Infrared Radiation Transmittance and Pilot Vision Through Civilian Aircraft WindscreensVan B. NakagawaraRonald W. MontgomeryCivil Aerospace Medical InstituteOklahoma City, OK 73125

Wesley J. MarshallU.S. Army Center for Health Promotion and Preventive MedicineAberdeen Proving Grounds, MD 21010

June 2008

Final Report

DOT/FAA/AM-08/15Office of Aerospace MedicineWashington, DC 20591

OK-08-1997

NOTICE

This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest

of information exchange. The United States Government assumes no liability for the contents thereof.

___________

This publication and all Office of Aerospace Medicine technical reports are available in full-text from the Civil Aerospace Medical Institute’s publications Web site:

www.faa.gov/library/reports/medical/oamtechreports/index.cfm

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Technical Report Documentation Page 1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

DOT/FAA/AM-08/15 4. Title and Subtitle 5. Report Date

June 2008 Infrared Radiation Transmittance and Pilot Vision Through Civilian Aircraft Windscreens 6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No. Nakagawara VB,1 Montgomery RW,1 Marshall WJ2

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

11. Contract or Grant No.

1FAA Civil Aerospace Medical Institute P.O. Box 25082 Oklahoma City, OK 73125

2U.S. Army Center for Health Promotion and Preventive Medicine Aberdeen Proving Grounds, MD 21010

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered

Office of Aerospace Medicine Federal Aviation Administration 800 Independence Ave., S.W. Washington, DC 20591 14. Sponsoring Agency Code 15. Supplemental Notes

16. Abstract INTRODUCTION: In support of a Department of Homeland Security project, the Federal Aviation Administration´s Civil Aerospace Medical Institute measured the optical transmittance properties of aircraft windscreens. This paper focuses on windscreen transmittance in the infrared (IR) spectral region (780 – 4000 nm) of the electromagnetic spectrum. METHOD: Transmission measurements were performed on eight aircraft windscreens. Three windscreens were from large commercial jets (MD 88, Airbus A320, and Boeing 727/737); two from commercial, propeller-driven passenger planes (Fokker 27 and the ATR 42); one from a small private jet (Raytheon Aircraft Corporation Hawker Horizon); and two from small general aviation (GA), single-engine, propeller-driven planes (Beech Bonanza and Cessna 182). The two GA aircraft windscreens were plastic (polycarbonate); the others were multilayer (laminated) composite glass. RESULTS: The average transmittance for both glass laminate and plastic windscreens in the IR-A region (780 – 1400 nm) varied considerably (47.5% ± 11.7%), with glass windscreens consistently attenuating more IR than plastic windscreens. The average difference in transmittance between the two materials fluctuated (27.3% ± 15.9%) throughout the first half of the IR-B spectrum (1400 – 3000 nm) up to approximately 2200 nm when transmittance dropped below 7%. The average transmittance for glass and plastic windscreens became negligible beyond 2800 nm. CONCLUSION: Aircraft windscreens provide a level of protection from potential ocular and skin hazards due to prolonged or intense exposure to IR radiation. The amount of protection is dependent on the type of windscreen material, the wavelength of the radiation, and angle of incidence. On average, laminated glass windscreens attenuate more IR than plastic. Additional research is recommended to confirm that the measured transmittance values for this sample of windscreens are typical of all aircraft windscreens currently in service and to evaluate the potential threat posed by new applications, such as IR lasers, in navigable airspace.

17. Key Words 18. Distribution Statement

Aviation, Laser, Radiation, Eye Health Document is available to the public through the Defense Technical Information Center, Ft. Belvior, VA 22060; and the National Technical Information Service, Springfield, VA 22161

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price Unclassified Unclassified 14

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

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ACKNOWLEDGMENTS

The C�v�l Aerospace Med�cal Inst�tute’s V�s�on Research Team w�shes to thank Dav�d Sl�ney, Pro-

gram Manager of the Laser/Opt�cal Rad�at�on Program at the U.S. Army Center for Health Promot�on

and Prevent�ve Med�c�ne, and staff phys�c�sts Terry Lyon and Stephen Wengra�t�s for the�r support

and expert�se. In add�t�on, we thank the members of C�v�l Aerospace Med�cal Inst�tute’s B�odynam�cs

Research Team for construct�ng the w�ndscreen cart used to conduct th�s �nvest�gat�on.

