ICNIRP GUIDELINES · ICNIRP Guidelines REVISION OF GUIDELINES ON LIMITS OF EXPOSURE TO LASER...

of 11 /11
INTERNATIONAL COMMISSION ON NONIONIZING RADIATION PROTECTION ICNIRP PUBLICATION – 2000 ICNIRP GUIDELINES REVISION OF GUIDELINES ON LIMITS OF EXPOSURE TO LASER RADIATION OF WAVELENGTHS BETWEEN 400 nm AND 1.4 μm PUBLISHED IN: HEALTH PHYSICS 79(4):431440; 2000

Embed Size (px)

Transcript of ICNIRP GUIDELINES · ICNIRP Guidelines REVISION OF GUIDELINES ON LIMITS OF EXPOSURE TO LASER...

INTERNATIONALCOMMISSIONONNONIONIZINGRADIATIONPROTECTION

ICNIRPPUBLICATION2000

ICNIRPGUIDELINESREVISIONOFGUIDELINESONLIMITSOFEXPOSURETOLASERRADIATIONOFWAVELENGTHSBETWEEN400nmAND1.4mPUBLISHEDIN:HEALTHPHYSICS79(4):431440;2000

ICNIRP Guidelines

REVISION OF GUIDELINES ON LIMITS OF EXPOSURE TOLASER RADIATION OF WAVELENGTHS BETWEEN 400 nm

AND 1.4 mm

International Commission on Non-Ionizing Radiation Protection*

INTRODUCTION

SINCE THE publication of the ICNIRPGuidelines on Limitsof Exposure to Laser Radiation of Wavelengths between180 nm and 1,000mm (1996), recent research has madeit appropriate to update the laser retinal protectionguidelines for ultrashort (sub-nanosecond) pulse dura-tions and for continuous-wave (CW) exposures lasting10 s or longer. These revisions are limited to the retinalhazard spectral region (400 nm to 1,400 nm). No changesin the limits are recommended for any exposure durationbetween 1 nanosecond (ns) and 10 s for intrabeamviewing (nor to 0.7 s for viewing extended sources).

Studies of laser-induced retinal injury from mode-locked laser pulses have been carried out for more thantwo decades (Goldman et al. 1977); however, untilrecently threshold data have not appeared to be consis-tent, nor have the underlying damage mechanisms forsub-nanosecond (sub-ns) laser-induced injury been wellunderstood. The Commission organized a task group onophthalmic biophysics to review the scientific data andcurrent knowledge of retinal injury mechanisms to con-sider recommending an extension of the laser guidelinesto pulse durations less than 1 ns. The result of this reviewis that the Commission believes that for ultrashort laserpulses there is now a reasonably consistent explanationof the non-linear optical phenomena that occur in the eyewhich cause retinal damage. Thus it is appropriate torecommend limits of exposure for pulse durations be-tween 100 femtoseconds (fs) and 1 ns.

Inconsistencies had been discovered in CW laserexposure limits (ELs) when these limits were applied tointentional viewing of light emitting diodes (LEDs) anddiode lasers. Consequently, the Commission also re-quested the ophthalmic biophysics task group to studythe validity of the current guidelines for CW exposures.Prior to the extension of the scope of some laser safetystandards to apply to LEDs, general guidance in all lasersafety standards was never to view a laser beam directly(intrabeam viewing) and serious efforts to derive accu-rate ELs for durations exceeding;10 s did not occur.Indeed, the previous effort had been to simplify theexpression, combining both thermal and photochemicaldamage mechanisms. This approach necessitated verylarge safety factors to accommodate both injury mecha-nisms within a single mathematical formulation.

The task group reviewed the past criteria and theeffects of eye movements, source size, pupillary re-sponse, spectral absorption, and the two competingretinal damage mechanisms (photochemical and thermo-chemical) upon the potential for retinal injury whenviewing a CW laser beam or any small light source. Thegroup recommended splitting the ocular ELs into duallimits: one for photoretinitis (photochemical) and one forretinal burns (thermochemical injury) to eliminate highlyinconsistent safety factors. The review showed that theimpact of eye movements is greatest for small lightsources, but least for large sources. This dual-limitapproach follows the same approach used to evaluateincoherent sources.

BACKGROUND

GeneralThere are a number of biophysical and physiological

factors that influence the derivation of the ocular ELs forlaser exposures. Cell injury from laser pulses of 1 ns to atleast 0.25 s is caused by thermo-mechanical damage ofcell structures or thermal denaturation of cell proteins.Heat flow from the absorption site plays an importantrole in the redistribution of incident energy. Thus,whether a cell is located in either the irradiated or anadjacent area will determine its temperature rise and the

* ICNIRP Secretariat, R. Matthes, c/o Bundesamt fur Strahlen-schutz, Institut fur Strahlenhygiene, Ingolstadter Landstrasse 1,D-85764 Oberschleissheim, Germany; The ICNIRP Task Groupconsisted of C. P. Cain (USA), D. Courant (France), D. A. Freund(USA), B. A. Grossman (USA), P. A. Kennedy (USA), D. J. Lund(USA), M. A. Mainster (USA), A. A. Manenkov (Russia), W. J.Marshall (USA), R. McCally (USA), B. A. Rockwell (USA), D. H.Sliney (USA, Chair), P. A. Smith (UK), B. E. Stuck (USA), S. A. Tell(UK), M. L. Wolbarsht (USA), and G. I. Zheltov (Belarus). Note: Anumber of other scientists took part in this effort through comments atother meetings and by correspondence; among these were F. Cheney(USA), L. McLin (USA), J. Ness (USA), K. Schulmeister (Austria),R. M. Steinman (USA), E. Sutter (Germany), and H. Zwick (USA).

