in the Chemical laboratory Edited by NORMAN V. STEERE, School of Public Health, University o f Minnesota, Minneapolis, Minn., 55455
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XXV. Laser Laboratory Design and Personnel Protection from High Energy Lasers* LEON GOLDMAN. M . D . , Director, and PETER HORNBY, Directing Physicist, Medicol Loser loboratory Supported by the John A. Hartford Foundotion at the Children's Hospital Research Foundotion, Cincinnati, Ohio. W o r k done in port under grant # O H - 0 0 1 1 8 - 0 4 from the U.S. Public Health Service. Because of the increasing importance of all phases of laser research, i t is becoming necessary to develop laboratories which incorporate specific featues to protert personnel from laser radiation. I n s n at,tempt to develop a laser laboratory which not ouly accommodates existing systems b ~ t talso has facilities far future laser developments, one must consider what safety feat,ures are necessary for lasers presently being used and also for those now being developed. The development of high energy laser systems increa5e-s the problems of personnel protection. Although the longt,erm ellects of even low energy laser reactions in living tissue are not as yet known, many research laboratories now have high eenerxy laser systems. We have considered .as high energy lasers those with exit energies of 50 joules or more gt. wavelengths of 6943 and 10,600 A. Systems have been developed with antputs in the thousnnds of joules and with energy densities in the 100,000 joules/cm2 range. Lasers operated in the Qawitched mode can now generate power outputs as high as gigawatts. High output. CW lasers may soon provide power outputs of htlndred.; of watts. With the generation of second and fourth harmonics, laser generation in the ultraviolet range is noK. possible. With such significant energy and power outpots, it is obvious that personnel working nit,ll lasers must be ~wotecled. I n brief, the protection program centers about the f r J l a ~ i n g laboratory design nnd aperntionsl features: (1) Personnel control-eye, exposed skin, and inlmlntion. (2) Area control-avoidance of spectral refiretanre, proper ventilation, and avoidance of elertried shock. It is not a1w;tys possible to separate rigidly the personnel control and area control, hot the distinction is made to
emphasize the need for the development of special laser laboratories, rather than the use of laser equipment in any available laboratory space. The contmrtion of weh special laboratories with their m m y safety precautions has been detailed before (I, 8). Normally in a research laboratory, laser systems cannot be enclosed bemuse of the continual need for changing optical arrangements and making minor adjusb ments during test programs. Thus, to protect the personnel from direct laser beam impacts, a warning system must be incorporated which warns of the charging of the laser power supply cappacitars and allows the operating personnel sufficient time to leave the aperilbing bench and turn away and cover their eyes before the actual firing. It has been the experience of this rc9earch group that accidental premature discharge of t,he flash lamps, particolnrly a3 the flaqh lamps become well used, can occur; bherefore a sbrict rule of operation should be that all work on the laser benches st,ops with the advent of the warning lights.
Eye Protection
Leon Goldmon, M.D., ir Chcirmon of !he Deportment of Dermotalagy. Coilege oi Medicine, univerrity of Cincinnati, ond Direc. tor of Dermatology in the Children's HOI. pita! of Cincinnoti, Ohio, or well 0 5 Directol of the Medical Loser Laboratory of the Children's Horpitol Research Foundotion, He it oiro a con$ultont to Occupationo' Heoith, US. Public Health Service, and w m Co-Choirmon of the Conference on the Lase! of the New York Academy of Science*, Moy 1964, ond of the Gordon Research Con. ference in Larerr in Medicine ond Biology held ot Andover, New Hampshire, in AuguS 1965. Dr. Galdmon is the author of "Loser Come! Research." in press, D b e published by Springer-Veriog, and of o chapter on lore! research in "Progress in Clinical Cancer.' He i. the author of "Biomedical A~dicotion! .. of the Lamr." now in preparation.
The first concern in laser prot,ection continues to be the eye. Full protection of the eyes means shielding not only from impact of the direct beam but also from the significant amount of reflection from surfaces. Eye protection is divided essentially into the following phases: (1) The design of laser systems to develop as much ns possible t,he closed svstem teehnioue. (2) The avoidance, a? f a as possible, of highly refleetant surfaces on and adjacent to the target areas. (3) Personnel protection in the farm of protective devices for the eyes. (4) Constant reappralral of all proposals for area and personnel protection. (5) Continued eye examinations and records for all o ~ o r a t ~ ocrsonnel. ne The latter provision is important especially t,o det,ermine the long-term eff'ects of exposure. Since laser instrumentation production is still not assembly-line, investigative models will continue to be used, both in the research laboratory and in pilot (Continued n page ASS#)
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'Portions of this article dealing primarily with laboratory design are reprinted with permission from Archives of Envimnmental H~alth, 10, 493 (1965), copyright 1965 by the American hledical Association, and the major portion is reprinted x,ith permission from the American Indusirial Hygiene Association Journal, 26, 553 (1965).