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CONTENTS

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

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Infrared radIatIon transmIttance and PIlot VIsIon through cIVIlIan aIrcraft WIndscreens

INTRODUCTION

An a�rcraft’s w�ndsh�eld (�.e., w�ndscreen) �s v�tal for enhanc�ng and protect�ng the p�lot’s v�s�on. The transm�ttance of a w�ndscreen mater�al can affect v�sual performance wh�le prov�d�ng protect�on from harm-ful electromagnet�c rad�at�on. Transm�ttance may be determ�ned by calculat�ng the rat�o of the total radiant or luminous flux that �s transm�tted to that wh�ch �s �nc�dent on the surface of the w�ndscreen. A h�gh rat�o �nd�cates that �nc�dent rad�at�on �s transm�tted effic�ently through the w�ndscreen, wh�le a low rat�o denotes lesser transm�ss�on.

Opt�cal rad�at�on �s defined as the part of the elec-tromagnet�c spectrum that �ncludes ultrav�olet (UV), v�s�ble, and �nfrared (IR) rad�at�on. The Comm�ss�on Internat�onalè de l’Ecla�rage (CIE) Comm�ttee on Pho-tob�ology has prov�ded spectral band des�gnat�ons that are conven�ent for d�scuss�ng b�olog�cal effects. These d�v�s�ons �n the opt�cal spectrum are �llustrated �n F�gure 1. Opt�cal rad�at�on can also be d�v�ded �nto two general reg�ons w�th respect to the�r potent�al for eye damage: the ret�nal hazard reg�on and the non-ret�nal hazard reg�on. The wavelengths of the ret�nal hazard reg�on �nclude v�s-�ble l�ght (380 – 780 nm) and near IR (780 – 1400 nm) or IR-A rad�at�on. The ret�nal hazard reg�on �dent�fies those bandw�dths that are transm�tted through the opt�cal

med�a of the eye (cornea, aqueous humor, crystall�ne lens, and v�treous humor) and focused onto the ret�na. The non-ret�nal hazard reg�on refers to wavelengths that are mostly absorbed by anter�or ocular t�ssues, w�thout s�gn�ficant transm�ss�on to the ret�na. These bandw�dths �nclude UV rad�at�on from 100 nm to 380 nm (UV-C, UV-B, and UV-A) and the IR bands w�th wavelengths greater than 1400 nm (IR-B and IR-C).

Excess�ve exposure to opt�cal rad�at�on �s a concern to �ndustr�al hyg�en�sts, safety eng�neers, and publ�c health offic�als for the�r potent�al as a hazard to health and safety. As�de from natural sources of rad�at�on, such as the sun and cosm�c background rad�at�on, many man-made sources of opt�cal rad�at�on ex�st and are becom�ng �ncreas�ngly access�ble to the general publ�c. Excess�ve exposure to these sources can also lead to adverse phys�-olog�cal consequences. Examples of these sources �nclude lasers, mercury-vapor and xenon halogen lamps, weld�ng dev�ces, and �nfrared and germ�c�dal lamps. These sources are frequently found �n office sett�ngs, water treatment plants, hosp�tals, research laborator�es, photo-etch�ng product�on l�nes, graph�c arts fac�l�t�es, mach�ne shops, tann�ng salons, and even �n homes.

Much has been wr�tten about the dangers assoc�ated w�th exposure to excess�ve levels of v�s�ble and UV rad�a-t�on �n the Nat�onal A�rspace System (NAS) (1,2,3). L�ttle has been reported concern�ng the potent�al hazards from

Figure 1. Optical radiation spectral bandwidths (CIE, 1970).

