Laser Safety (1975)

[Music] the word laser triggers fears about death rays disintegrators and other beam transported catastrophes but what we need is truth about lasers not science fiction we need to replace unreasoning fear with informed caution so that the full capabilities of one of man's most remarkable inventions can be unleashed in support of the army's mission [Music] more and more weapon systems that incorporate laser functions are being introduced yearly on land and in the air when certain controls are established and certain precautions are observed lasers can be used without undue constraint and at minimal risk to personnel so that rational judgments and decisions concerning laser operations can be made in test maintenance training and operational environments we'll first characterize the nature of laser light define the actual hazards that are of concern and then review the controls and precautions that should be established for the safety of all those in a position to be exposed to laser radiation the laser's capabilities can help multiply the army's tactical effectiveness in the field to help achieve that goal let's now trade silly fiction for hard facts the facts you'll need to use the laser wisely and safely in the service of the nation [Music] you can think of a laser basically as a light source but it produces light so different from ordinary light that it's beyond most people's experience the light we're used to comes from hot bodies like the sun incandescent light electric arcs or electric currents passing through a gas and exciting the gas itself or a coating inside the tube to emit light as an electromagnetic phenomenon light is energy and is capable of doing work powering a satellite or a light meter activating the growth cycle in living tanning the skin starting the process we know as vision by energizing the photoreceptors in the retina of our eyes the eyes serve as detectors for the energy in that portion of the electromagnetic spectrum we label as visible light we can't detect ultraviolet or infrared wavelengths because the eye is opaque in these regions but it's the ultraviolet energy from the sun that tans or burns us and from the welder's ark that can damage our eyes and the infrared from the heat lamp that sears us if we fail to take proper precautions so in this sense there is some hazard from ordinary light and adjacent parts of the spectrum but it is a fair indication of the extraordinary properties of laser light that for billionths of a second it can achieve intensities many times greater than the sun and is therefore potentially even more hazardous in the absence of suitable safeguards and precautions ordinary light energy is spread over spectral bands but lasers concentrate their energies at single wavelengths and so are called monochromatic or single color devices these characteristics high intensity and monochromaticity together with minimum beam divergence and coherence account for the hazardous nature of laser radiation these properties arise from the way in which the laser's energy is produced light of any kind is produced as electrons are first excited by incoming energy and as they then de-excite themselves giving off or radiating energy in the process the tiny packet of energy involved in both ends of this process is called a photon the electron can remain in the excited state for a portion of a second before it gives up the energy it has received this is called spontaneous emission if however another photon arrives while the first is still excited the photon is triggered or stimulated to de-excite itself instantaneously producing a second photon that has the same energy wavelength phase and direction of the incident photon the incoming photon is not absorbed however and so there are now two this process is known as stimulated emission under ordinary conditions the greatest numbers of electrons are in the ground or unexcited state and only a small percentage are involved in the spontaneous emission of light before stimulated emission can occur the population must be inverted that is the highest percentage of electrons must be in the excited state this is accomplished by supplying a massive input of energy which in effect pumps enough electrons to the excited state so that the highest number of electrons is in the excited state this condition favors stimulated emission and results in energy being released this energy is radiated randomly in all directions in the laser however mirrors are added at each end of what now becomes an optical cavity and they immediately establish a preferred direction for the radiation release by the stimulated emission a photon cascade now develops along the axis between the mirrors as the level of energy being produced avalanches upward this is light amplification in the laser one mirror is usually partially transparent and so a large percentage of the amplified light passes through to create the laser beam but the remainder is reflected back into the cavity to start another cascade fully reflected by the other mirror the cascade traverses the cavity again constantly building in intensity until it too results in a brilliant pulse of light light amplification continues as long as the population inversion lasts the entire process is called lazing after light amplification by stimulated emission of radiation excitation can be achieved by electrical discharge and this can be continuous as in the case of continuous wave gas lasers or this excitation or pumping can be achieved by pulses of high intensity light from flash tubes or other lasers however it is performed the excitation and de-excitation of electrons is dependent upon discrete energies and frequencies and the output of the laser is directly related to the frequency at which the excitation occurs in the ruby laser for example the output wavelength is exactly 694.39 which is red in color it is important to understand that almost all of the power output of the laser is concentrated at this single wavelength power from a conventional light source is distributed over a band of wavelength so that the intensity at any given wavelength is much less than that produced at the same wavelength by a laser a single wavelength oscillating in a resonant cavity such as that in a laser develops an output that is said to be coherent or in phase both in terms of time and in spatial relationship between waves this coherence is maintained in the output beam whereas in ordinary light any coherence that is present is lost very near the radiator and the output is said to be incoherent additionally beam divergence in the laser is minimal measured in mills or several seconds of art intensity or more exactly irradiance on the target is strongly maintained however the light beam from an ordinary light source spreads rapidly due to spherical radiation and intensity drops off rapidly as well a very large proportion of the laser's spectacularly high intensity is still present at very long ranges measurable in kilometers in tactical laser systems because of this high energy the beam may be powerful enough at these long ranges to cause eye damage and under certain conditions even reflections can be hazardous