(Manuscript received30 March 2000; revised manuscript re-ceived2 May 2000,accepted3 June2000)

0017-9078/00/0Copyright 2000 Health Physics Society

431

duration for which that cell is at an elevated temperature(Barnes 1974; Hillenkamp 1989; Mainster et al. 1970;Priebe and Welch 1978; Sliney and Wolbarsht 1980;Sliney 1999). This time-temperature history is crucialbecause of the dependence of thermo-chemical denatur-ation of proteins on the peak temperature and the entireduration (of many milliseconds or seconds) that thetemperature is elevated. Therefore, the geometry of theretinal exposure will influence the retinal lesion size andthe threshold in much the same way regardless of theexposure duration of a laser pulse, since most of thethermal denaturation of proteins occurs after the pulse,i.e., during the cooling period (Courant et al. 1989a,1989b; Sliney and Wolbarsht 1980). Despite the presenceof non-linear effects, a similar spot-size dependence alsoexists in the sub-ns time regime (Goldman et al. 1977;Cain et al. 1996; Rockwell et al. 1997). This permits theuse of a single scaling factor (CE) to adjust the EL from100 fs to at least 10 s. The correction factorCE is set asthe angular subtense of the sourcea divided byamin (1.5milliradian). Histological studies of the sites of small andlarge retinal image size exposures at 530, 580, and 1064nm show effects upon the neural retina and in the retinalpigment epithelium. Although the damage in the pig-mented epithelium was centered on the melanin granules,it was apparently unrelated to linear absorption by themelanin granules (Toth et al. 1996, 1997; Goldman et al.1977).

Sub-nanosecond exposuresGuidelines for ELs have not previously existed for

sub-ns exposure because of controversies over tissuedamage mechanisms; only a very tentative interim guide-line to limit peak irradiance to the EL for 1 ns has beenrecommended (ICNIRP 1996). Studies of laser-inducedretinal injury from mode-locked laser pulses in the10300 picosecond (ps) time domain and sub-ps expo-sures in the femtosecond time regime have been carriedout in several research programs, and the scientificdatabase is now sufficient for the first time to recommendELs for pulse durations between 100 fs and 1 ns. Thedevelopment of ELs in the sub-ns time domain has beenvery difficult because of different interaction mecha-nisms of laser radiation with biological tissues (Cain etal. 1997; Roach et al. 1999; Toth et al. 1997). Thedamage produced by non-linear effects does not followthe same relationship with wavelength, with pulse dura-tion, and with retinal image size as do thresholds fromthermal and thermoacoustic damage mechanisms. Forthis reason, it had been necessary to perform a number ofstudies of non-linear damage mechanisms (Cain et al.1997; Gerstman et al. 1996; Hammer et al. 1997;Rockwell et al. 1997). Likewise, the range of potentialeffects that could occur below the ophthalmoscopicallyvisible threshold had to be studied by histological tech-niques. The consequences of exceeding the threshold,i.e., supra-threshold effects, also must be understoodprior to setting ELs. Fig. 1 shows the ophthalmoscopi-cally determined threshold damage levels from severalstudies in the sub-ns regime and also shows the newrecommended guidelines. A safety factor of approxi-mately 10 to 20 was maintained in the derivation of theselimits, the exact magnitude depending upon the level ofuncertainty in the measured damage thresholds. Betweenapproximately 10 ps and 1 ns, self-focussing of theocular media further concentrates laser energy into nar-rower beams or filaments which increases the retinalradiant exposure and thereby lowers the thresholds (Cain

Table 1. Values fortmin.

Wavelength range tmin

315400 nm 1 ns4001,050 nm 18 ms

1,0501,400 nm 50 ms1.41.5 mm 1 ms1.51.8 mm 1 s1.82.6 mm 1 ms2.61,000 mm 100 ns

Fig. 1. Laser retinal injury thresholds and 1996 exposure limits for visible and near infrared (NIR) Nd:YAG laserwavelengths, expressed as corneal radiant exposure.

432 Health Physics October 2000, Volume 79, Number 4

et al. 1997; Roach et al. 1999). The time-dependence ofself-focussing is complex and non-linear, but was best fitby a functionf(t0.75) (Figure 1). Other non-linear opticalmechanisms appear to play a role in retinal injury in thesub-ps region (Roach et al. 1999; Rockwell et al. 1997).

CW exposuresSeveral physiological factors come into play for CW

exposures (generally taken as greater than 0.25 s) wherethe persons visual task will limit the exposure durationand the retinal area illuminated. Physiological factors,

such as pupillary activity, eye movements, breathing,heart-beat, blood-flow, other bodily movements, andvisual task behavior become important (Andre-Deshayset al. 1988; Eizenman et al. 1980). In addition, at shortvisible wavelengths, another injury mechanism (photo-chemical) plays an important role, and synergism be-tween photochemical and thermal injury mechanisms caninfluence the EL. All of these factors were considered inrevising the ELs for exposure durations greater than 10 s.

The ICNIRP task group examined a number ofstudies of fixational eye movements, and reached ageneral conclusion regarding image wander over thefovea and macula (Epelboim et al. 1995, 1997; Epelboim1998; Eizenman et al. 1980; Ferman et al. 1987). Nessand colleagues (1997, 2000) then conducted additional,highly specific, worst-case visual fixation measurementsof image wander when a subject was asked to fixate ona very small, fixed target. The subjects were trained,mostly young observers with good vision. Employing themethod developed by Ness et al. (1997), the task groupattempted to confirm the earlier studies by conservativelysimulating the worst-case viewing conditions. The eye-movement studies of Ness et al. (1997, 2000) clearlyshowed that the retinal image can remain remarkably stillfor a few seconds, but that within 30 s the central retina(the fovea) must move to other points in space forcognition. Two viewing conditions were studied: with astabilized head (as could occur with ophthalmic instru-ment applications of lasers), and without head restraint(normal conditions) while volunteers attempted to fixateon a small point light source. The results were evalu-ated in a variety of ways. The fixation history of thesmall retinal image diameterdr (,30 mm) was plotted asa function of time to determine the integration of retinalexposure dose. Figs. 2 and 3 illustrate the accumulated

Fig. 2.Distribution of laser energy on the retina produced by eye movements over a 10-s period for a non-human primateunder anesthesia (left) and the distribution in the human eye during normal, unrestrained conditions (right) [adaptedfrom Ness et al. (2000)]. The spot size at 50% peak irradiance points increased from a nearly circular 30mm pattern(left) to an elliptical pattern (right) with an average cross-sectional dimension of 75mm (right). Energy was alsoscattered outside the central image.

