Tutorial Guide: Laser Measurement Techniques Guide For Hazard Evaluation. Part 2Terry Lyon Citation: Journal of Laser Applications 5, 37 (1993); doi: 10.2351/1.4745329 View online: http://dx.doi.org/10.2351/1.4745329 View Table of Contents: http://scitation.aip.org/content/lia/journal/jla/5/2?ver=pdfcov Published by the Laser Institute of America Articles you may be interested in Wave Propagation in Arrays of Scatterers Tutorial: Part 2 Acou. Today 6, 13 (2010); 10.1121/1.3373152 Structural Acoustics Tutorial—Part 2: Sound—Structure Interaction Acou. Today 3, 9 (2007); 10.1121/1.2961152 Understanding laser hazard evaluation J. Laser Appl. 7, 99 (1995); 10.2351/1.4745379 Tutorial Guide: Laser Measurement Techniques Guide For Hazard Evaluation. Part 1 J. Laser Appl. 5, 53 (1993); 10.2351/1.4745324 Thermal diffusivity measuring technique for hazardous materials J. Appl. Phys. 44, 1420 (1973); 10.1063/1.1662387

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Sensing, Measurement & Control

TUTORIAL GUIDE - Part 2

Laser Measurement Techniques GuideFor Hazard Evaluation

Terry LyonLaser Microwave Division

U.S. Army Environmental Hygiene Agency

Key Words: Military Laser Systems Guide, LaserMeasurement, Hazard Evaluation.

ABSTRACTA tutorial guide is presented to provide uniform guidance

when performing radiometric measurements upon laser systemsto perform a hazard evaluation. This guide was requested byand presented to the Department of Defense (DOD) LaserSystem Safety Working Group (LSSWG) to serve as astandardized method when performing hazard evaluationsprimarily upon tactical and training military laser systemsincluding laser rangefinders and designators. Technicalinformation is provided in detail to select a suitable detector,preform radiometric measurements, assess laser system pointingerrors, calculate laser protective eyewear optical density,calculate beam divergence, and predict peak beam irradiancesor radiant exposures at any distance from the laser. The guideidentifies the primary health and safety consultants within DODwho are responsible to evaluate the potential hazards frommilitary laser systems.

This guide is intended to serve less experienced personnel bypointing out some common measurement errors and detectorlimitations which can lead to erroneous results. The guide alsodirects the reader to review the requirements of MIL-STD-1425A when a military laser system has been exempted fromthe requirements of the federal regulation for laser products, 21CFR 1040. This guide was intended to facilitate acceptance oflaser hazard evaluations performed by other services and to aidplanners of joint service laser exercises.

Part 2.

GENERAL LASER RANGE EQUATION

Gaussian Beam Distribution

Laser beams do not have well-defined edges as they exit fromthe laser cavity. Most are more intense at the center of the beamand gradually decrease outward. A Gaussian beam is one inwhich the irradiance variation follows the "error" function asgiven below in Equation (1):

-4p2 (1)

E Eoe DL2

where p = DL/2 when E = Eo/e. This equation is presentedgraphically by the solid curve of Figure 1. The p represents thedistance radially from the center of the beam and DL is thediameter at 1/e peak irradiance points.

The initial beam diameter, a, is defined for hazard evaluationin such a manner that it contains 63 percent of the beam powerwhich is the area under the curve. This value of beam diameteris referred to as the diameter at 1/e points. By using the 1/e-points beam diameter and the total output power or energy, the

(Continued next page)

EDITOR'S NOTE:

Tutorial Guide - Part 1 appeared in JLA V.5, No. 1, Spring 1993, pp. 53-58. Inaddition to the reasons identified in his abstract, author Terry Lyon notes theGuide will be particularly useful to laser manufacturers who build and designmilitary laser systems which are exempt from certain Federal PerformanceStandards because they are used in combat situations.

Further, the Guide will be useful in development of the proposed ANSIZ136.4-Measurements and Instrumentation. Co-chaired by Aaron A. Sanders

and Thomas R. Scott, both of the National Institute of Standards and Tech-nology, the subcommittee is at work on the initial draft.

Part 1 included sections: Hazard Evaluation, Measurement versus Theory,Detector Types, Measurement Consideration, Compliance with MIL-STD-1425A, Laser System Pointing Errors, Laser Protective Eyewear, and LaserBeam Divergence.

