Advanced technology

Laser hazardsNew standards of safety issued by the Bureau of

Radiological Health mean laser costs may begin to rise

Since the laser was invented in 1960, there has been dustries with proven disorders. Another viewed theconsiderable concern for the safety of those who either standard as an overreaction to rampant consumerism.use or happen to be near these sources of spectrally On the other hand, laser representatives who sanc-pure, high-intensity radiation. The appearance of tion if not welcome Federal intervention see it as asome form of regulation was therefore inevitable and chance to play a viable role in standardizing theirnot wholly unexpected. A compulsory guideline-in own industry-indeed, many leading laser scientiststhe form of a proposed Laser Safety Performance have made valuable contributions to the proposedStandard issued by the U.S. Food and Drug Admin- BRH regulation. Others see a Federal standard as theistration's Bureau of Radiological Health-was best alternative to the wide latitude in laser safetypublished in the Federal Register in July and, unless that is provided for by various State labor and healthaltered or amended by public commentary, will be- codes.come law within 18 months of its final publication. Although everyone agrees in principle that the pub-Industry response to the proposed BRH regulation lic-consumer and employee alike-must be safe-

of laser emission has been varied. Laser spokesmen guarded from any potential danger, there are quite a

CLASER MEDIUM_ ^ A ~~MAXIMUM OUTPUT. .

> g ~~~~~~PULSESPECIFICATION

_ | t ~~~AV O I D D I R E C T} \ ~~EX P O S U R E T O B E A M

This Laser product complies with DHEWRadiation Safety Standards foraClass III Laser product. 21CFR Part 278.2XX.

opposed to vigorous control cite the almost impecca- number of laser experts who think that many of theirble safety record (there have been 12 reported in- views were ignored on some of the basic issues. Thestances of laser injury to humans over the past ten feeling that the Government had its mind set onyears, none of which have been verified in the visible passing some form of regulation, regardless of howCW area for lasers operating at 5 mW and under) of overzealous the safety factors or overcautious the ben-laser use over the past decade and question the need efit-risk analysis, has many persons thinking they areto police against nonexistent dangers. One advocate faced with a fait accompli. Questions have even beencalled attention to other areas of "real" or "potential" raised as to whether BRH is exceeding the prescribeddanger and decried the "misdirected" efforts of a reg- jurisdiction of its legal mandate.ulatory agency that could be ministering to other in- The reactions of a product manager for one of the

largest laser manufacturers to the proposed BRH stan-dard may sum up what many in his position are think-

Marce Eleccion Staff Writer ing: "In writing previous product standards on micro-

32 IEEE spectrum AUGUST 1973

wave ovens and X-ray tubes, BRH has been concerned is absorbed by the neuroectodermal tissue and 40 per-with controlling unwanted emissions.... In writing a cent by the pigment epithelium. Since power densitylaser standard, BRH is now trying to control the desired is difficult to measure (some researchers say theyradiative output. Their efforts to generate uniform can) at the retina itself, measurements must be takenproduct categories in labeling requirements and inter- at the cornea and calculated from transmission andlock are certainly appropriate. Their efforts to control focusing characteristics.the output and uses of lasers and the operational fea- At near UV, the eye responds in nearly the sametures are misguided." way as it does to visible light, the exception being a

marked decrease in vision between 3800 A and 4200 AHazard evaluation because of a strong absorption at the lens of the eye.

Extreme exposure may cause cataractogenesis.It is generally recognized that the area of laser haz- In the UV-B and UV-C bands, radiation is absorbed

ard evaluation is a fairly difficult one for which to de- by the cornea and conjunctiva, causing keratocon-velop a standard. This is true not only from a techno-logical standpoint, but because the few instances of sunctivitus (better known as "welder's flash"),' which

laserinjuyhaeno proided nvesigatrs wth aalso seems to occur as an effect of snow blindness.

laser injury have not provided investigators with a At IR wavelengths greater than 1.4 jlm (IR-B andlarge enough sample of empirical data (much of IR-C), absorption of incident radiation occurs in thewhich must be extrapolated from studies of animals cornea and aqueous humor; beyond 1.9 ui, absorp-or injuries sustained from human interreaction with tion is restricted to the cornea alone. At near-IR andhigh-intensity radiation sources like the sun and nu- visible wavelengths, iris heating caused by absorptionclear fireballs). As a result, much of the analysis of is thought to play a major role in forming lenticularlaser hazards depends on subjective judgments. It is opacities such as cataracts although quantitativethe industry viewpoint that, in developing standardsthat would substantially affect such a major technolo-gy as lasers have become, any attempt at regulation Sclerashould be based upon actual evidence. Pigment layer Choroid