Fig. 3. Distribution of laser energy for 100-s unrestrained viewing[adapted from Ness et al. (2000)]. The minimal spot size was about135mm. This leads to a 2.3 reduction in risk from thermal injury.This result is consistent with the 150 to 200mm elliptical lesionsproduced when an observer stares at a welding arc (Naidoff andSliney 1974).

433ICNIRP Guidelines INTERNATIONAL COMMISSION ON NON-IONIZING RADIATION PROTECTION

radiant exposure in the retinal image area. This exposuredirectly predicts the extent and position of photochemicalretinal injury, since retinal radiant exposure determinesthe risk of photoretinitis for a given wavelength. Whilethese data permitted the determination of the photoreti-nitis (blue-light hazard) EL value, the determination ofthe thermal hazard ELs was more complex, since thermalretinal injury thresholds decrease for increasing retinalspot size.

The determination of the potential for retinal ther-mal injury for long fixation times required an examina-tion of three factors:

1. The time-averaged retinal irradiance;2. The thermal effects that result from the way eye

movements transform a fixed retinal image into theequivalent of a repetitive-pulse exposure; and

3. The manner in which image size affects the retinalthermal injury threshold due to radial heat flow duringand after the exposure.

The time-averaged retinal irradiance must not ex-ceed the CW irradiance permitted for a fixed image. Thethermal repetitive-pulse exposure obeys what is referredto as theN20.25 additivity rule where the net effect ismodified as the number of pulsesN raised to the20.25power (N20.25). This rule is expressed in the ICNIRPGuidelines (1996) by the correction-factorCp, whichreduces the EL for a single pulse in a train of pulses.However, if multiple pulses exist with a durationtmin(Table 1), those pulses are treated as if they were a singlepulse.

These interrelated factors were dealt with by trialand error and by examining a few worst-case conditions.If the retinal image moved very little during the entireviewing time, the average irradiance condition domi-nated. However, for viewing with an unrestrained head,the retinal image moves dramatically, and the sameretinal area may be exposed a number of times during theentire viewing sequence at a low duty cycle, and theN20.25 additivity function dominates. The instantaneousimage size was also considered in these calculations,which modified the results.

Correction factors for eye movements, as it turnedout, were only important for viewing durations exceeding10 s. Only the thermal mechanism is important atdurations less than 1 s for small images. Although thephysiological eye movements known as saccades dospread the absorbed energy in minimal retinal images (ofthe order of 25mm or less) within the 0.1 to 10 s timeregime, the limits recommended in these guidelinesprovide an added safety factor for this viewing condition.At 0.25 s with unrestrained head viewing, the meanretinal spot illuminated is spread out to approximately 50mm. By 10 s, the illuminated retinal zone becomesapproximately 75mm and the added safety factor for theminimal image condition becomes 1.7 over a stabilizedeye, with the spot-size dependence taken into account.By 100 s, it is rare to achieve an illuminated zone(measured at 50% points) as small as 135mm and this

means that there is, in effect, an additional safety factorof 2.3 or more above the minimal image condition. Thedata from eye-movement studies and retinal thermalinjury studies were combined to derive a break-point inviewing time T2 at which eye movements compensatedfor the increased theoretical risk of thermal injury forincreased retinal exposure durations if the eye wereimmobilized. Because the thermal injury threshold, ex-pressed as radiant power entering the eye, decreases asthe exposure durationt raised to the20.25-power (i.e., areduction of only 44% per tenfold increase in exposureduration), moderate increases in the exposed retinal areawill compensate for the increased risk for longer viewingtimes. The increase in retinal area of irradiation resultingfrom greater eye movements with increased viewing time(Velichovsky et al. 1996) takes longer to compensate forthe reduced impact of thermal diffusion with largerextended sources. Thus for increasing angular subtenseathe break-pointT2 increases from 10 s for small sourcesto 100 s for larger sources. Beyond 100 s there is nofurther increase in risk of thermal injury for small andintermediate size images. The specification of limits andmeasuring conditions specified here attempt to followthese variables with some simplification leading to aconservative determination of risk.

A pupil size of 7 mm is unrealistically large forlengthy viewing durations, leading to a still greater safetyfactor for all image sizes. It is conservatively assumedthat retinal thermal injury thresholds vary inversely withretinal image size (stabilized) between approximately 25mm and 1 mm (corresponding to angles of 1.5 to 59mrad). There is a transition range from 1 mm to approx-imately 1.7 mm, and beyond 1.7 mm (i.e., visual anglesgreater than 100 mrad) there is virtually no spot-sizedependence when limits are expressed at the retina.When expressed at the cornea, the ELs become a con-stant radiance for angles greater than 100 mrad.

For photochemically induced retinal injury there isno spot size dependence for a stabilized image. Unlikethermal injury mechanism, the thresholds for photochem-ical injury are highly dependent upon wavelength and areexposure dose (radiant exposure) dependent; i.e., thethresholds expressed as irradiance decrease inverselywith the lengthening of exposure time. Photochemicalretinal injury from welding arcs subtending angles ofaround 11.5 mrad show typical lesion sizes of the orderof 185200mm (corresponding to visual angles of 1112mrad), clearly demonstrating the influence of eye move-ments during fixation. This and other studies of eye-movements during fixation underpinned the derivation ofELs to protect against photochemical retinal injury.These studies also led to the ELs to be specified as anirradiance or radiance defined for a cone angle ofacceptance (field-of-view, or FOV) of 11 mrad for anexposure duration between 10 and 100 s. Hence, sourceswith an angular subtensea less than 11 mrad weretreated as equivalent to small, point-type sources. Thisapproach was not strictly correct when expressing an ELas an irradiance measurement of an 11-mrad source is not

434 Health Physics October 2000, Volume 79, Number 4

equivalent to irradiance averaging over a field-of-view(g) of 11 mrad, unless the source had a rectangular(top-hat) radiance distribution. Thus, in this revision ofthe guidelines, a distinction is made between angularsubtense of a source and radiance (or irradiance) averag-ing for photochemical EL values. For viewing times inexcess of approximately 3060 s, the influence ofsaccadic eye motion during fixation is generally over-taken by behavioral movements determined by visualtask, and it is quite unreasonable to assume that a lightsource would be imaged solely in the fovea for a durationlonger than 100 s. For this reason, the averaging anglegis increased linearly with the square-root oft. Theminimal angular subtenseamin correctly remains at thereference angle of 1.5 mrad for all exposure durationsused in thermal retinal hazard evaluation. However, forphotochemical retinal hazard assessment, the concept isactually different, as the angleg is a linear angle definingthe cone angle of acceptance for averaging irradiance,and it is important to note that this applies only forextended sources greater than approximately 11 mrad.