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expenments, even in the field of biomedical research. Many of these instruments will he open with the laser beam freely oxposed on its course from the lascr head to the target area. Area control means the development of dark, dull non-reflecting surfaces in the labmatory, the use of extensive heavy black felt drapes, and the use of devices to attempt to focus on the target area and t o observe the effects on the target area without direct observt~tionof the target. These devices inehde reflectant focusing devices with diffusing screens and remote closed circuit television viewing.
There artre a large number of parameters involved in determining what the energy thresholds would he for causing retinal burns. Color of the retina, wavelength of incident light, degree of beam collimation, acceptance area of the eye (dark or light oriented), condition of the lens (i.e., how well it brings the laser beam to focus on the retina), energy density of the incident beam, and specific absorption characteristics of the eye other than the retina are but some of the variables involved. A generalized formula for determining the threshold energy for retinal damage, whieh incorporates most of the parameters, has been developed by Solon (3). One can utilize this formula to develop threshold conditions for special
cases. Rather than develop this formula, we will concentrate more on utilieing actual data collected a t this Laboratory on threshold energies and then determine under the worst conditions that could occur what aualitv of filter would be required to give adequate protection to the personnel. Personnel protection for the eye requires a goggle which has sufficient protective material and whieh is so fitted that stray light exnnot come in from any angle. This is especially impartsnt for high energy laser work. This means the development essentially of s. welder's type of goggle with an efficient filtering glass. We still recommend Schott BG-18 glass with an optical jensity of 9 a t 6943, 10,600 and 6328 A (He-Ne gas laser) (4). I t is difficult to make a general statement on what constitutes the maximum incident energy density permissible an the glass. However, we w e the figure of 100 joules/cm* for our glasses because a t that value cracking and cratering become consistently evident. However, the glasses should he designed to give one-shot protection in the event of an accidental direct beam exposure. Swope and Koester (6) have recommended the use of a second filter with a lower absorption coefficient (Schott BG-38 glass) in front of the Sehatt BG-18 glass to present a higher threshold for crazing and for breakage of the filter. I n our work with argon gas lasers of 3 to 4 watts ootput, we have used amber colored plastic sheets to protect against the int,ense fluorescence developed a t t,he target area in tissue. These shields were recommended and are used by the argon gas laser research group a t Bell Telephone Laboratories. Protection is d s o necessary against the ultra, violet radiation that is thrown off from the sides of the tnhe. The helium-neon gas laser output a t 6,328 & . is effectively filtered using the BG-18 glass. However, this and other ga? lasers, as well as junction diode la9er8, generate many wavelengths in the infrared whieh, because of their fairly Low outputs, do not cause any strong sensation when absorbed directly into the eye. Sufficiently large doses, however, can cause irreparable damage to the eye; as much caution should he exercised with these lasers as with the high energy lasers. I t is imperative that personnel do not look down the barrel of any laser when the laser is operating or, in the case of a high energy laser, when there is a charge on the power supply capacitors.
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sudjected to h e r irradistian, and t h e n surveyed far damage. Protection may be studied also by measoring experiments with various photosensitive devices. Schlickmsn and Kingston (6) have developed a dosimeter to measure the energy of the reflected laser pulse. This is used in an attempt to prevent eye damage by indicating radiation data which has reached the point where it can cause damage to the eye. Calibration is done by experiment on the eyes of rabbits. Finally. it mav be done in the least desirabie fashion by the evaluation of the operating personnel after use of the pro-
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tective glawen. The evah~ation h y the operating personnel of the prot,ertive glasses means that, there shodd he detailed eye examinslions by xu ophthalmologist., preferably olte who is familiar with laser technology. This eye examiunt,ion should include a detailed examinetion and a fttndus pirtnre so that a record may be available. Infrared color photography with special film from the Esstman Kodsk Company (Kodak Ektsehmme Infrsred Aero, film, T v ~ e8443) (7) has been used bv u s to st& reflectance from large1 areas with h e r impact. This type of photogmphy done in s. dwk room using a wide-angle lens can show the detailed pattern of reflectance on personnel, inchding pratective glasses and exposed skin. M m y mmors havo been heard nbout types of corneal opacities or mtaritets caused by high energy and high peak power Q-swilching. We have unt seen any as yet or confirmed reports or ahserved these in e.xperiment,alanimals. For biomedical purposes in dirert impact.? abont the face of patients, we have used extra black cloth awerings for the eyes, in addition t o the protective goggles. A double felt curtain withstands up to 100 jonles/cml from a direct impact withe$ appreriable t,ransmission a t 6943 A (ruby wavelength). However, such !nitberial transmits on the a r d y of In'?, of the incident light tLt 10,600 A according t o transmittance curves done recently by Buckley. This s h o d d be considered in prot,eetion programs with high energy neodymium layers. I t is the experience of the patients who have had finch irnpaels t,hat some reddish light is st,ill perceived, as i t is with the protective glasses alune by refraction of the light a? it, passes into the tissue). I t is recommended that a3 a rule personnel not work in the dark since the difference in eye sreeptsnce area between being in thedark and in thelight is 16:l. I n summary, then, for eye protertion this program shodd be armslsnt, and a part of (,he personnel protertion in all its phases. Even with low euergy lasers and with gnn lasers, the quostiou irf eye protection mnst he considered and any programs must be continually rsevaluated in the light of the eont,inrdng studies on Lhe immediate and delayed reactions of the eye to laser radiation. Related eye protection is the transmission of light to the eye or orbit by soft tissue, the transilluminat,ing effect v i t h h e r s . This is importaut in sreidents which may occur from impacts about bhe fare, such as with premature firing of the flash tubes, et,c., and in twrtt,ment impacts of personnel rtboni, the heed and neck of a. patient, including laser dental surgery. We have attempted to study this in a patient by measures to avoid surface reflectance and, as mentinned ahove, wit,h detailed photography, especially with the new Kodak colored infrared film. Black cloths and vrtrions other materials are used to attempt to protort the eyes (Calinued on page A338)
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and the face from the reflected beam and the photographs are taken of the impact in darkness. The ~hotographs of the protective areas are taken before, during and after impects with either Kodarhrome, Ektachrome, or the new experiment,al Kodak infrared odored film. Exposure conditions for this film have been determined by Buckley (7). I n brief, in exposures about the forehead and the buccal cavity for laser dentistry, it appears that there is transilluminstion of the sinuses and also of the soft.tissues. . As'yet;no permanent sequelae can he recognized. This is an important study and should be done before the biomedical applieatirms of the laser become significantly more widespread and espeoidly before the laser is used for actual transilluminationprocedures.