In the prelaser era, scientists recognized ultraviolet P lae(WV) radiation less than 3200 A (0.32 gm) as the prin- \tncipal optical hazard to human tissue and eyes.' Be- ,2.... Retinacause of this, optical materials with high absorption Conjunctivabelow this value were used to enclose a radiation

C ctiva

source. With the laser, the hazard range extended to Fovea

every frequency at which a high-intensity beam could Pupil /be emitted (at present from 860 A to far-infrared).Tb deal with optical emissions, the Commission In-

ternationale de l'Eclairage (CIE) in 1970 established Opticband designations for the UV and IR spectrum to en- nervehance the discussion of biologic effects. These bandsare: UV-A, 3150-4000 A; UV-B, 2800-3150 A; UV-C,

Blind spot,1000-2800 A; IR-A, 7800-14 000 A; IR-B, 14000-30000 Blind\sBlind spoA; and IR-C, 3-1000 ITm. Aqueous

Biological damage resulting from interaction with humorlaser radiation has a number of results, includingphotochemical (actinic), thermal, and pressure ef-fects, acoustic and ultrasonic shock waves, plasma Cornegeneration, ultraviolet emission, and even generationof free radicals.2 Of these, the first three mechanisms Iris Vitreous humorhave important consequences on tissue, organs, andeyes, producing such damage as tissue ionization, mo- [1] The human eye, the most susceptible of the body organslecular rearrangement, blood vessel occlusion, corneal to laser radiation damage, is shown here in its many parts;opacity, retinal lesion, blindness, and death. of particular importance is a convergence factor that in-

Ocular damage. In the visible and IR portion of creases incident laser energy by 1O5 from lens to retina.the spectrum, some investigators believe that damageto eye tissue is mainly thermal in nature, most likelyresulting from a denaturation of proteins and inacti- data supporting this theory are scarce.

vation of enzymes,3 although there is disagreement at Recommended limits of laser radiation to the eyeBRH with this view. At these wavelengths, which are have been published by various organizations. Tabledominated by the ruby laser (6943 A) and to a lesser I itemizes some of these for intrabeam viewing.degree Argon and He-Ne gas lasers and the metal Skin damage. In general, serious skin injury fromvapor lasers, the eye is almost 100 percent transpar- laser radiation can be caused at high power levels;ent, increasing the susceptibility to such damage as however, in the visible (4000-8000 A) and near-IR re-

retinal burn or lesion. This effect is further compli- gions, eye injury is usually a more important consid-cated by the fact that the energy density of the irra- eration (see Fig. 2). Besides wavelength emission, thediating light converging on the retina is concentrated threshold for skin injury varies with pigmentation andby a factor of 105 over that passing through the pupil. exposure duration; recommended safe exposure levelsOf the light that reaches the retina, about 60 percent from various sources are given in Table II. Figure '3

Eleccion Laser hazards 33

I. Recommended limits for ocular exposure to laser radiation (0.4-1.4 /rm) for a 7-mm pupil1

Intrabeam Viewing of a Collimated BeamCorneal Radiant

Laser Wave- Exposure, Corneal Irradiance,Organization lengths, nm Exposure Duration t J*cm-2 W.cm-2

U.S. Departments of the Army 400-1400 5-50 ns 10-7and Navy (February 1969) Approx. 1 ms 10-6

Continuous 10-6ACGIH (1971) 694.3 1 ns to 1 llS 10-7

694.3 1 /istoO.1s 10-6400-750 >0.1 s 10-5

U.S. Department of Labor 632.8 Incidental (1 s) 10-2329 CFR 1518.54 (1971) Continuous 10-26

U.S. Department of the Air 400-700 1 0-1 00 ns 1.3 X 1 o-6Force (September 1971) 200 Aus to 2 ms 10-5

2-10ms 5X10-310-500 ms 2.5 X 10-3

1064 10-100ns 6 X 10-6200Asto2ms 5X10-52-1 0 ms 2.5 X 10-210-500ms 1.3X10-2

ANSI Z-136 proposed 400-700 1 ns to 18 Aus 5 X 10-7(February 1972) 18 As to 10 s 1.8 X 10-3-t