The impact of eye movements is dramatic forminimal retinal spot sizes and permits a leveling of thethermal EL fora , 1.5 mrad to a constant irradiance of1 mW cm22 in the visible spectrum (400700 nm) fort .10 s. However, as would be expected, there is only asmall impact for a source size of 100 mrad, and theplateau of no further risk of retinal injury due to eyemovements does not occur until 100 s. For photochem-ical injury, eye movements had already been incorpo-rated into the visible laser limits for an exposure durationbetween 10 and 100 s. Beyond 100 s, it is probablyunreasonable to assume that fixation could realisticallytake place, and it was concluded that the limits for all butvery large sources (greater than 100 mrad) should endthere. This same rationale was followed by the Commis-sion in the derivation of the guidelines for incoherentsources (ICNIRP 1997).

No changes in the measurement apertures (whichaverage irradiance and radiant exposure) are recom-mended. The use of a 7-mm pupillary aperture wasretained in the derivation of the thermal guidelines for allwavelengths, even for visible wavelengths, since thethermal damage mechanisms dominate in the near-infrared and red end of the visible spectrum, where thepupillary response is weakest or non-existent. However,for short wavelengths in the visible, a 3-mm aperture wasused (as previously) for the derivation of the photochem-ical limits, since pupillary constriction will be present atthese wavelengths. However, because of physiologicalmovements over several seconds of fixation, the 7-mmaperture averaging is still appropriate. In other words,although a 3-mm pupil was assumed in the derivation ofthe photochemical limits, to assess exposures of the eyeone is to use a 7-mm pupil. This same approach wasfollowed previously in the derivation of visible ELs forthe different wavelengths.

PROPOSALS FOR REVISED ELS

Sub-ns exposure ELsFor sub-ns exposures, the Commission recommends

the guidelines for single-pulse exposures at pulse dura-tions between 100 fs (10213 s) and 1 ns (1029 s) bereduced to the values in Table 2.

Research is also underway in the temporal region of 10 to100 fs in the visible and IR-A bands. (Note: The spectralcorrection factors CA and CC account for reduced retinalexposure in the IR-A band and are defined in Table 3.)

CW exposure ELsSince the risk of retinal injury from viewing a CW

light or near-infrared source depends upon three vari-ablestime, wavelength and angular subtense (Lund etal. 1998; Sliney 1996; Zuclich et al. 1998)and theangular dependence is not constant for all wavelengthsand exposure durations, the Commission concluded thata single EL function for some wavelengths was notpossible. The Commission concluded that ELs for timesgreater than 1 s required a dual limit approach forshort-wavelength light as followed for the incoherentoptical radiation ELs. The ELs are therefore expressed asthe lowest of two values:

1. For the CW thermal EL, the Commission recom-mends a limit of 18CACEt0.75 J m22 (1.8 CACEt0.75 mJcm22) as now used forl . 700 nm (whereCA 5 1.0from 400 nm to 700 nm); and this would apply to allwavelengths in the retinal hazard region (i.e., from400 to 1,400 nm) up to a duration that will now betermedT2 where the movement of the image over theretina dominates and limits the possibility of a thermalinjury, rather thant 5 1,000 s, where near-infraredlimits currently become constant irradiance values;and

2. For the CW photochemical (blue-light or photoreti-nitis) EL, the Commission recommends a limit of 100CB J m22 defined for a measurement cone-angle-of-acceptance (FOV,g) of 11 mrad fort # 100 s. Fort .100 s, an increasing FOVg 5 1.1 =t is applied.These limits lead to the result that for any smallsource subtending an angle less than 11 mrad, this

Table 2. Exposure guidelines.

Wavelength range Pulse duration t Exposure guideline EL

400 to 1,050 nm 10213 to 10211 s 1.5 CA 3 1024 J/m2

(100 fs to 10 ps) (1.5 CA 3 1028 J/cm2)400 to 1,050 nm 10211 to 1029 s 2.7 CA t0.75 3 104 J/m2

(10 ps to 1 ns) (2.7 CAt0.75 J/cm2)1,050 to 1,400 nm 10213 to 10211 s 15 CC 3 1024 J/m2

(100 fs to 10 ps) (15 CC 3 1028 J/cm2)1,050 to 1,400 nm 10211 to 1029 s 27 CC t0.75 3 104 J/m2

(10 ps to 1 ns) (27 CCt0.75 J/cm2)Other wavelengths 10213 to 1029 s Maintain irradiance limit

of the 1996 1-ns ELa(100 fs to 1 ns)

a This is the current, 1996 interim guideline. Research to establish ELs inthese spectral regions is planned, as very few data exist (Wantabe et al.1991) for the skin or cornea.

435ICNIRP Guidelines INTERNATIONAL COMMISSION ON NON-IONIZING RADIATION PROTECTION

Fig. 4. The guideline EL for a minimal-image, 400 nm to 450 nm blue-laser exposure is a single value, independent ofsource-size, averaged over an 11 mrad field-of-view for 10100 s exposureswhereas the guideline for 650-nm redlaser exposures varies with source size. Note that source size directly plays a role only in the retinal thermal ELs. Forphotochemical limits fort . 100 s, the measurement cone angleg increases with time.

Table 3. Proposed changes to CW laser guideline exposure limits.