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exposure to h�gh levels of IR rad�at�on. Th�s �s l�kely due to the percept�on that there �s m�n�mal r�sk of �njury to av�ators and the fly�ng publ�c from art�fic�al and natural sources of IR rad�at�on �n the NAS. Th�s �s generally true for natural sources of IR rad�at�on, �nclud�ng the sun. Most of the sun’s ultrav�olet rad�at�on below 300 nm �s absorbed by atmospher�c ozone (O

3) and oxygen (O

2),

wh�le most of the v�s�ble and IR rad�at�on str�k�ng the earth’s atmosphere reaches the surface. The earth’s surface absorbs v�s�ble l�ght and re-em�ts much of the energy as IR rad�at�on back �nto the atmosphere (4). Certa�n gases �n the atmosphere, ch�efly water vapor (H

2O) and

carbon d�ox�de (CO2), absorb IR and re-rad�ate �t �n all

d�rect�ons (5). W�thout the atmosphere to capture ther-mal energy from v�s�ble and IR rad�at�on, the est�mated average temperature of the earth would be a fr�g�d 0º F, rather than a comfortable 59º F (6).

Under normal c�rcumstances, naturally occurr�ng at-mospher�c IR rad�at�on �s generally safe for ocular t�ssues and sk�n. Prolonged and/or repeated exposure to �ntense sources of IR, however, can result �n ret�nal, corneal, and sk�n burns, as well as IR-�nduced cataracts. For normal outdoor act�v�t�es, appropr�ate sunglasses and sunscreen �s all that �s necessary; however, w�th extreme exposure, spec�al precaut�ons are requ�red. For example, astronomers are adv�sed to never v�ew a solar ecl�pse w�thout a filter that attenuates all UV rad�at�on, 99.997% of v�s�ble l�ght, and 99.5% of IR-A (7). Glassblowers and anyone rout�nely work�ng w�th molten mater�als should take s�m�lar precau-t�ons to protect the�r eyes from excess�ve exposure to IR rad�at�on (8,9). It �s commonly accepted that commerc�al p�lots who fly at h�gh alt�tudes for prolonged per�ods of t�me should �nvest �n sunglasses that protect the�r eyes from exposure to both UV and excess�ve amounts of �ntense v�s�ble l�ght (10,11). Ongo�ng sc�ent�fic research and emerg�ng technolog�es that employ IR lasers for use �n the NAS have ra�sed concerns about the poss�b�l�ty of av�at�on personnel and the publ�c be�ng acc�dentally exposed to harmful levels of IR rad�at�on.

The Department of Homeland Secur�ty (DHS) Counter-Man Portable A�r Defense System (MANPADS) Spec�al Project Office requested that the C�v�l Aerospace Med�cal Inst�tute’s (CAMI) V�s�on Research Team evalu-ate the potent�al hazards to av�at�on personnel and the general publ�c posed by the IR rad�at�on em�tted from these Counter-MANPADS systems. The measurement of a�rcraft w�ndscreen transm�ttance was part of th�s laser safety assessment. Sc�ent�sts from the Un�ted States Army Center for Health Promot�on and Prevent�ve Med�c�ne (CHPPM), Aberdeen Prov�ng Ground, MD, were enl�sted to prov�de techn�cal support. Based on the data prov�ded by the CHPPM (12), an FAA report was publ�shed descr�b�ng w�ndscreen transm�ttance for

UV rad�at�on and v�s�ble l�ght (13). The current report documents transm�ttance propert�es of a set of a�rcraft w�ndscreens through the IR reg�on (780 – 4000 nm) of the electromagnet�c spectrum.