10-104 s 10-2>104 S 10-6

700-1060 1 nsto18,us 5C1X 10-718 /is to 10 s 1.8C1 X 10-3.t10-100s C1X102

700-800 100 - [104/ C1 X 10-2(X - 699 nm)] s

>[104/(X699 nm)] s C1 (X -699,nm) X 10-6

800-1060 >100s C1X 10-41060-1400 1 ns to1O00Ms 5 X 10-6

100 ,s to 10 s 9 X 10-3*t10-100 s 5 X 10-2>100 s 5 X 10-4

Note: t is in seconds, A is wavelength in nanometers, and C1 = exp [(X - 700 nm)/224]}

demonstrates the variability of skin reflectance in the What's the story?visible and near-IR spectrum. The present proposed compulsory laser safety stan-

It is known that the skin becomes increasingly sen- dard began with a concepts document issued by BRHsitive in the UV range, with wavelengths in the UV-B in October 1971. As part of the research data, a jointband penetrating the surface more deeply, producing State-Federal survey of 288 (out of 30 000) classroomsevere burn or erythema.' Moreover, chronic exposure lasers was conducted in seven States during 1972. Re-to UV radiation has been found to accelerate skin sults of this study indicated that laser outputs wereaging and is believed to increase the risk of devel- not consistent (e.g., ratings of 1 mW were actuallyoping certain types of skin cancer, attested to by 0.19-3.00 mW) and that no laser was marked with itsepidemiologic studies of solar UV-B levels at various output power. In addition, it was found that 72 per-altitudes and latitudes. Although quantitative data cent of the lasers examined were operated without warn-from such studies do exist and are used by BRH, no ing signs and 59 percent lacked warning labels.threshold data are yet available. Since 92 percent of all lasers used in high schoolsAt IR wavelengths, the principal adverse biologic and 79 percent of those in colleges are helium-neons,

effects seem to be infrared cataracts, flash burns of BRH reviewed the literature for reports on the possi-both the skin and cornea, and heat stress. In the IR ble hazards from this type of device. Specifically, itrange beyond 14 000 A, as well as in the near-UV, haz- was found that an accepted value of 7 mW (quoted inardous levels for both skin and eye are comparable if The Physics Teacher as the power required from athe injury mechanism is thermal. He-Ne laser to produce a retinal burn* in experimental

Skin tissue, a heterogeneous mixture of layers and animals) was misleading. Other data turned up bynumerous inclusions such as blood vessels and hair BRH included independent reports that rhesus mon-follicles, is primarily composed of water, hence rela- keys exposed to 2 mW for 1 second sustained'vi'sibletively transparent to near-JR laser light. For this rea- retinal burns. Further, it was pointed out that delete-son, and because most absorption occurs in pigment rious changes in retinal cells can be observed with agranules and blood vessels, internal organs are partic-ularly susceptible to radiation. *Generally referred to as minimum ophthalmically visible lesions.

34 IEEE spectrum AUGUST 1973

microscope after eye exposures to laser beams at pow- equipnent be published in the Federal Register. Al-ers perhaps as much as 100 times lower than that though limited opposition was voiced to the X-rayproducing visible retinal burns. [A representative of standard, EIA and American Association of Physicsthe Electronic Industries Association (EIA) has stated Teachers representatives did protest the "unneces-that minute retinal changes 100 times below the sarily" restrictive measures in some laser categories.2-mW level can also be caused by photographic flash- According to BRH, an important consideration inbulbs, auto headlights, and sunlight.] How damaging developing the laser standard centered about the Bu-these changes really are is debatable, since the process reau's position that Class II (see box, page 38) lasersof seeing itself implies some retinal change. "should be required to permit human access to noAlthough the BRH survey of classroom lasers pro- more than I mW of visible laser radiation," whereas

duced no evidence of laser-induced injury (it actually EIA and other laser representatives urged that a max-wasn't looking for any), it did indicate that there imum of 5 mW be imposed for this class of lasers sowere "gross examples of rhisuse," alluding to the as to include the widely used survey and alignmentwidespread condition of direct interaction between lasers (currently under Class III provisions, Fig. 4).laser emission and student (in some cases deliberate), In testimony before BRH hearings last November,as well as the lack of adequate warnings. Since the EIA Laser Section chairman Robert L. Mortensonproposed laser standard will not apply to devices categorized the under-5-mW continuous-wave (CW)presently in use, BRH has issued guidelines for a visible laser as not only beneficial and safe, but num-laser safety action program to "all users of lasers in erically the most important group of lasers since 95the classroom environment." percent of all lasers in the United States (estimated

at over 100 thousand) fall within it (see Fig. 5). InFederal action addition, he stated that a 5-mW output is essential