Wavelengthrange (nm) Exposure duration Exposure limit (EL)a Comments

4001,400 nm 1 ns , t , 1 s No Change in limit values,except to eliminate theincreased value of amin fort . 0.7 s.b

Original limits well established asthermal damage mechanism.However, eye movement researchrequired a change.b

400600 nm 1 s , t , 100 s H 5 10 CB mJ/cm2 where FOVg 5 11 mrad, (i.e., V 51024 sr), and equivalent to:L 5 100 CB J/(cm2 sr) whereFOV g 5 11 mrad.

Photochemical: Assumes FOV g 511 mrad which is over-conservativefor t . 100 s; therefore, a newproposal for meas. FOV g 5 1.1 =tmrad for t . 100 s (V 5 t msr)

100 s , t , 104 s E 5 0.1 CB mW/cm2 (i.e., L 5100 CB J/(cm2 sr) for meas.FOV g 5 1.1 =t (mrad,important when a . 11mrad).

First dual limit to protect againstphotochemical injury (i.e.,photoretinitis).

400600 nm 1 s , t , T2 s H 5 1.8 t0.75 CE mJ/cm2 Based only upon thermal effects.T2 s , t , 30 ks E 5 1.8 CE T220.25 mW/cm2 for

a . 1.5 mradSecond dual limit to protect against

retinal thermal injury.6001,400 nm 1 s , t , T2 H 5 1.8 CA CE CC t0.75 mJ/cm2 Original limits over-stated

large-image thermal damagemechanisms, and eye movementsnot adequately addressed for CWexposures.

E 5 1.0 CA CC mW/cm2 for a, 1.5 mrad

T2 , t , 30 ks E 5 1.8 CA CC CE T220.25

mW/cm2 for a . 1.5 mrad

a CB 5 1.0 forl , 450 nm (referred to as the newC3 in IEC terminology);CB 5 100.02(l2450) for l . 450 nm; andT2 5 10[10(a21.5)/98.5]limited to T2 5 10 s fora , 1.5 mrad and 100 s ifa . 100 mrad;CA 5 1.0 for l , 700 nm;CA 5 100.002(l2700) for 700 nm, l ,1,050 nm;CA 5 5.0 for 1,050, l , 1,400 nm; andCC 5 1.0 for l , 1,150 nm;CC 5 100.018(l21,150) for 1,150, l , 1,200 nm;CC5 8.0 for 1,200 nm, l , 1,400 nm;CE 5 a/amin wherel is expressed in nm anda is expressed in milliradians (mrad).b amin now is 1.5 mrad for all thermal retinal hazard ELs.

436 Health Physics October 2000, Volume 79, Number 4

Table 4. Laser exposure limits for the eye.

Wavelength l (nm)Exposure duration

t (s)Exposure limit EL(J/cm2 or W/cm2)

Exposure limit EL(J/m2 or W/m2) Restrictions

Ultraviolet

Dual limits for 180600 nm ultraviolet laser exposures at t . 1 ns:Photochemical180 to 302 1 ns to 30 ks 3 mJ/cm2 30 J/m2 Aperture sizes:

1 mm for t , 0.3 s1.5 t0.375 mm for

0.3 s , t , 10 s3.5 mm for t . 10 s

303 1 ns to 30 ks 4 mJ/cm2 40 J/m2304 1 ns to 30 ks 6 mJ/cm2 60 J/m2305 1 ns to 30 ks 10 mJ/cm2 100 J/m2306 1 ns to 30 ks 16 mJ/cm2 160 J/m2307 1 ns to 30 ks 25 mJ/cm2 250 J/m2308 1 ns to 30 ks 40 mJ/cm2 400 J/m2309 1 ns to 30 ks 63 mJ/cm2 630 J/m2310 1 ns to 30 ks 0.1 J/cm2 1.0 kJ/m2311 1 ns to 30 ks 0.16 J/cm2 1.6 kJ/m2312 1 ns to 30 ks 0.25 J/cm2 2.5 kJ/m313 1 ns to 30 ks 0.40 J/cm2 4 kJ/m2314 1 ns to 30 ks 0.63 J/cm2 6.3 kJ/m2315 to 400 10 s to 30 ks 1.0 J/cm2 10 kJ/m2Thermal180 to 400 1 ns to 10 s 0.56t 0.25 J/cm2 5.6t 0.25 kJ/m2

Visible

400 to 700 100 fs to 10 ps 0.015 CE mJ/cm2 0.15 CE mJ/m2400 to 700 10 ps to 1 ns 2.7 CE t 0.75 J/cm2 27 CE t 0.75 kJ/m2400 to 700 1 ns to 18 ms 0.5 CE mJ/cm2 5 CE mJ/m2400 to 700 18 ms to 10 s 1.8CE t 0.75 mJ/cm2 18CE t 0.75 J/m2Dual limits for 400600 nm visible laser exposures at t . 10 s (all for 7-mm limiting

aperture)Photochemicala400 to 600 10 s to 100 s 10 CB mJ/cm2 100 CB J/m2 for a , 11 mrad (g 5 11

mrada)400 to 600 100 s to 30 ks 0.1 CB mW/cm2 1 CB W/m2 for a , 11 mrad400 to 600 100 s to 10 ks 0.1 CB mW/cm2 1 CB W/m2 for a . 11 mrad (g 5 1.1

t 0.5 mrad)400 to 600 10 ks to 30 ks 10 CB mW/(cm2 sr) 100 CB W/(m2 sr) (See Notea)Thermala (all for 7-mm limiting

aperture)400 to 700 10 s to 30 ks 1.0 mW/cm2 10 W/m2 for a , 1.5 mrad400 to 700 10 s to T2 s 1.8 CE t 0.75 mJ/cm2 18 CE t0.75 J/m2 for a . 1.5 mrad400 to 700 T2 s to 30 ks 1.8 CE T220.25 mW/cm2 18 CE T220.25 W/m2 for a . 1.5 mrad