An Appendix featuring useful CIE Radiometric and Photometric units isincluded.

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FIGURE 1. Ideal Gaussian beam distribution and a rectangularbeam profile which has the same peak irradiance.

RelativeIrradiance

4(.n.. Beam Diametern0.1

Rectanglular Profile

Beam Profile

peak irradiance or radiant exposure in the beam can becomputed from Equations (2):

1.274) ,

a2 (2)

The beam diameter from any distance from the laser, DL is,therefore, given by:

DL = a2+(r-ro)212 (4)where "a" could represent either the diameter of the beam at thelaser exit port or the diameter of an external beam waist ifpresent. The "r" represents the distance to the second measuringpoint, and "ro" represents the distance to an external beam waist,if any.

Normally the beam divergence specified by the laser manu-facturer or the developer is given to 1/e2 points which allowsone to determine the average irradiance downrange rather thanpeak irradiance. The peak irradiance is more of interest from thestandpoint of a potentially hazardous exposure to the skin oreyes. For a gaussian beam the divergence specified at 1/e 2points can be divided by 42 to obtain the diverge at 1/e points.

Often the beam divergence can be checked by visuallyobserving the beam spot size on a target board with a gridpattern. Accounting for the fact that the eye is a nonlineardetector, the effective beam diameter to lie-points may beapproximately 0.7 times the observed diameter. The divergencein radians is simply the effective diameter divided by thedistance between the laser and the target.

The Laser Range EquationThe range equation in Equation (5) predicts the beam irradi-

ance, E, at any distance from the laser using the output lasercharacteristics and the beam divergence [8].

E —

H.1.27Q ,

a2 E1

'

27 013 + (r-ro)2

4:10 2 (5)

(note that 1.27 = 4 /This concept can be visualized more clearly with a rec-

tangular profile in Figure 1 which has the same total area underthe curve as the Gaussian but with a dimension of 1/e. Note thatthe peak value is the same between the two distributions.Assuming that most laser beams are Gaussian yields areasonable approximation of beam diameter even if the beamshape is not truly Gaussian.

Beam DivergenceLaser beam divergence should be determined at 1/e points

since the peak beam irradiance or radiant exposure can then bedetermined at any distance from the laser. One method todetermine beam divergence is to measure the 1/e-diameter at adistance away from the laser and then calculate the angle atwhich the beam spreads from:

2 2DL - ar - ro (3)

where ro is the distance from the laser to an external beamwaist, if any. If an external beam waist does exist, then "a"refers to the diameter at the waist rather than the initial beamdiameter.

This equation applies strictly for peak data only and provides afair approximation for irradiance data measured through anaperture. Atmospheric effects may exaggerate the peak data anduse of an aperture will average somewhat the localizedconcentrations within the laser beam which are created by theatmosphere.

Atmospheric attenuation can have a significant effect, espe-cially at longer ranges, even in clear, nonturbulent air. Theeffects from the atmosphere are less noticeable within severalhundred meters. Saturation of the magnitude of scintillationoccurs at approximately 700 m. At greater or lesser ranges thevariation of localized irradiance (or radiant exposure) valuesdecreases. The range equation contains the term, e -ur, to allowfor the attenuation loss introduced by the atmosphere, where .tis the atmospheric attenuation factor. The theoretical peakirradiance at any range is the output power (reduced byatmospheric absorption) divided by the area of the beam to lie-points.

Likewise, Equation (5) can be rearranged to solve for the ef-fective beam divergence, 4), from measured values of peak beamirradiance or radiant exposure. Irradiance or radiant exposuredata obtained at long ranges can be used to determine the beamdivergence using a graphical best-fit approach while selecting arealistic atmospheric attenuation coefficient for the

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FIGURE 2. Percentage of power transmitted through a circularaperture when placed centered within a Gaussian laser beam asa function of relative aperture diameter.

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0 0 1 2 3

APERTURE DIAMETER / BEAM DIAMETER

w

coz

UJLU CC

0 CCa. IA2 a.(

CO 0

< 0I- CC

i-LLOz0

U-

measurement environment. Other general laser range equationstake into account the entrance aperture size of the detectorrelative to the beam diameter and are published in the literature.See References [6, 7, 8] for additional details.