In May, BRH's Technical Electronic Product Radia- for using lasers in ambient sunlight.tion Safety Standards Committee suggested that In response, F. Alan Anderson, acting chief of thetheir proposed radiation safety performance standards genetic studies section of BRH's Division of Biologicalfor laser products and nonmedical cabinet X-ray Effects, cited that the safety record of lasers up to 5

I 'I'"!TTII. Proposed laser guidelines forL skin protection from various organizations4

Human100 Continuous-

7 _ - _ -- wave (UV Non-0- Q-80 _ . x,> . excepted), switched, switched, Other Classi-

o 60 / Organization W/cm2 J/cm2 J/cm2 fication

60r l t lIllWeston(1965),I ~~~~~~British Min-

F 40| istry ofAviation 0.1 0.1 0.1 Not specified

20 \ \ Electronic En-.1 -_\\.gineering/ i l, l ~.; r l_lAssociation0Q o 0.8 10 1.2 1.4 of Great

Wavelength,prn Britain[21 Mean values for the transmission of equal intensity light (1966) 0.1 0.1 0.1 Not specifiedthrough human, monkey (rhesus), and rabbit ocular media U.S. Atomichave been derived by many investigators and appear Energy Com-throughout the literature. mission,

Nevada Op-erations Of-fice (1967) 1.0 0.1 0.1 WV visible, IR

[31 The spectral reflectance of both dark and fair human skin Aica Con-has been documented over a wide range of values.5 American Con-

v --_j I T I ! T j r l* - -n I I T ference ofGovernmen-

0.6- tal IndustrialFair complexion Hygienists

0.5- (ACGIH)X 0.4- _.V)l (1968) 5.0 0.05 0.005 Visible

Bell (1968),t 0.3- _ Cincinnati.) o .- - Dark pigmentation Laser Safety

l0.2 Conference 1.0 0.1 0.1 Not specified0.1- / U.S. Depart-

ments of the

02 0.4 0.81 2 4 6 810 20 40 Army andNavy (1969) 0.1 0.1 0.01 Visible, IRWavelength. Mm

E1 CeCeiOJI .Laser haza lds 35

1_ mW "results from the safe use of a potentially haz-0,5 _ ardous device and not from the uncontrolled use of a

safe device." Reflecting the conclusions of both theBureau and the American National Standards Com-mittee (whose Z-136.1-1973 Standard on the Safe Useof Lasers was approved on April 26), Dr. Andersonnoted that damage to the retina "can be expected from

1-2 _visible emissions from laser products that exceed onemilliwatt total power into one eye."

If the proposed BRH standard is enacted in its

10-3- present form, it would classify all lasers in terms ofwavelength, power output, and emission duration, andestablish a system of cautionary warnings (see sample,page 32), protective housings, and other features

10-4 designed for human protection. As a "product-performance safety standard," its legal basis is

3 Public Law 90-602, which is the Radiation Control for10-5 _ Health and Safety Act of 1968. According to BRH,

the standard itself would only prohibit the manufac-ture and sale of a defective laser product, although"the user is free to buy anything he wants." In effect,

10-6 - BRH is saying that it is protecting the user and notbanning lasers. That they can indeed regulate in thisarea has been attested to by the HEW General Counsel,

10 7 temporarily negating any claims to the contrary.The Department of Health, Education, and Welfare

Class is not the only government agency concerned withcontrolling the use of laser products. The Department

o-s _4--- - of Labor's Occupational Safety and Health Adminis-tration (OSHA), created by the Occupational Safety

0.25 -- 0.4 -- 0.7 - 1.4 13.0 and Health Act of 1970, also has an interest in theUltraviolet Visible Near-infrared Far-infrared safe use of lasers-but for a slightly different reason.