Near Infrared, IR-A

700 to 1,050 100 fs to 10 ps 0.015 CA CE mJ/cm2 0.15 CA CE mJ/m2 7 mm limiting aperture700 to 1,050 10 ps to 1 ns 2.7 CA CE t0.75 mJ/cm2 27 CA CE t0.75 mJ/m2700 to 1,050 1 ns to 18 ms 0.5 CA CE mJ/cm2 5 CA CE mJ/m2700 to 1,050 18 ms to 10 s 1.8 CA CE t0.75 mJ/cm2 18 CA CE t0.75 J/m21,051 to 1,400 100 fs to 10 ps 0.15 CC CE mJ/cm2 1.5 CC CE mJ/m21,051 to 1,400 10 ps to 1 ns 27 CC CE t0.75 J/cm2 270 CC CE t0.75 kJ/m21,051 to 1,400 1 ns to 50 ms 5 CC CE mJ/cm2 50 CC CE mJ/m21,051 to 1,400 50 ms to 10 s 9.0 CC CE t0.75 mJ/cm2 90 CC CE t0.75 J/m2700 to 1,400 10 s to 30 ks 1.0 CA CC mW/cm2 10 CA CC W/m2 for a , 1.5 mrad700 to 1,400 10 s to T2 s 1.8 CA CC CE t0.75 mJ/cm2 18 CA CC CE t 0.75 J/m2 for a . 1.5 mrad700 to 1,400 T2 s to 30 ks 1.8 CA CC CE T220.25

mW/cm218 CA CC CET220.25 W/m2

NTEb 100 mW/cm2

for a . 1.5 mrad

Far Infrared

1,400 to 1,500 nm 1 ns to 1 ms 0.1 J/cm2 1 kJ/m2 Aperture sizes:1,400 to 1,500 nm 1 ms to 10 s 0.56t 0.25 J/cm2 5.6t 0.25 kJ/m2 1 mm for t , 0.3 s1,500 to 1,800 nm 1 ns to 10 s 1.0 J/cm2 10 kJ/m2 1.5 t0.375 mm for1,801 to 2,600 nm 1 ns to 1 ms 0.1 J/cm2 1 kJ/m2 0.3 s , t ,10 s1,801 to 2,600 nm 1 ms to 10 s 0.56t 0.25 J/cm2 5.6t 0.25 kJ/m22,601 nm to 1 mm 1 ns to 100 ns 10 mJ/cm2 100 J/m2 3.5 mm for t . 10 s2,601 nm to 1 mm 100 ns to 10 s 0.56t 0.25 J/cm2 56t 0.25 kJ/m21,400 nm to 1 mm 10 s to 30 ks 100 mW/cm2 1 kW/m2

a For small sources subtending an angle of 1.5 mrad or less, the visible dual limit ELs from 400 nm to 600 nm, for times greater than10 s, reduce to the thermal limits for times less thanT1 and to photochemical limits for longer times.T1 5 10 s forl , 450 nm;T1 510 3 100.02(l2450) for 450 nm, l , 500 nm; andT1 5 100 s forl . 500 nm. The photochemical retinal hazard limit may also beexpressed as an integrated radianceL 5 100 CB J/(cm2 sr); T2 see Table 5.b NTE 5 Not to exceed; see Table 5 for definitions of constants.

437ICNIRP Guidelines INTERNATIONAL COMMISSION ON NON-IONIZING RADIATION PROTECTION

radiance limit corresponds to a corneal irradiance of 1.0CB W m22 (0.1 CB mW cm22) for t . 100 s. Thisphotoretinitis limit is only required for wavelengthsbetween 400 nm and 600 nm. From a photobiologicalstandpoint, the action spectrum used for incoherentsources, B(l) should be used (Ham and Mueller 1989);however, for simplicity, it was concluded that CB shouldhave a value of 1.0 between 400 and 450 nm, and thena power function between 450 nm and 600 nm:CB 5100.02(l2450) (wherel is expressed in nm). The ELs aretherefore similar to, but are slightly more conservativethan, the incoherent limits to account for the uncertain-ties in the photoretinitis action spectrum which are morecritical for monochromatic radiation. The break pointwhere intra-beam (minimal-image) photochemical limitsapply rather than 10 W m22 (1.0 mW cm22) is againtermed T1 as in previous guidelines, since it is thetransition from dominantly thermal to dominantly pho-tochemical injury.

Fig. 4 shows how the guideline values appear for tworepresentative wavelengths: where photochemical mech-anisms dominate (450 nm) and where thermal mecha-nisms dominate (650 nm).

Repetitive exposuresEach of the following three general rules should be

applied to all repetitive exposures as occur from repeti-tively pulsed or scanning laser systems:

1. The exposure from any single pulse in a train ofpulses shall not exceed the EL for a single pulse ofthat pulse duration;

2. The exposure from any group of pulses (or sub-groupof pulses in a train) delivered in timeT shall notexceed the EL for timeT; and

3. The exposure from any single pulse within a group ofpulses shall not exceed the single-pulse EL multipliedby a cumulative-thermal correction factorCP 5 N20.25,

Table 5. Laser exposure limits for the skin.ab

Wavelength l (nm)Exposure

duration t (s)Exposure limit EL(J/cm2 or W/cm2)

Exposure limit EL(J/m2 or W/m2) Restrictions

Ultraviolet

180 to 400 1 ns to 30 ks Same as Eye EL

Visible and IR-A

400 nm to 1,400 nm 1 ns to 100 ns 20 CA mJ/cm2 200 CA J/m2 3.5 mm limiting aperture400 nm to 1,400 nm 100 ns to 10 s 1.1 CA t 0.25 J/cm2 11 CA t 0.25 kJ/m2