METHOD

Several a�rcraft w�ndscreens were sh�pped from var�ous a�rcraft ma�ntenance fac�l�t�es to CAMI’s V�s�on Research Laboratory. E�ght w�ndscreens were selected from those ava�lable for test�ng. Three were from large commerc�al jets (MD 88, A�rbus A320, and Boe�ng 727/737), one was from a smaller pr�vate jet (Raytheon A�rcraft Corporat�on Hawker Hor�zon), two were from commerc�al, propeller-dr�ven passenger planes (Fokker 27 and the ATR 42), and two were from smaller, s�ngle-eng�ne propeller general av�at�on planes (Beech Bonanza and the Cessna 182). The Beech and Cessna w�ndscreens were full w�ndsh�elds and made of a s�ngle-layer polycarbonate mater�al, rather than the lam�nated glass that compr�sed the other s�x.

Instrumentat�on for test�ng �ncluded:

1) EG&G model 580 spectro-rad�ometer systems (w�th UV, v�s�ble, and IR grat�ngs and hous�ngs); sort�ng filters; models 580-22A, 580-23A, and 580-25A photod�ode detectors; and Palentron�c AR582F �nd�cator un�t.

2) Internat�onal L�ght Model 1700 rad�ometer w�th SED 623 broadband detector.

3) Oph�r LaserStar rad�ometer system, w�th model 3A-P-SH thermop�le detector.

4) Narrow pass filters: 1450 nm, 1540 nm, 1940 nm, 2050 nm, 2100 nm, 2200 nm, 2300 nm, and 2380 nm.

5) Long pass filters: 1600 nm and 2500 nm.6) Sapph�re w�ndow: transm�ss�on from UV to 4000

nm.7) L�ght sources: deuter�um lamp, 100-watt �ncan-

descent l�ght, 250-watt heat lamp.8) L�ght box and a�rcraft w�ndscreen cart.9) M�scellaneous laboratory mounts, filters, filter

holders, and equ�pment.10) Perk�n Elmer UV/VIS/NIR model Lambda 900

spectrometer system.11) Cold m�rror and extended-range hot m�rror.

Transm�ss�on measurements on the var�ous w�ndscreens were performed �n a sem�-darkened room. Two large tables were used: one for the l�ght sources and the other to place the var�ous opt�cal detectors. A custom-made w�ndscreen cart (F�gure 2) was used to sl�de the w�ndscreens �n and out of the beam path between the two tables separat�ng the l�ght sources and detectors.

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Three monochromator systems were placed s�de-by-s�de and al�gned w�th the appropr�ate l�ght source, wh�ch was placed �n a metal enclosure (F�gure 3). For each w�ndscreen and each spectral reg�on, a basel�ne measure-ment was made w�th the w�ndscreen moved to one s�de and then repeated w�th the w�ndscreen placed between the l�ght source and the detector. To confirm the data collected by the EG&G spectro-rad�ometers, the two polycarbonate (plast�c) w�ndscreens were cut and tested �n a Lambda 900 spectrometer, result�ng �n good agree-ment. An attempt was made to cut a sample from the compos�te glass w�ndscreens, but craz�ng of the sample made transm�ss�on measurements �mposs�ble.

For v�s�ble and near-�nfrared transm�ss�on (400 – 1250 nm) measurements, an ord�nary �ncandescent 100-watt l�ght bulb was suffic�ent for an �llum�nat�on source due to the h�gh transm�ss�on of the w�ndscreens for v�s�ble and near-IR rad�at�on. For wavelengths farther �n the �nfrared, a 250-watt heat lamp was used for an �llum�na-t�on source. The EG&G Model 580-22A detector was used for measurements �n the spectral reg�on (400 – 800 nm), and the Model 580-23A detector was used �n the

spectral reg�on (800 – 1400 nm). For wavelengths longer than 1400 nm, a sapph�re w�ndow and a ser�es of narrow pass filters were employed w�th a thermop�le detector. Narrow pass filters were not ava�lable from 1550 nm to 1950 nm. Due to heat�ng of components �n the opt�cal l�ne of s�ght between the source and detector, not all background rad�at�on could be el�m�nated. Therefore, the reported results for very low values of transm�ss�on, espec�ally those for wavelengths greater than 2100 nm, reflect a max�mum transm�ss�on, rather than an actual transm�ss�on measurement.