Wavelength,jArn Whereas BRH's role is limited to that of overseeing4An EIA interpretation oftheBRH proposedclassifica the practices of manufacturers, OSHA has a commit-

[4] AEI nepeao of the BRH proposed classificationsof CW lasers by wavelength and power. In the January 14 ment aimed at protecting the safety of the nation'sdraft of the standard, BRH prohibited manufacture and sale employees. At present, OSHA does have minimal re-of lasers for demonstration, surveying, leveling, ranging, or quirements for the use of lasers in the constructionalignment at power levels above Class I or il limits. The industry. When asked whether further safety stan-March 6 draft of the BRH proposed standard allows for such daswilbsefootremoysofaerevc,uses under a new limit (called Class IIIA by EIA) of 5 mW, dards wll be set for other employers of laser devices,so long as power densities do not exceed 2.5 mW/cm2. OSHA informed Spectrum that, although they could

not predict their substance or form, other stan-dards would certainly be considered. One of the in-

III. Comparison of some widely puts in establishing such standards has already beenused photometric and radiometric ter provided for in Section 61 of the 1970 Act, which

states that extragovernmental interests can petitionQuantity System Unit Abbreviation the Secretary of Labor to create regulations for par-

ticular areas involving employee safety. Under thisRadiance Radiometric watts per stera- w/cma sr provision, EIA is currently preparing a laser safety

dian and document for consideration by OSHA, based on the

meter premise that a device is not a hazard unless used in aIrradiance Radiometric Watts per W/CM2 hazardous manner. Consequently, with prescribed

square centi- guidelines and precautions for the safe use of lasersmeter (operator training, etc.), such a standard could con-

Radiant exitance Radiometric Watts per W/cm2 ceivably provide for higher-power emissions thansquare centi- presently prescribed in the proposed BRH standard.meter

Radiant intensity Radiometric Watts per stera- W/sr Laser parametersdian

Luminance Photometric Footlambert or fL, cd/M2 Two types of measurements can be useful in deter-candelas per mining the effects of laser radiation: one dealing withsquare meter laser output, the other describing actual interaction of

Illuminance Photometric Footcandle or fc, lx, im/i2 laser energy with matter.5 Unfortunately, the latterlux (lumens type of analysis is difficult for many reasons, includingper square m) variances in refraction index and absorption, difficulty

Radiant exposure Radiometric Joules per J/cm2 of sensor placement, short event times, small tempera-square cm ture increments, and the necessity for nondestructive

36 IEEE spectrum AUGUST 1973

measurement. Hence, any successful investigation of r 40000laser radiation effects must deal directly with output °parameters. Of these, most are unimportant in deter- 30000mining laser predilection for damage, the most signifi-cant parameters being energy, power, and beam di- 20000vergence. Although these are easily quantified by oldand established radiometric methods, complications inmeasurement arise from such factors as high laser C 10oo0power output, short emission duration, and subtle in-attention to systemic error (seemingly unimportant, Drbut well documented in the literature). The problem is Fj 0 l 2 3 4 5further complicated by the fact that, even if precision Total power output mW(or reproducibility) is attained, systemic error may still [5] Number of S5-mW CW lasers emitting in the visible spec-

trum that were marketed during 1961-1971 (as released inlead to inaccurate results. EIA Special Report MS-1-IA). Industry sources estimate thatAnother problem that arises is the variability of such units comprise over 95 percent of the total number of

pulsed laser output, which has been recorded as high lasers shipped during this period.as +6() percent, partially attributable to measure-ment error, but also due to poor-quality laser rods,dielectric coatings, degradation of bleachable dyes,changes in flashlamp energy, and inadequate cooling. ric and photometric terms and units that are used. InSuch wide variability can lead to a lack of testing some literature, the effect of radiant energy on tissuerepeatability, and requires a reliable monitoring sys- is described in terms of power per unit area (powertem to obtain accurate results. density, W/cm2) or energy per unit area (energy den-

In understanding the quantitative results of radia- sity, J/cm2); these terms are also known as beam in-tion, it is necessary to identify the various radiomet- tensith.4 At other times, luminance is used. Table III

Proposed accessible emission limits to be used in classifying laser products (BR H)

