 mW seldom, if ever, would actually enter the eye.This may not be assumed to be the case for the small laserpointers at very close range.

    COMPARISONS WITH CONVENTIONAL SOURCES

    To better appreciate the potential hazards of He-Nelasers, this small laser should be compared to other brightlight sources. For light safety purposes, two light sourcesfamiliar to people in the entertainment industry are usefulto compare with the He-Ne laser: the sun and a 1000 WPAR spotlight. To make a comparison, one must firstunderstand how such light sources cause harm to the retinaand how they are focused on the retina. Fig. 2 shows howa laser beam is focused to a very small, intense spot on theretina, whereas, a spotlight or the sun is imaged as a

    LAMP

    FIGURE 2. The parallel rays of a laser are focused toa minimal "point" image on the retina. By contrast,a conventional lamp source is imaged as an extendedsource.

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  • ElectricWelding Arc or

    Carbon Arc

    1.0

    3[VDT)

    10'

    Laser 11mW

    101

    10'

    Tungsten Maften SteelFilament

    Pyrotechnic Flare

    Frostedlncandescent Lamp

    Outdoor

    Fluorescent Daylight 2Lamp

    Candle

    BlueLight

    Hazards

    .2 -g0E

    cisa0k, Hazard

    1 0"

    10'

    LASER SAFETY 163

    larger, less concentrated spot. But to make a fair compari-son, one must determine not only how small is the image,but how much light (i.e., how many mW) enters the eye.

    The sun is the brightest light source that one is normallylikely to view, even momentarily. When the sun is high inthe sky, the light level (irradiance) falling on the earth isabout 100 mW cm-2 (or about 100 W ft-2). If the eyes lookdirectly at the sun the pupils normally close down to atleast 2 mm in diameter. The area of a 2 mm diametercircle is 0.03 cm-2; hence by multiplying 100 mW cm-2 by0.032 cm2, one obtains the optical power entering the eye:3.2 mW. The size of the retinal image is determined bymultiplying the angle subtended by the sun (1/2-degree)expressed in radians (0.0094 rad.) by the effective focallength of the eye (17 mm). The diameter of the sun'simage is 0.16 mm, or 160 !tm. The retinal image of a laser,if perfectly focused by the eye, is about 10-20 pm, butsuch a small image can only occur when the eye is focusedfor viewing at a distance [3]. Although a laser's light canbe concentrated to a very small image, a CW laser like theHe-Ne will produce a much larger "heat image" becauseof heat flow away from the absorbing site. For this reason,3 mW from the sun or 3 mW from a laser are actuallycomparable from the standpoint of eye hazards, since bothlight sources will produce similar zones of heated retinaltissue.

    A PAR lamp is a sealed-beam tungsten-halogen lampused as a spotlight. Testing one such lamp (type FFN)operating at 1000 W, showed the light beam irradiance tobe 10 mW cm-2 at a distance of 12 ft (3.7 m or 370 cm). Ata distance of 3 ft (91 cm) the irradiance was 100 mW cm-2.Therefore, using the same calculational procedure fol-lowed for viewing the sun, one would find 3.2 mW enter-ing a dilated pupil at 3 ft, and 0.32 mW entering a 2 mmpupil. However, standing in stage lighting is not the sameas standing outdoors, since the total ambient light in-fluences the pupil diameter. Therefore, the pupil sizeindoors under stage illumination may be 3 mm. In thatcase, 7.1 mW would enter the eye at 3 ft and 0.7 mWwould enter the eye at a 12 ft viewing distance. The imagesize of the spotlight, however, is larger than that of thesun. The parabolic reflector of the spotlight was 18 cm indiameter, and the filament filled the reflector (this istermed "flashed") at 3.7 m. The angle subtended by thelight source in radians was therefore: (18 cm)/(370 cm) =0.049, and the retinal image diameter would be:(0.049)(17 mm) = 0.83 mm or 830 iem. The image area ofthe spotlight was therefore much larger than that of thesun, which in turn was much larger than the worst-caseimage diameter of the laser beam.

    Figure 3 is a graphical representation of the irradiance atthe retina for all of the light sources. Notice that although

    2' 5' 10' 30' 12 2 4 10 20

    Angle Subtended by Light Source

    FIGURE 3. Comparison of the retinal irradiances of differentlight sources. The retinal irradiance of non-frosted tungstenlamps is normally of the order of 0.1 W cm-2, whereas theirradiance of the sun is nearly 10 W cm-2. But if all of thepower from a 1 mW He-Ne laser enters the eye, it canproduce 100 W cm-2 in a 20 pm image if distant objects are infocus on the retina. When the eye is accommodated forviewing nearby objects indoors, the retinal image from thelaser is far greater and the retinal irradiance would be com-parable with that from viewing the sun [3]. Because of heatflow and eye movement, retinal hazards depend not only onirradiance, but also image size.

    viewing the 1-5 mW He-Ne or diode laser produces thehigher retinal irradiance, the sloping line which representsrisk is also near the sun, although well above the PARlamp.

    CONCLUSIONS

    Although one should not aim a laser pointer into a lectureaudience at powers above 1 mW, the likelihood is vanish-ingly small that an accidental intrabeam exposure of theeye will result in retinal injury if the radiant power is lessthan 5 mW (i.e. Class 3A) if the exposure is momentary(i.e. less than 0.25 s). However, we remain somewhatconcerned that there may be some rare individuals (pro-tanopes) who have a decreased visual sensitivity at diodewavelengths near 670 nm, and these individuals, if per-mitted to stare into the laser, could greatly exceed permis-sible exposure limits and perhaps sustain a small retinalinjury. For this reason, we strongly urge greater caution beexercised with their use. Obviously, the problem of thesesmall laser pointers will disappear in a couple of years

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  • 164

    SLINEY AND DENNIS

    when green or orange diode laser pointers appear on themarket, since these could readily be used at the Class 2limit It is hoped that the manufacturers will take advan-tage of the increased visibility and reduce the outputpower of pointers accordingly rather than push the allow-able power to the limit The fundamental lesson to belearned is that although the retinal thermal injury hazardvaries little across the visible spectrum, the eye's sensitiv-ity varies by nearly two orders of magnitude, and a choiceof more central visible wavelengths near 550 nm will al-ways provide better performance for aiming and alignmentlaser products.

    REFERENCES

    1. Center for Devices and Radiological Health, Federal Perform-ance Standards for Laser Products, Title 21, Subchapter J,

    Code of Federal Regulations (USA), part 1040, US Food andDrug Administration, Rockville, MD 20859 USA, 1989.

    2. American National Standards Institute, Safe Use of Lasers,American National Standard Z-136.1(1993), 120 pp. Issued bythe American National Standards Institute (ANSI), 11 West42nd St., New York, NY 10036, USA, and published by LaserInstitute of America, 12424 Research Parkway, Suite 125,Orlando, FL 32826, USA, 1993.

    3. Sliney, D.H. and Wolbarsht, M.L. Safety with Lasers andOther Optical Sources, A Comprehensive Handbook. 1000 pp.New York: Plenum Publishing Corp., 1980.

    4. International Electrotechnical Commission, IEC Standard825-1, Radiation Safety of laser Products and EquipmentClassification, Requirements and User's Guide. Geneva, IEC:1993.

    5. Ham, W.T., Jr, Geeraets, W.J., Mueller, H.A., Williams,R.C., Clarke, A.M. and Cleary, S.F. Retinal burn thresholdsfor the heliumneon laser in the rhesus monkey. Arch. Oph-thalmol ., 84(12), 797-808, 1970.

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