Due to the we�ght of the w�ndscreens and the need to repos�t�on the detectors and l�ght source for var�ous spectral reg�ons, a s�ngle basel�ne was not pract�cal. There-fore, a new basel�ne was usually created for each set of measurement cond�t�ons for each w�ndscreen. Measur�ng two of the polycarbonate w�ndscreens under both field and laboratory cond�t�ons served to val�date the measurement method used on-s�te for all w�ndscreens measurements and also to �dent�fy potent�al problem areas.

RESULTS

The transm�ttance values for �nd�v�dual glass lam�nate w�ndscreens are summar�zed �n F�gure 4 and those for the two plast�c w�ndscreens �n F�gure 5.

The average transm�ttance data for both glass lam�nate and plast�c w�ndscreens are plotted �n F�gure 6. In the IR-A spectral reg�on (780 – 1400 nm), average transm�ttance of the two mater�als var�ed (average d�fference = 47.5% ± 11.7%), w�th glass w�ndscreens cons�stently attenuat-�ng more IR rad�at�on than the�r plast�c counterparts. S�m�larly, the average d�fference �n transm�ttance fluctu-ated (27.3% ± 15.9%) throughout the first half of the IR-B spectrum (1400 – 3000 nm) up to approx�mately 2200 nm, where the transm�ttance for both mater�als

Figure 2. Custom-made windscreen cart for manipulating aircraft windscreens.

Figure 3. Detector and light source configuration.

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Figure 4. Transmittance of individual glass windscreens.

Figure 5. Transmittance of individual plastic windscreens.

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dropped below 7%. The average transm�ttance for both glass and plast�c w�ndscreens gradually decreased unt�l �t became v�rtually �mmeasurable from 2800 nm through 4000 nm.

DISCUSSION

S�nce September 11, 2001, many analysts see the use of MANPADS as a terror�st’s weapon of cho�ce for str�k�ng U.S. a�rl�nes, espec�ally as �mprovements �n a�rport secur�ty reduce the chances for smuggl�ng explos�ves or �ncend�-ary dev�ces onto planes. There are as many as 500,000 shoulder-fired m�ss�les �n m�l�tary arsenals around the world and from 5,000 to 150,000 �n the hands of up to 30 non-state organ�zat�ons, accord�ng to a report by the Congress�onal Research Serv�ce (CRS) (14). An analys�s by the CRS �nd�cates that, �n the last 26 years, MANPADS have been used �n attacks on 35 c�v�l�an a�rcraft, of wh�ch 24 were shot down, k�ll�ng more than 500 people. Most of these �nc�dents took place �n war zones, pr�nc�pally Afr�ca, Sr� Lanka, and Afghan�stan (15).

M�l�tary a�rcraft have been equ�pped w�th counter-measures that draw off a m�ss�le’s gu�dance system as �t attempts to lock onto the heat s�gnature of the a�rcraft’s eng�nes by deploy�ng flares from under the a�rcraft. After an Israel� passenger jet surv�ved an attempt by al-Qaeda to shoot �t down over Kenya �n 2002, El Al Israel A�rl�nes �nstalled flare-based Counter-MANPAD systems on all �ts a�rcraft (16). But the fear of collateral damage from fires, should the flares be deployed by m�stake, makes

th�s solut�on less than opt�mal for most c�v�l av�at�on author�t�es, �nclud�ng those �n the Un�ted States.

The Counter-MANPADS program, as �t �s known, began �n January 2004 when DHS selected three teams from a field of 24 to compete �n a study to determ�ne how �nfrared counter measures (IRCM) could be adapted for use on large c�v�l�an transport a�rplanes. The systems currently under development for U.S. carr�ers are housed �n pods mounted on the unders�de of an a�rcraft’s fuselage and employ IRCM to d�srupt the gu�dance systems of surface-to-a�r m�ss�les (F�gure 7).