Wave-length, Emission Durationnm Interval, s Class Class II Class III Class IV

>250 S3.0 X 104 S2.4 X 10 I(k,)(k2) J* Not applicable >Class but >Class Illthrough S2.4 X 10 4 (k1)(k2) J400 >3.0 X 104 S8.0 X 10- 10 (kl)(k2) W Not applicable >Class I but >Class IlIl

1. 55 X 10 3 (k1)(k2) W>400 >1.0 X 10 9 S2.0 X 10 7 (kl)(k2) J Not applicable >Class I but >Class IlIlthrough to <10(H 3) (k,)(k2) J-cm 21400 2.0 X 10- 5 to a

5 ~~~~~~~~~~~~~maximum>2.0 X 10 5 7.0 X 10-4 (kl)(k2)(t3 4) J Not applicable value ofto 1OJ-cm 2

2.5 X 10-'>2.5 X 10 -1 to 10 7. 0 X 10 4(k1)(k2)(t3 4) J Class but Class II >Class Ilil>10 to 104 S3.9 X 10 -3(k1) (k2) J S 1.0 X 10 3 but>104 S3.9 X 10-7(k,)(k2) W .(kl)(k2) W** .5.0 X 10 W

OR>1.0 X l0 9 10(k1)(k2)(tl 3) J.cm 2*sr 1 Notapplicable

to1.0 X 10

>1.0 X 10 .20(kl)(k2) J.cm 2*sr-1 Not applicableto1.0 X 104

>1.0 X 104 .2.0 X 10 3(k1)(k2) W.cm 2-sr 1 Not applicable1400 >1.0 X 10-9 S7.9 X 10- 5(kl)(k2) J Not applicable >Class >Class Ililthrough to but13000 10 X 10 -7 10J-cMr 2

>1.0 X 10-7 S4.4 X 10-3(kl)(k2)(tl 4) J Not applicable Class I butto S10J.cm 2

1.0 X 10>1.0 X 10 S7.9 X 10 4(k,)(k2) W Not applicable >Class I but >Class IlIl

S5.OX10 1W*Class emission limits for >250 nm to 400 nm shall not exceed the Class emission himts tor > 1400 nm to 13000 nm for comparable emission duration intervals.**Class II applies to emission within the wavelength range of >400 nm through 700 nm for emission durations of >0.25 second.Note: t = magnitude of inherent emission duration in seconds J = joule

k, and k2 = wavelength dependent correction factors (obtainable from BRH) W = wattJ-cm 2 = radiant exposure W-cm 2 = irradiance

W-cm 2'-sr = sourceradiance

Eleccion I.aser hazards 37

Laser Safety Performance StandardAccording to Richard W. Peterson of BRH, the laser radiation exceeding the upper limits of Class IlIl and,

safety standard, first drafted in 1971, has evolved since there exists a definite hazard to both skin andfrom one designed to control the use of lasers in the eye, a Class IV product must be posted with theclassroom environment to an extremely broad perfor- admonition "Avoid Eye or Skin Exposure to Direct ormance standard that is applicable to "manufacturers Scattered Radiation." An indication of the emissionof all laser products." In many ways, the proposed levels that will be allowed under the new BRH classifi-regulation parallels the new ANSI Z-136.1-1973 laser cations is given in the table on page 37.safety standard, which served as a major input to Aside from the labeling already cited, other warn-BRH. ings are required by the standard. These include maxi-

In the March 6 draft of the proposed standard, four mum output ratings, disclosure of open apertures, andclasses of lasers would be created to allow manufac- various cautionary and danger warnings.turers to categorize a product based on its potential In addition to protective housings on all four classeshazards to a user. Although BRH simply labels these of lasers to prevent human irradiation by levels in ex-classes through IV, from the most safe to the most cess of the prescribed limits, BRH has required thehazardous, ANSI classifies five groups of laser de- use of interlocking and anti-interlock-defeating tech-vices-nonhazardous or exempt (emission of less niques designed directly into the finished laser prod-than the recommended limit for intrabeam viewing), uct. Essentially, a safety interlock must be providedlow power (safe for momentary viewing), medium for each portion of the protective housing that can bepower (presenting an intrabeam viewing hazard but removed by the user during normal operation or main-not generating hazardous diffuse reflections), high tenance thereby allowing excess to hazardous radia-power (probability of hazardous diffuse reflections as tion. In the event the design of a safety interlock al-well as a fire hazard), and enclosed. Since BRH is lows the user to defeat the system manually, the man-concerned with only those emissions to which human ufacturer must incorporate visible or audible warningsaccess is possible, there is no difference between of this interlock override during laser operation.their Class (safe) laser and ANSI's "enclosed" cate- Further, any Class IlIl or IV laser must include angory, which from a product standpoint leaks no radia- externally accessible electrical connector having twotion; BRH therefore does not separate the two. contacts that, when not joined, preclude human ac-