400 nm to 1,400 nm 10 s to 30 ks 0.2 CA W/cm2 2 CA kW/m2

Far infrared

1,400 nm to 1 mm 1 ns to 30 ks Same as Eye EL for 1,400 nm to 1 mm 3.5 mm limiting aperture

a Notes for all EL tables: Angles: All values ofa andg in milliradians (mrad). Wavelength: All values of wavelength (2) in nanometers(nm). Time: All values oft in s; 1 ks5 1,000 s, and 30 ks5 8 h.1. Spectral Correction Factors:CA 5 1 for l 5 400 to 700 nm;CA 5 10[0.002(l2700)] if l 5 7002 1,050 nm; CA 5 5.0 if l 5 1,0511,400nm; CB 5 1 for 400 nm, l # 450 nm;CB 5 100.02(l2450) if l 5 400700 nm;T2 5 10[10(a21.5)/98.5] s; for 1.5 mrad, a , 100 mrad;T2 5 10 s fora , 1.5 mrad and T2 5 100 s if a . 100 mrad;CC 5 1 for l # 1,150;CC 5 100.0181(l21,150) for 1,150, l , 1,200;CC 5 8 for 1,200# l , 1,400.2. Angular subtensea of a source and limiting cone angle measuring field-of-viewg: amin is 1.5 mrad for all thermal retinal hazardexposure limits.g 5 11 mrad fort # 100 s,g 5 1.1 t0.5 mrad for 100 s, t , 10,000 s, andg 5 110 mrad fort . 104 s. T2 510[10(a21.5)/98.5] such thatT2 5 10 s fora , 1.5 mrad and 100 s ifa . 100 mrad.3. Extended Source ELs: For extended-source viewing of laser radiation (e.g., diffuse reflection) between 400 nm and 1,400 nm, thethermal ELs include the correction factorCE provided that the angular subtense of the source (measured at the viewers eye) is greaterthanamin, whereamin is:

amin 5 1.5 mrad for all thermal limits

CE 5 1.0 for a , amin

CE 5 a/amin for amin , a , 100 mrad.

CE 5 a2/~amin 3 amax! for a . 100 mrad whereamax 5 100 mrad.

The angle of 100 mrad may also be referred to asamax at which point the extended source limits can be expressed as a constantradiance using the last equation written in terms ofamax:

LEL 5 ~8.53 103!~ELpt source!J/~cm

2 sr! for t , 10 s.

LEL 5 100CBJ/(cm2sr) for t . 1s

b For retinal photochemical limits, where the radiance is measured over a cone angle of acceptance (field-of-view)g which increasesfrom 11 mrad for times less than 100 s to 110 mrad for times greater than 10 ks.4. Terminology: The term Exposure Limit (EL) is used by ICNIRP. The same values are termed MPEs (Maximum Permissible ExposureLimits) by ANSI and IEC, and termed TLVs (Threshold Limit Values) by ACGIH. Essentially all groups have the same limit values.

438 Health Physics October 2000, Volume 79, Number 4

whereN is then number of pulses. This rule appliesonly to ELs to protect against thermal injury, whereall pulses delivered in less thantmin are treated as asingle pulse (Table 1). The inflection point,tmin is 1 nsfor wavelengths between 315 and 400 nm, is 18ms forwavelengths between 400 and 1,050 nm, is 50ms forwavelengths between 1,050 and 1,400 nm, is 1 msfor wavelengths between 1.4 and 1.5mm and between1.8 and 2.6mm, is 1 s between 1.5 and 1.8mm, and is100 ns for wavelengths between 2.6 and 1,000mm.

RESTRICTIONS

The guidelines apply to normal, awake, task-oriented viewing conditions and normally cannot beapplied directly to ocular exposure from ophthalmicinstruments or head-mounted ocular illuminators, wherethe impact of normal head movements is neutralized.Tables 4 and 5 provide the final updated summary of alllaser ELs for eye and skin.

Acknowledgments:The support received from the International RadiationProtection Association, the World Health Organization, the InternationalLabour Office, the European Commission, and the German Government isgratefully acknowledged.

REFERENCES

Andre-Deshays, C.; Berthoz, A.; Ravel, M. Eye-head couplingin humans. I. Simultaneous recording of isolated motorunits in dorsal neck muscles and horizontal eye movements.Experimental Brain Res. 69:399406; 1988.

Barnes, F. S. Biological damage resulting from thermal pulses.In: Wolbarsht, M. L., ed. Laser applications in medicine andbiology. New York: Plenum Press; 1974: 205221.

Cain, C. P.; DiCarlo, C. D.; Rockwell, B. A.; Kennedy, P. K.;Noojin, G. D.; Stolarski, G. J.; Toth, C. A.; Roach, W. P.Retinal damage and laser-induced breakdown produced byultrashort-pulse lasers. Graefes Arch. Clin. Exper. Ophthal-mol. 234:2837; 1996.

Cain, C. P.; Noojin, G. D.; Hammer, D. X.; Thomas, R. J.;Rockwell, B. A. Artificial eye for in-vitro experiments oflaser light interaction with aqueous media. J. Biomed.Optics 2:8894; 1997.

Courant, D.; Court, L.; Abadie, B.; Brouillet, V. Retinaldamage thresholds from single-pulse exposures in the visi-ble spectrum. Health Phys. 56:637642; 1989a.

Courant, D.; Court, L.; Sliney, D. H. Spot size dependence oflaser retinal dosimetry. In: Mueller, J. G.; Sliney, D. H., eds.Dosimetry of laser radiation in medicine and biology,Bellingham, WA: SPIE Press; SPIE Institute Series, Vol.IS5:156165; 1989b.

Eizenman, M.; Hallett, P. W.; Frecker, R. C. Power spectra forocular drift and tremor. Vision Res. 25:16351640; 1980.

Epelboim, J.; Kowler, E.; Steinman, R. M.; Collewijn, H.;Erkelens, C. J.; Pizlo, Z. Gaze-shift dynamics and two kindsof sequential looking tasks. J. Vestibular Res. 3:421442;1995.

Epelboim, J.; Steinman, R. M.; Kowler, E.; Pizlo, Z.; Erkelens,C. J.; Collewijn, H. Gaze-shift dynamics and two kinds ofsequential looking tasks. Vision Res. 37:25972607; 1997.

Epelboim, J. Gaze and retinal-image-stability in two kinds ofsequential looking tasks. Vision Res. 38:37733784; 1998.