EXIT BEAM DIAMETER

Beam ProfileAn initial task important to a laser hazard evaluation is to

determine the shape of the laser beam. A visual examination,using appropriate image converters and protective measures,can be used as a rough guide to evaluate the beam profile. Ifsufficient power exists, a rough picture of the output beamshape can be recorded on a thermally sensitive paper placedperpendicular to the beam size.

Clear Exit ApertureIt is often useful to measure the clear aperture diameter for

future reference. The clear exit aperture can normally bemeasured with a metric ruler placed against the face of theoutput lens. Operation of the laser can be used to confirm thatthe exit lens limits the output beam.

Beam Diameter MeasurementIf the beam is fairly circular, an estimate of the beam diam-

eter at 1/e-peak-irradiance-points can be approximated bymeasuring the total output power and then centering a variablediameter circular aperture over the output. When the aperture ispositioned to maximize the reading and adjusted to pass 63percent of the total radiant power, the diameter should be nearlythe 1/e-diameter referred to as the effective emergent beamdiameter.

Gaussian BeamsIf the output beam is strictly Gaussian in shape then its

diameter might also be estimated by dividing the manufacturer'sspecified effective beam diameter at 1/e points by 12. The exitbeam diameter specified by the developer is normally to 1/e2

points which allows one to predict the average irradiance ratherthan peak value. Figure 2 illustrates the fraction of a Gaussianbeam that passes through a circular aperture.

Non-Gaussian BeamIf the beam is not circular, then further analysis would be

required depending upon the degree of departure from aGaussian shape and the size of the beam. Non-Gaussian laserbeams can be treated as Gaussian beams in terms of effect uponthe eye or skin in the far field to determine an effective exitbeam diameter and an effective beam divergence. Irradiance orbeam radiant exposure data should be relied on in the near fieldsince existing laser range equations may not adequately predicta potentially hazardous exposure.

RADIANT POWER or ENERGY

Collateral RadiationFirst, it is necessary to determine if all the output radiation is

contained in only one beam and to account for any pump lightor extraneous (collateral) radiation which may add un-necessarily to the measured output. When a laser system isfound to emit multiple beams, each beam must be evaluatedindependently. Simply viewing the beam falling upon a whitetarget in a dark environment with the unaided eye or with anear-infrared viewer provides a quick check to assess pumplight and to detect other off-axis secondary beams. A laserattenuating filter may be necessary to permit safe viewing of thelaser beam reflection. The attenuating filter should pass mostvisible radiation while attenuating the laser wavelength.

Infrared lasers can be checked by other forms of thermallysensitive materials. If pump radiation is detected, a suitablemethod should be used to account for it during the laser beammeasurement.

Measurement of Power or EnergyThe first task when making a measurement of power or

energy is to locate the beam. Locating a potentially hazardouslaser beam, even when visible, can be difficult while wearinglaser protective eyewear. Laser beams operating from 400 to1,100 nm can be viewed with near-infrared conversion devicesthrough protective eyewear. Near-infrared phosphor cards canbe excited from a source of ultraviolet to view a fluorescentspot marking the beam location. Liquid crystal sheets havesufficient sensitivity to locate many far-infrared laser beams.Various thermal papers have been used to locate high peakpower beams from pulsed visible and near-infrared lasers. Otherinvestigator's senses have been employed to locate laser beams.A snapping sound has been heard from high peak powerNd:YAG lasers which emit below the ELs for the skin. Thepalm of the hand has been used to follow a fairly low power

(Continued next page)

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CO2 far-infrared laser beam. Fluorescence has been observedfrom skin when testing a 1,540 nm Erbium laser operating at alevel considered safe for skin exposure.

When measuring the beam, ensure that the entire beam isfalling on the active surface of the detector. Systems with alarge exit beam may require the use of a lens to collect all thelaser emissions for the detector. Be careful not to damage thedetector or any detector input optics with the focused beam.Also be sure to check the detector for linearity.

The detection system with the lens should be calibrated as aunit since the addition of the lens can be complicated to analyzetheoretically.

Other measurement errors can be created by polarization ofthe laser beam in combination with optical components placedin the measurement train. Initially one should check for thepresence of beam polarization by rotating a crystal polarizer infront of the detector. A variation in transmitted power or energysuggests that the beam is polarized. The maximum and mini-mum values during rotation should be recorded for futurereference. Gelatin polarizers should not be used outside of theirwavelength region which is generally confined to the visible.