specular or mirror-like surfaces change the direction of the beam and retain beam divergence characteristics shiny metals reflect virtually all the energy at angles of incidents nearly perpendicular glass and plastic reflect a few percent and transmit the rest at grazing angles very high percentages of the incident energy are reflected from such surfaces as water ice glass and other smooth glossy flat surfaces the point here is that specular reflection should be thought of as containing substantial portions of the energy present in the incident being after striking a diffuse surface the hazard associated with reflected radiation is greatly reduced or eliminated since radiation in the initial beam is scattered in many directions but in spite of this there may be enough energy left at close ranges to be hazardous in clear air with strong temperature gradients typified by desert terrain scintillation may create both hot spots and dark spots the bright spots indicating an intensification of energy present locally in the beam path laser reflections from fog rain snow and dust can be of consequence but mostly at close ranges within approximately one meter water drops are specular surfaces but they are strongly curved and so dissipate energy effectively due to the highly coherent nature of the laser beam interference effects that is the reinforcement and canceling of reflective energy produce the speckling that is characteristic of laser reflection such reinforcement produces locally higher intensities which have been taken into account in the existing protective standards a final consideration is the fact that lasers operate in different time domain and this affects the energy that is both generated and delivered to the target as its name implies power output from a continuous wave or cw laser is continuous usually a low power device the cw laser can nevertheless be dangerous if exposure time at the target is long biological effects can accumulate and damage to tissues can result a pulsed laser delivers its energy in millionths of a second the total energy is compressed in time and so power density at the target is very high and can be dangerous the q-switched laser concentrates its full energy in an even shorter time billionths of a second power density at the target increases accordingly and the biological effects can be extremely dangerous the outputs of pulsed and q-switched lasers can be varied at the desired pulse repetition frequency in the pulsed laser the state of the cavity remains unchanged and the pulsing rate is established by the rate of firing of the energy source in the q switched laser the lasing action is spoiled or inhibited from completion by blocking the output from the cavity until maximum power is developed at which time the entire quantity is released in a single sharp pulse in this rangefinder for example q switching is achieved by means of a rotating prism firing of the energy source is timed so that it precedes the instant at which the position of the prism optically opens the cavity and transmits the accumulated energy the force of the laser's energy is especially vivid near the output this laboratory argon laser has its output invisible light and the backstop of material withstands its force but the light in the beam acts immediately on sample material when the energy is focused by a lens flammable material burns instantly the lens of the eye serves to concentrate the laser radiation at the retina just as the argon laser radiation is being concentrated at the target by the laboratory land it thus becomes easier to understand why lasers can inflict serious damage to the extent of destroying human vision or producing severe burns on the skin at closer ranges good reasons never to look at a laser beam or its reflections or to intercept it with the hand or body and we can now see why the use of external optical systems such as binoculars or optical sights aggravates the danger even further making the user more vulnerable to the radiation such equipment should therefore never be pointed into a beam given the high intensities and other extraordinary characteristics of laser light and the increasing use in the army of laser devices the probability of injurious exposure to laser radiation is also increasing so adequate safeguards must be established for the protection of all personnel who may be exposed this can be achieved efficiently through the use of the approval safety requirements and guidelines contained in the department of army bulletin control of hazards to health from laser radiation tb med 279 comprehensive data are supplied so that relationships between incident illumination reflectivity characteristics and biological effects of laser irradiation can be determined and safe operational conditions established the bulletin and technical manuals for specific laser equipments also provide detailed guidance for safety measures to be taken in dealing with hazards common to electro optical equipment this includes explicit data on hazards such as electrical shock x-rays cryogenic temperatures and so on for the laser beam safety factors inherent in each formula and equation have been arrived at through scientific studies of the biological effects of laser radiation on the human eye in general we can consider that these effects are mainly thermal and the degree to which laser energy will be reflected transmitted or absorbed depends upon the various properties of the tissue involved damage to numerous eye structures can result from laser irradiation and the location of the damage is related to the laser wavelength short ultraviolet and far infrared are absorbed primarily at the cornea and the type of damage can range in severity from slight pain to severe pain and clouding of the cornea with obstruction of light entering the eye long ultraviolet visible and near infrared energy is refracted at the cornea and lens and absorbed by the retina here the degree of damage depends upon where the laser energy is directed damage may be minimal as in the case of an oblique beam entrance which causes a lesion in the peripheral retina and may go unnoticed but if the beam hits the region of critical focusing in the macula or more especially in the fovea there can be partial but permanent loss of vision laser radiation of all wavelengths is partially absorbed by the skin the effects varying according to energy output some lasers are considered non-hazardous while others can cause reactions varying from mild reddening to blistering and charring although present-day military equipment does not have the power to inflict injury at long ranges some lasers can cause injury at short ranges carbon dioxide lasers for example operate at a wavelength that is not transmitted to the retina but it can still cause corneal or skin damage at close range or at high energy to ensure the safety of all personnel working in laser areas tb med 279 should be consulted and its safety requirements observed all personnel subject to exposure to laser radiation should receive medical surveillance with particular attention given to any change in visual capacity warning signs and labels should be used whenever practical to warn