Skin Protection Another part of the concern in personnel protection with the high energy h e r is the exposure of the skin. The direct impact of high energy lasers may cause considerable damage to the skin, especially where it is pigmented. Recently, studies have been done to measure this reflectance by s rapid scanning spectrophotometer. The skin hm been checked in visible light and with examination under Wood's filter to detect any early pigmentation, dryness, or scaling. There is, as yet, little data known of repeated exposures of exposed areas from reflectance. As yet, in our Laboratory, with exposure periods varying up to more than two years, no changes from chronic exposure to the skin have been found. I n one research worker (8),an individual with a hyper-reactive atopic skin, deliberate attempts were made to sensitize this individual. At present, even low energy densities in the nature of 0.23 joules of exit energy and 52.3 joules/cm2 energy density in a target area of 0.004 cm' produce significant reactions in the skin. Where patients are receiving impacts with the laser, the personnel in the laboratory who will be called on to assist in positioning the patient and will be exposed to numerous impacts, should attempt to keep their hands and face protected. Black felt or leather gloves or felt coverings can be used on the hands. The face should he turned away from the target area. As indicated above, pictures of the impact in a dark room with infrared colored film reveal the extent of meas of exposure about the target zone. Only with long-term studies will it be possible to determine what the chronic radiation effects artre on the exposed skin. I n high energy laser treatments of skin malignancies, the skin around the target area has been protected by single or posslble to determme what the chronlc radiation effects are on the exposed skin. I n high energy laser treatments of skin
6bdio snoirra &e- &a. --mpica? i t p i j n m k tions of dye mixtures are also under study far protection of the skin. The increesed absorption of the laser beam in blood means also that direct
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impacts should he avoided on the superficial blood vessels in the skin of laboratory personnel. Detailed hematologic ex* minations in our laboratories have failed to reveal any anemis. in laser personnel over a. period of two years.
Air Pollution Air contamination is also a problem, espeeidly as it relates to the use of liquid nitrogen coolants and dry nitrogen purges. The concentration of nitrogen vapor could become significant in confined spaces with a consequent reduction of oxygen concentration. Preliminary measurements in our Laboratory by the Occupational Health Field Headquarters of the Public Health Service have shown no low oxygen levels. The increasing use of water-cooled lasers in preference to t,he nitrogen-cooled types may reduce the frequency of the use of nitrogen. Liquid nitrogen also produces burns when handled in a careless manner. I n our experiments with peak power outputs of 100 to 150 megawatts, the search by our radiation physicist far x-ray generationfromvarious targets, bath metallic and non-metallic, failed to elicit any evidence of this, although such generation has been reported to us. The methods of our analysis were crude far this type of technique, i.e. essentially a Geiger counter and the use of X-ray sensitive film. Recently, in much more significant experiments, Schwartz ( 9 ) has reported no radiation traces from peak power outputs of 20 to 30 megawatts in a. cloud chamber. However, this report did not analyze the interaction with metals. Ozone is produced a t times about the flash lamps and concentrations of this could build up with high repetition rat,e lasers. Routine chest films of the staff of the Laser Lahoratory have shown no findings.
Electrical Shock I n the research laboratory, the possihilities of electrical shock are great when space is a t a minimum. Some high energy systems may require upwards of 50 capacitors. The energy stored a t high voltage in these capacitors when dissipated through a human conductor will cause severe shock and massive This caution applies thermal burns. also to the cavities of some lasers which are maintained a t the high voltage side of thesystem. Shielding of the capacitors should he mandatory, not only to protect the personnel from accidental contact with the capacitors hut also to protect them from the possibility of capacitor explosion which in its effect resembles a small hand grenade exploding. Often laboratory models of lasers have open electrical circuits which are hazards (1). I n the finished oroducts. all electrical circuits 5hoold he housd' in p