Ferman, L.; Collewijn, H.; Jansen, T. C.; van den Berg, A. V.Human gaze stability in the horizontal, vertical and torsionaldirection during voluntary head movements, evaluated witha three-dimensional scleral induction coil technique. VisionRes. 27:811828; 1987.

Gerstman, B. S.; Thompson, C. R.; Jacques, S. L.; Rogers,M. E. Laser-induced bubble formation in the retina. LasersSurg. Med. 18:1021; 1996.

Goldman, A. A.; Ham, W. T., Jr.; Mueller, H. A. Oculardamage thresholds and mechanisms for ultrashort pulses ofboth visible and infrared laser radiation in the rhesusmonkey. Exp. Eye Res. 24:4546; 1977.

Ham, W. T.; Mueller, H. A. The photopathology and nature ofthe blue light and near-UV retinal lesions produced bylasers and other optical sources. In: Wolbarsht, M. L., ed.Laser applications in medicine and biology. New York:Plenum Press; 1989: 191246.

Hammer, D. X.; Jansen, E. D.; Frenz, M.; Noojin, G. D.;Thomas, R. J.; Noack, J.; Vogel, A.; Rockwell, B. A.;Welch, A. J. Shielding properties of laser-induced break-down in water for pulse durations from 5 ns to 125 fs. Appl.Opt. 36:56305640; 1997.

Hillenkamp, F. Laser radiation tissue interaction. Health Phys.56:613616; 1989.

International Commission on Non-Ionizing Radiation Protec-tion. Guidelines on limits of exposure to laser radiation ofwavelengths between 180 nm and 1,000mm. Health Phys.71:804819; 1996. (This reference is the rationale docu-ment for the laser ELs, upon which the more recent laserELs are based.)

International Commission on Non-Ionizing Radiation Protec-tion. Guidelines on limits of exposure to broad-band inco-herent optical radiation (0.38 to 3mm). Health Phys.73:539554; 1997. (This reference is the rationale docu-ment for the visible and infrared ELs for broad-band lightsources, and provides the most recent rationale for thermaland photochemical retinal injury ELs.)

Lund, D. J.; Edsall, P. R.; Fuller, D. R.; Hoxie, S. W. Bioeffectsof near-infrared lasers. J. Laser Appl. 10:140143; 1998.

Mainster, M. A.; White, T. J.; Tips, J. H.; Wilson, P. W.Transient thermal behavior in biological systems. Bull.Math. Biophys. 32:303314; 1970.

Naidoff, M. A.; Sliney, D. H. Retinal injury from a weldingarc. Am. J. Ophthalmol. 77:663668; 1974.

Ness, J.; Lund, J.; Zwick, H.; Scales, D.; Stuck, B. Eyepositions during fixation, an evaluation. Proceedings of theInternational Laser Safety Conference. Orlando, FL: LaserInstitute of America; 1997.

Ness, J. W.; Zwick, H.; Stuck, B.; Lund, D. J.; Molchany,J. M.; Sliney, D. H. Eye movements during fixation:implications to laser safety for long duration viewing.Health Phys. 78:19; 2000.

Priebe, L. A.; Welch, A. J. Asymptotic rate process calcula-tions of thermal injury to the retina following laser irradia-tion. J. Biomech. Eng. 100:4954; 1978.

Roach, W. P.; Johnson, T. E.; Rockwell, B. A. Proposedmaximum permissible exposure limits for ultrashort laserpulses. Health Phys. 76:349354; 1999.

Rockwell, B. A.; Hammer, D. X.; Hopkins, R. A.; Payne, D. J.;Toth, C. A.; Roach, W. P.; Druessel, J. J.; Kennedy, P. K.;Amnotte, R. E.; Eilert, B.; Phillips, S.; Noojin, G. D.;Stolarski, D. J.; Cain, C. P. Ultrashort laser pulse bioeffectsand safety. Proceedings of the International Laser Safety

439ICNIRP Guidelines INTERNATIONAL COMMISSION ON NON-IONIZING RADIATION PROTECTION

Conference. Orlando, FL: Laser Institute of America3:159165; 1997.

Sliney, D. H. Laser effects on vision and ocular exposurelimits. Applied Occupational and Environmental Health11:313319; 1996.

Sliney, D. H. Retinal injury from laser radiation. NonlinearOptics. 21:118; 1999.

Sliney, D. H.; Wolbarsht, M. L. Safety with lasers and otheroptical sources. New York: Plenum Publishing; 1980.

Toth, C. A.; Narayan, D. G.; Cain, C. P.; DiCarlo, C. D.;Rockwell, B. A.; Roach, W. P. Histopathology of ultrashortlaser pulse retinal damage. Invest. Ophthalmol. Vis. Sci.37:3170; 1996.

Toth, C. A.; Narayan, D. G.; Cain, C. P.; Noojin, G. D.; Winter,K. P.; Rockwell, B. A.; Roach, W. P. Pathology of macularlesions from subnanosecond pulses of visible laser energy.Invest. Ophthalmol. Vis. Sci. 38:22042213; 1997.

Velichovsky, B.; Pomplun, M.; Rieser, J. Attention and com-munication: eye-movement-based research paradigms. In:Zangemeister, W. H.; Stiehl, H. S.; Freska, C., eds. Visualattention and cognition. Amsterdam: North Holland-Elsevier; 1996: 125154.

Wantabe, S.; Anderson, R. R.; Brorson, B.; Dalickas, G.;Fujimoto, J. G.; Flotte, T. J. Comparative studies of femto-second to microsecond laser pulses on selective pigmentedcell injury in skin. Photochem. Photobiol. 53:757762;1991.

Zuclich, J. A.; Zwick, H.; Schuschereba, S. T.; Stuck, B. E.;Cheney, F. E. Ophthalmoscopic and pathologic descriptionof ocular damage induced by infrared laser radiation. J.Laser Appl. 10:114120; 1998.

f f

440 Health Physics October 2000, Volume 79, Number 4

INTRODUCTIONBACKGROUNDGeneralSub-nanosecond exposuresCW exposures

PROPOSALS FOR REVISED ELSSub-ns exposure ELsCW exposure ELsRepetitive exposures

RESTRICTIONSREFERENCES