The IRCM system uses mult�ple-wavelength lasers that em�t rad�at�on �n the �nfrared port�on of the electromag-net�c spectrum. Although the redundant safeguards and the relat�vely long exposure t�mes that would be neces-sary to �nfl�ct �njury reduce the r�sk to p�lots, a�r traffic controllers, ground crews, and the publ�c, IR em�ss�ons from these laser systems can be hazardous to ocular t�s-sues and sk�n under certa�n c�rcumstances. The type of damage an IR laser may cause depends on several factors, �nclud�ng the energy del�vered per un�t area, durat�on of exposure, and wavelength. W�th�n the Nom�nal Ocular Hazard D�stance, near-�nfrared (IR-A) laser rad�at�on may damage the ret�na, wh�le m�ddle- and far-�nfrared laser rad�at�ons (>1,400 nm) can �njure the cornea and, to a lesser extent, the crystall�ne lens, prov�ded the appl�cable max�mum perm�ss�ble exposure (MPE) l�m�t �s exceeded. Damage to t�ssue from laser rad�at�on �s usually due to heat�ng (thermal effects); however, photochem�cal �njury may also occur. Most photochem�cal effects are l�m�ted

Figure 6. Average transmittance for glass and plastic windscreens.

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to shorter wavelengths; whereas, thermal effects can oc-cur at any wavelength �n the opt�cal rad�at�on spectrum. A summary of the adverse b�olog�cal effects that can result from excess�ve exposure to var�ous wavelengths of rad�at�on �s �llustrated �n F�gure 8. Only a very small percentage of rad�at�on reaches the ret�na beyond 1,400 nm, due to absorpt�on by t�ssues �n the anter�or port�on of the eye. M�d-�nfrared rad�at�on (IR-B) may penetrate anter�or t�ssues of the eye deep �nto the crystall�ne lens, caus�ng corneal burns and �nfrared cataracts. Far-�nfrared rad�at�on (IR-C) �s pr�mar�ly absorbed by the cornea and can result �n corneal burns and blurred v�s�on. The major danger to the sk�n from lasers operat�ng �n the IR reg�on of the spectrum �s thermal burns. Other poss�ble �njur�es �nclude erythema and rad�ant heat stress. In some cases, symptoms may appear 6 – 12 hours after exposure and d�sappear gradually after 24 – 36 hours, leav�ng no permanent damage (8).

The present study found that w�ndscreens can prov�de vary�ng degrees of protect�on from IR rad�at�on exposure depend�ng on the type of w�ndscreen mater�al and wave-length of the rad�at�on. Plast�c mater�al transm�tted up to 40% more rad�at�on at 780 nm. In other words, glass lam�nate w�ndscreens blocked between 20 and 60% more IR rad�at�on than plast�c from 780 nm through 2,100 nm. Above 2,200 nm, both glass and plast�c w�ndscreens reduced IR transm�ss�on to 7% or less, and the d�ffer-ence became pract�cally �mmeasurable from 2,800 nm through 4,000 nm.

The reduced transm�ss�on of w�ndscreen mater�als at certa�n wavelengths can help to protect a p�lot from ocular t�ssue damage. Opt�cal dens�ty (OD), wh�ch can be calculated from transm�ttance (T) (�.e., OD = Log

10(1/T)), �s the capac�ty of an opt�cal element to

absorb (attenuate) rad�at�on of a g�ven wavelength. The opt�cal dens�ty for the average glass (OD

glass) and

Figure 7. Conceptual illustration of a civilian Counter-MAN-PADS deploying (invisible) IRCM to disrupt the guidance systems of surface-to-air missiles.

Figure 8. Potential adverse biological effects from excessive exposure to optical radiation Sliney & Wolbarsht, 1980).