personnel of protective procedures required for safety government and industry standards have classified lasers into five safety related categories class 1 consists of very low power lasers with completely harmless radiation class 2 consists of low power lasers which require precautions against continuous direct viewing of the beam class 3 consists of medium power lasers which require appropriate protective safety goggles during viewing of the beam class 4 consists of high power lasers which can damage the eye or skin of an unprotected viewer protective eyewear and clothing are required class 5 consists of low medium or high power lasers which are safely enclosed as long as the potentially hazardous optical radiation is contained during laser operation these lasers are harmless safety calculations performed according to prescribed procedures should form the foundation for all laser safety and control measures for they take into account the significant variables likely to affect the health and safety of all personnel hazard evaluation should be performed for each circumstance in which lasers are to be used so that the controls and safety measures selected actually protect against the potential hazards involved both audible and visual warnings and establishment of closed areas may be advisable for example in a laboratory or maintenance area where accidental exposure is possible a countdown before firing may be desirable under no conditions look into a laser when it is capable of being fired accidental pulsing may occur avoid looking into the primary beam and it is just as important to avoid looking at specular reflections of the beam including those from lens surfaces and there are numerous hazards arising from equipment auxiliary to the lasers themselves such as power supplies where voltages are over 15 000 volts the equipment may produce x-rays gas-filled tubes or other containers for compressed gases may explode with acoustic energy that may themselves be dangerous optical radiation such as ultraviolet light from discharge tubes can be hazardous and toxic gases from materials used in or exposed to lasers can cause serious injury and lasers sometimes include cryogenic fluids intensely cold materials which can inflict injury if proper protective gear is not used work with lasers should be done in areas of high general illumination so that the iris of the eye closes down thereby limiting the effect of stray or accidental laser exposure the laser beam should be discharged into a background that is non-reflective and fire resistant areas near the target should be painted black to absorb stray radiation when exposure levels exceeding those arrived at by reference to tb med 279 are anticipated safety eyewear designed to attenuate laser light to values known to be safe should be worn during firing the eyewear should be labeled so its specific characteristics can be identified it is also important to be sure that eyewear designed for specific wavelengths are not used with lasers of different wavelengths the filter may safely block one but fully transmit the other in two-sided training exercises in the field employing laser equipment safety eye protection must be worn by all personnel for other training exercises eye protection is required only for personnel down range within the laser beam path calculation should be carried out that define danger areas and the protective areas around them suitable safeguards must then be physically established at the test or training site personnel must be excluded from the beam path either direct or reflected to a distance where beam intensity or beam radiant exposure is within permissible levels erection of physical barriers is an obvious means of control others include administrative restrictions and carefully controlled limitations on laser beam traverse it is also essential that all specular reflecting surfaces even those of a limited surface area be removed from areas close to the laser also all flat shiny specular reflecting surfaces on the target such as glass and mirrors should be masked or removed prior to laser ranging standardized personnel safety control should be exercised at the laser firing position only those targets which have a backstop capable of absorbing or dissipating laser radiation should be ranged on this may include natural terrain features such as trees and hills or mountains the backstop may also consist of man-made features such as earthworks and timber structures specular laser reflections from water normally do not require additional range controls for ground personnel however it should be recognized that laser energy reflected from water at low grazing angles maintains high energy levels in the downrange path the physical controls that are established must effectively exclude all personnel from the reflection area tracking of non-target vehicular or airborne traffic should be prohibited and there are certain precautions that should be observed when airborne laser operations are involved as in the case of laser designator and seeker teams reflection of laser illumination from still water ponds or other bodies of water and substantially horizontal pieces of glass or other flat shiny materials may be hazardous to personnel aloft due to the position of the designator aircraft with respect to the laser beam and its specular reflection the hazard is less for personnel aboard the source aircraft except when the beam is directed straight down the geometry of the beam path is such that the beam bounces away in other directions consequently the hazard is greater for those aboard aircraft flying tandem with the designator those aboard the seeker aircraft for example in test and training situations where airborne operations may involve specular reflections from lasers then consideration should be given to the use of safety eyewear by those in aircraft adjacent to the laser source and flying in tandem and in all cases care must be taken to avoid illuminating personnel on the ground it must be accepted of course that the procedures and controls outlined here are applicable for test or training exercises only and may be impossible to accomplish in the tactical environment in the past few minutes we have reviewed the basic characteristics of the laser in terms of the safe operation of these powerful and versatile devices laser radiation is potentially hazardous because of its high intensity minimum beam divergence coherence and monochromaticity it is therefore essential to observe the requirements of tb med 279 before employing lasers in the shop or in the field make all required calculations and establish all specified safeguards for the protection of personnel from hazardous laser radiation and from dangers associated with laser support equipment never look at the primary beam or its reflections where dangerous levels are anticipated where safety eyewear designed to protect against the specific type of laser radiation never allow the laser to be fired under circumstances in which unprotected personnel may be inadvertently exposed whether you operate the laser or control its use by others plan first for safety and then unleash the laser for all the tasks [Music] you