The investigator should observe changes in the laser outputcharacteristics while varying user controls and record any fluc-tuations. It is also desirable to measure variations in laser outputpower or energy with power supply voltage or current and am-bient temperature. Minor variations will have little effect upon ahazard evaluation as their influence upon NOHD and protectivefilter OD would normally be slight.

A representative sample of laser devices should be evaluatedfor differences in laser characteristics which may affect lasersafety. Complete data from each device is not necessary unlesssignificant variations are detected. Detailed measurementsshould be performed upon devices with the greatest power orenergy and tightest beam divergence. The hazard analysisshould incorporate the worst-case parameters from severaldevices.

The radiometer and laser should be checked for properoperation before and after the laser measurement. A calibrationcheck may be especially important if the radiometer werehandled roughly during transit or in the field.

IRRADIANCE and BEAM RADIANT EXPOSURE

Measurement of Irradiance and Radiant ExposureThe irradiance or beam radiant exposure should be measured

at the laser output using a 7-mm and a 5-cm to 8-cm diametercircular aperture in front of the detector and at variouslogarithmic distances away from the laser. To obtain the peakvalue of irradiance or radiant exposure in the beam, theinvestigator should manually scan the detector aperture withinthe beam for a maximum value. Be careful to control anyspecular reflection from the detector window which could bedirected to an area with unprotected personnel.

Measurements should not be performed when atmosphericturbulence is severe. A detector aperture of around 2.5-cm helpsprovide an average reading at long distances. When turbulenceis severe it may be very difficult to measure the irradiance or

radiant exposure downrange and dozens of measurements maybe required.

Field measurements should be avoided when the temper-atures are near freezing or when the humidity is high to preventproblems with the instrumentation.

Check of Field DataBefore leaving the field measurement environment it is

necessary for the investigator to perform a quick check toensure that the far-field data does follow the inverse square law.This can be accomplished by plotting the calculated values ofirradiance or beam radiant exposure versus range on log-loggraph paper. The data should follow a smooth curve with a far-field slope of approximately 2.

A calculation of beam divergence should be performed usingthe irradiance or radiant exposure data, effective beam diameter,radiant power or energy, and atmospheric attenuation coef-ficient. The divergence to lie should not be less than the dif-fraction limit, eDd for a circular aperture of diameter, d:

4:13d = 1.22(6)

OTHER EXPOSURE CONDITIONS

Diffuse ReflectionsThe potential for retinal injury from viewing a diffuse re-

flection of the beam must also be determined when performinga hazard evaluation. Such a viewing condition might occurwhen a natural object is located within the beam path at closerange to the laser. When a laser device is incapable of pro-ducing a diffuse reflection hazard to the eye, this negativefinding should be reported to prevent unwarranted fears.Likewise, if a potential diffuse reflection viewing hazard existsonly when a lens is used to focus the beam onto a diffuse target,this finding should be reported. Such information may beimportant to laser maintenance personnel who might routinelyfire the laser through a lens.

Skin ExposureThe potential for skin injury from direct exposure to the beam

must be also be evaluated. Just as for the diffuse reflection case,the evaluation should report negative findings and specialharmful situations, such as when a lens is used to focus thebeam on the skin.

Multiple WavelengthsA method has been reported for the evaluation of lasers

which emit at multiple wavelengths. This method evaluates thecombined effect from the multiple wavelengths upon eachorgan site. The techniques for evaluating multiple wavelengthlasers are beyond the scope of this guide and are not describedhere. Reference [9] contains more details.

Specular ReflectionsFor a conservative analysis specular reflections of the beam

are treated like the direct beam. Realistically, the reflected beammay be more divergent than the incident beam. References [7]and [10] contain more details.

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APPENDIX A. USEFUL CIE RADIOMETRIC UNITS 1 '2

Term Symb01

Defining equation SI Unit andabbreviation

Radiant Energy QQ•= g` edt. f

Joule (J)

Radiant EnergyDensity

W. d(2„W. - —

Joule per cubicmeter (J in- 3)

dV

Radiant Flux(Radiant Power)

(1)0,P0 -

dQ, Watt (W)•

dt

Radiant Exitance Me de. Watt per square meter (W-m -2)M. -

dA= fL.cos8y1Q

Irradiance or RadiantFlux Density (Dose

Rate in Photobiology)

E. de.E.=

Watt per squaremeter (Win')--

dA'

Radiant Intensity L de. Watt per steradian(W sr')I. -

42

Radiance' Le d240 . Watt per steradi anper square meter

(Wv sr i in-2)L. =

dil-dAvos0

Radiant Exposure(Dose in

Photobiology)

H. d(2. = E, dt

Joule per squaremeter (Jm')H. = —dA f

Radiant Efficiency`(of a source)

nn p= —n.