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plast�c (ODplast�c

) w�ndscreens are presented �n F�gure 9. For a part�cular wavelength, a h�gh transm�ttance value results �n a low OD. Conversely, an opaque mater�al w�th a transm�ttance near 0% would have a very h�gh OD. The w�ndscreens exam�ned �n th�s study exh�b�ted IR transm�ttance between 1.3 and 91.5%, result�ng �n OD values that ranged from 1.9 to 0.04, respect�vely, w�th lam�nated glass w�ndscreens attenuat�ng more IR rad�at�on than plast�c w�ndscreens. In F�gure 9, two 6th-order polynom�al approx�mat�ons were ut�l�zed to fill the gaps between the spec�fic wavelengths and bandw�dths measured us�ng the var�ous �nstruments and filters. It can be �nferred that the OD values for wavelengths between 3,000 nm and 4,000 nm are at least 1.9 and 1.6 for glass and plast�c, respect�vely.

Table 1 compares several IR lasers of d�fferent wavelengths. All lasers l�sted are repet�t�ve pulsed w�th frequenc�es of 15 Hz, average power output of 300 m�l-l�watts (mW), d�vergence of 1 m�ll�rad�an (mrad), energy

per pulse of 20 m�ll�joules (mJ), and pulse w�dth of 12 nanoseconds (ns). The last four columns conta�n the Nom�nal Ocular Hazard D�stance (NOHD), Nom�nal Sk�n Hazard D�stance (NSHD), and OD values for ocular t�ssues (OD

eye) and sk�n (OD

sk�n) for these lasers. These

results were calculated us�ng the LHAZ V4.0 software, developed by The A�r Force Research Laboratory, Opt�cal Rad�at�on Branch, at Brooks AFB, TX. The NOHD and NSHD are the m�n�mum d�stances that must be ma�n-ta�ned from the laser source to avo�d exceed�ng the MPE, respect�vely, for an exposure of up to 10 s. For example, a Nd:YAG (1064 nm) laser w�th the output parameters l�sted below must be at a d�stance of at least 2,610 m to avo�d ocular t�ssue damage from a 10-s exposure (w�th atmospher�c attenuat�on �n clear a�r accounted for). Sk�n damage from th�s laser would be avo�ded beyond 5.2 m. Laser eye protect�on w�th an OD of 4.6 �s requ�red for ocular protect�on w�th�n the NOHD, wh�le an OD of only 0.5 �s requ�red w�th�n the NSHD.

Figure 9. Mean OD values for glass laminate and polycarbonate plastic wind-screens.

Table 1: Optical density values and hazard distances for selected IR lasers.

Laser Type Wavelength (nm)

MPE(µJ/cm2)

NOHD (m)

NSHD (m) ODeye ODskin

GaAs 840 0.2722 5,730 16.6 5.28 0.91Nd:YAG 1064 1.429 2,610 5.2 4.56 0.49 Nd:YAG 1330 11.43 938 5.2 3.66 0.49Cr2+:CdSe 2600 2,857 58.8 37.5 2.95 1.50 HeNe 3390 2,857 58.8 37.5 2.95 1.50CO2 10600 2,857 58.8 37.5 2.95 1.50

8

Table 2 �llustrates the reduct�on �n both NOHDs and NSHDs for the s�x selected lasers when the average OD values for glass and plast�c w�ndscreens were appl�ed to the hazard d�stances �n Table 1. Note that attenuat�on of IR rad�at�on by plast�c w�ndscreens reduced the NO-HDs of the three lasers operat�ng �n the ret�nal hazard reg�on (< 1,400 nm) by 4 to 28%, wh�le glass reduced these d�stances by 35 to 63%. Both glass and plast�c w�ndscreens prov�de suffic�ent attenuat�on to el�m�nate any ocular and sk�n hazards for the example lasers oper-at�ng �n the IR-B and IR-C reg�ons (> 1,400 nm) of the electromagnet�c spectrum.