Basically, Class lasers would be used in any en- cess to laser radiation greater than accessible emis-vironment and allow human access to no more than sion limits of Class lasers. Class IlIl and IV lasers0.39 IuW of continuous visible radiation (higher levels must also have a master control device with a key toare allowed for shorter durations; see table, p. 37) and render the device inoperable when removed. In addi-no more than 0.79 mW in the far IR spectrum. These tion, Class II, IIl, and IV devices are to have visible orlasers do not have to display warning labels,* and re- audible indication that energy is being applied, andquire special housings equipped with safety inter- one or more permanently attached mechanical meanslocking devices to shut down excessive radiation. (other than power supply switches) to reduce radia-

In Class II, any laser with visible light emission tion levels to less than that for Class lasers. Othercannot exceed 1 mW for continuous human exposure limitations include attenuation requirements on alland is regarded as unsafe for chronic viewing but with viewing optics, viewports, and display screens.a low probability of injury for single short exposures. On the face of it, it would'seem that the posting ofA label carrying the warning "Do Not Stare into Beam" a few signs to warn the public of a potential hazardwould be mandatory, as well as the laser medium. is analogous to the health warning displayed on the

As an accommodation to survey and alignment la- side of every pack of cigarettes, Discounting thatser users, Class IlIl of the standard allows CW power cigarette sales actually rose after a short period ofof up to 5 mW with the laser beam sufficiently wide public concern following the warning requirement, theto prevent more than 1 mW (or a power density no public benefit derived from such warnings far exceedsgreater than 2.5 mW.cm-2) from entering the bare the possible risk of losing sales. The real questioneye of a user. Other CW lasers in this class can emit at issue concerns the necessity of installing protec-no greater than 0.5 watt in either the visible or IR tive housings, interlocking devices, automatic cutoffs,range. Class IlIl lasers are considered to present high and audio-visual warnings on prescribed types of la-probability of eye injury even during a single short sers. Such requirements would increase the basicexposure, and low-power devices would carry warn- cost of those laser systems that do not already haveings against staring into the beam, with high-power such features (several do), which would have to bedevices displaying a warning against direct beam ex- passed on to the purchaser.posure. Laser medium and pulse duration are also Since the proposed standard exempts OEM prod-specified (see illustration, p. 32). ucts and is mandatory only for the manufacturer who

A Class IV product includes any laser that emits markets a consumer item, the main responsibility*A certification label "This product complies with the DHEW radiation for complying with the standard will be left prettysafety standards-21 CFR Part 278.2XX" must be affixed, however. much to the actual designer of a final laser system.

compares some of the more widely used photometric Applications in Medicine and Biology, pp. 125-162.and radiometric nomenclature; note that radiance 4. Sliney, D. H., "The development of laser safety criteria-biologi-cal considerations," in Laser Applications in Medicine and Biology,and luminance describe radiant energy leaving a pp. 163-238.source, and irradiance and illuminance represent 1ev- 5. Heffner, D. K., "Calibration of lasers-necessity and techniques,"a ~~~~~~~~~~~~~~~~inLaser Applications in Medicine and Biology, pp. 19-33.els at a given point in space.

REFERENCES Reprints of this article (No. X73-082) are available at1.zrdSliny, l.H.t,and Fresier,B.C., "Evallu9ation of optical radiation $1.50 for the first copy and $0.50 for each additional copy.

2. Riggle, G. C., Hoye, R. C., and Ketcham, A. S., "Laser effects on Please send remittance and request, stating article num-normal and tumor tissue," in Laser Applications in Medicine and ber, to IEEE, 345 E. 47 St., New York, N.Y. 10017, Att:Biology, M. L. Wolbarsht, ed. New York: Plenum, 1971, pp. 35-65. SPSU. (Reprints are available up to 12 months from date of3. Vassiliadis, A., "Ocular damage from laser radiation," in Laser publication.)

38 Eleccion-Laser hazards