Pi

unitless

Optical Density' D. D. = _login (c) unitless

1. The units may be altered to refer to narrow spectral bands in which the term is preceded by the word spectral and the unit is then per wavelengthinterval and the symbol has a subscript X. For example, spectral irradiance EA, has units of W • in • ni l or more often, W • cm-2.

2. While the meter is the preferred unit of length, the centimeter is still the most commonly used unit of length for many of the above terms andthe nm or pm are most commonly used to express wavelength.

3. At the source L = al and at a receptor L — dE

dA • cos8 di/ • cos0

4. Pi electrical input power in Watts.

5. ti is the transmission.

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USEFUL CIE PHOTOMETRIC UNITS

Term Symbol Defining Equation SI Units and Abbreviations

Luminous Energy(Quantity of Light)

Q„ Q. = f4) • v&lumen-second (1m-s) or talbot

Luminous Energy Density W,,Ny, = dQ

♦

dV

talbot per cubic meter (1m.s m -3)

Luminous Flux(Luminous Power)

4), dt4)♦ = 683f. • V(1) • dAdA

lumen (hn)

Luminous Exitance MyM,‘ = dJ- = IL,•cosO•dia

dA

lumen per square meter (1mm -2)

Illuminance(luminous flux density)

E„ d.,E„. = —dA

lumen per square meter Om In -2)or lux (lx)

Luminous Intensity(candlepower)

Iv 61),IV - dill

lumen per steradian (lmsr')or candela (cd)

Luminance ' 1-s,d2,

I.s, -,, lumen per steradian per square

meter (1m sr' m2)or candela per square meter

(cd m -2)

dO •dA -cos°

Light Exposure Hv dQ, r11'' = d—A- = i E

r • dt

lux-second (lx ,․ )

Luminous Efficacy(radiation)

KK 4)„

r.. = --4.

lumen per watt Om NO)

Luminous Efficacy 2

(broad band radiation)V(*) K K= =v(*)

K. 683

unitless

Luminous Efficacy '

(of a source)n..

n, = 4,,P,

lumen per watt (lm w')

Optical Density ° D„ 13,, = -log, • T, unitless

Retinal Illuminance s E, 13, = L, • Sp troland (td) = luminance (cdin2)times the pupil area in mm2

cu 1. At the source L = and at a receptor L = dE dA • cose da • cos()

2. Km = K at 555 nm.

3. P, is the electrical input power in watts.4. "C is the transmission.

5. S, = Area of the pupil in mm 2 .

REFERENCES

1. ANSI Z136.1-1993, Safe Use of Lasers. Laser Institute ofAmerica, Orlando, Fl.

2. TB MED 524, Control of Hazards to Health from Laser Radiation,20 June 1985.

3. Title 21, Code of Federal Regulations (CFR), 1991 rev. Part 1040,Performance Standards for Light-Emitting Products.

4. MIL-STD-1425A, Safety Design Requirements for Military Lasersand Associated Support Equipment, 30 August 1991.

5. NAVSWC TR 90-665, December 1990, Laser Safety Design Re-quirement Checklist Adapted from MIL-STD 1425.

6. Marshall, W. J., and Conner, P. W., "Field Laser Hazard Cal-culations," Health Physics 52(1): 27-37, January 1987.

7. Marshall, W. J., and Van DeMerwe, W. P., "Hazardous Ranges ofLaser Beams and Their Reflections from Targets," Applied Optics25(5): 605-611, 1 March 1986.

8. Marshall, W. J., "Determining Hazard Distances from Non-Gaussian Lasers," Applied Optics 30(6): 696-698, 20 February1991.

9. Lyon, T. L., "Hazard Analysis Technique for Multiple WavelengthLasers," Health Physics, Vol 49, No. 2: pp 221-226 (1985).

10. Marshall, W. J., "Laser Reflections from Relatively Flat SpecularSurfaces," Health Physics 56(5): 753-757, May 1989.

REPRINTS are available from the author:Terry Lyon, Physicist, U.S. Army Env. Hyg. Agency,Aberdeen Proving Ground, MD 21010-5422 (410) 671-5069

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