Av�at�on personnel who wear correct�ve spectacle lenses may add a small amount of m�d-IR attenuat�on to that prov�ded by w�ndscreens. The transm�ttance data

for clear ophthalm�c lens mater�als are plotted �n F�gure 10 (17). Note that both glass and plast�c lens mater�als transm�t approx�mately 90% of IR rad�at�on from 780 nm to 1,100 nm. Both crown glass and h�gh-�ndex (1.60) glass lenses cont�nue to transm�t h�gh levels of IR (> 83%) through 2,530 nm. Plast�c lenses (CR-39®, MR-6, and polycarbonate) transm�t 80% to 10% of the IR rad�at�on from 1,350 nm to 2,300 nm, respect�vely. Ne�ther glass nor plast�c ophthalm�c lens mater�als prov�de adequate protect�on from IR rad�at�on exposure �n the ret�nal hazard reg�on unless spec�al treatments are added to the lens mater�al.

Sunglass lenses des�gned for “general-purpose” use, made from ophthalm�c glass w�th gray, green or tan t�nts, prov�de h�gh IR attenuat�on throughout the ret�nal hazard

Table 2: The nominal hazard distances considering transmission losses through glass and plastic windscreens.

NOHD NSHD LaserType Plastic

(m) Change

%Glass

(m) Change

%Plastic

(m) Change

%Glass

(m) Change

%GaAs 5,490 4 3,740 35 15.5 7 6.1 63Nd:YAG 2,470 5 1,520 42 3.1 40 0 100 Nd:YAG 673 28 346 63 0 100 0 100Cr2+:CdSe 0 100 0 100 0 100 0 100 HeNe 0 100 0 100 0 100 0 100CO2 0 100 0 100 0 100 0 100

Figure 10. Transmittance of selected glass and plastic ophthalmic lens materials (adapted from: Torgersen, 1998).

9

reg�on (17). These lenses also meet ANSI Z80.3 - 2001 m�n�mum standards for transm�ttance of v�s�ble l�ght and s�gnal l�ght recogn�t�on. Average IR transm�ttance, as defined by ANSI Z87.1 – 1989 (780 – 2000 nm), for the darker t�nts are 25.4% for tan and 6.1% for both gray and green. Some manufacturers offer �nfrared protect�on �n “spec�al-purpose” des�gns (�.e., sports, safety, weld�ng, etc.) ava�lable under var�ous trade names. Many of these lenses, however, are expens�ve and d�fficult for the general publ�c to obta�n.

In summary, the study results �nd�cate that a�rcraft w�ndscreens prov�de some protect�on from exposure to IR rad�at�on. The amount of protect�on afforded by a w�ndscreen �s dependent on the type of mater�al and the wavelength of the rad�at�on. Generally speak�ng, lam�nated glass w�ndscreens attenuate more IR rad�a-t�on than plast�c w�ndscreens. Although normal levels of most naturally occurr�ng atmospher�c IR rad�at�on exposure pose no ser�ous threat, p�lots may w�sh to take added precaut�ons to avo�d prolonged exposure to exces-s�ve levels of IR rad�at�on. The opt�cal dens�t�es of glass lam�nate w�ndscreens can substant�ally reduce th�s r�sk. A lam�nated glass w�ndscreen and appropr�ate sunglass lenses afford good protect�on from excess�ve exposure to naturally occurr�ng v�s�ble l�ght and IR rad�at�on. Add�t�onal research �s recommended to confirm that the measured transm�ttance values for th�s small sample of w�ndscreens are typ�cal of all w�ndscreens currently �n serv�ce. F�nally, as appl�cat�ons for lasers that could be harmful to av�at�on personnel or passengers �ncrease, more research may be needed to assess the potent�al hazards assoc�ated w�th the�r use and determ�ne how best to m�t�gate the�r �mpact on av�at�on safety.

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