ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - NON-IONIZING RADIATIONS ppt

779 NON-IONIZING RADIATIONS Lasers, microwave ovens, radar for pleasure boats, infrared inspection equipment and high intensity light sources gener- ate so-called “non-ionizing” radiation. Electromagnetic radiations which do not cause ionization in biological systems may be presumed to have photon ener- gies less than 10–12 eV and may be termed “non-ionizing.” Because of the proliferation of such electronic products as well as a renewed interest in electromagnetic radiation hazards, the Congress enacted Public Law 90-602, the “Radiation Control for Health and Safety Act.” This Act has as its declared purpose the establishment of a national elec- tronic product radiation control program which includes the development and administration of performance standards to control the emission of electronic product radiation. The most outstanding feature of the Act is its omnibus cover- age of all types of electromagnetic radiation emanating from electronic products, that is, gamma, X-rays, ultraviolet, visible, infrared, radio frequencies (RF) and microwaves. Performance standards have already been issued under the Act for TV sets, microwave ovens and lasers. In similar fash- ion, the recent enactment of the federal Occupational Safety and Health Act gives attention to the potential hazards of non-ionizing radiations in industrial establishments. For the purposes of this chapter more formal treatment is given to ultraviolet (UV) radiation, lasers, and micro- wave radiation than the visible and infrared (IR) radiations. However the information on visible and IR radiation presented in the section on Laser Radiation is generally applicable to non-coherent sources. It should become obvious in reading the material which follows that the eye is the primary organ at risk to all of the non-ionizing radiations. NATURE OF ELECTROMAGNETIC ENERGY The electromagnetic spectrum extends over a broad range of wavelengths, e.g. from Ͻ10 Ϫ12 to Ͼ10 10 cm. The short- est wavelengths are generated by cosmic and X-rays, the longer wavelengths are associated with microwave and elec- trical power generation. Ultraviolet, visible and IR radia- tions occupy an intermediate position. Radiation frequency waves may range from 10 kHz to 10 12 Hz, IR rays from 10 12 ; 4 ϫ 10 13 Hz (0.72 m m), the visible spectrum from approxi- mately 0.7–0.4 m m, UV from approximately 0.4–0.1 m m and g - and X-radiation, below 0.1 m m. The photon energies of electromagnetic radiations are proportional to the frequency of the radiation and inversely proportional to wavelength, hence the higher energies (e.g.10 8 eV) are associated with X- and g -radiations, the lower energies (e.g.10 Ϫ6 eV) with RF and microwave radiations. Whereas the thermal energy associated with molecules at room temperature is approximately 1/30 eV, the binding energy of chemical bonds is roughly equivalent to a range of Ͻ1–15 eV, the nuclear binding energies of protons may be equivalent to 10 6 eV and greater. Since the photon energy necessary to ionize atomic oxygen and hydrogen is of the order of 10–12 eV it seems in order to adopt a value of approximately 10 eV as a lower limit in which ionization is produced in biological material. An extremely important qualifi cation however is that non-ionizing radiations may be absorbed by biological sys- tems and cause changes in the vibrational and rotational energies of the tissue molecules, thus leading to possible dissociation of the molecules or, more often, dissipation of energy in the form of fl uorescence or heat. In conducting research into the bioeffects of the non- ionizing radiations the investigator has had to use several units of measurement in expressing the results of his stud- ies. For this reason Appendix A, containing defi nitions of many useful radiometric terms has been included. Appendix B provides a simple means for expressing radiant exposure and irradiance units in a number of equivalent terms. ULTRAVIOLET RADIATION Physical Characteristics of Ultraviolet Radiation For the purpose of assessing the biological effects of UV radia- tion the wavelength range of interest can be restricted to 0.1–0.4 m m. This range extends from the vacuum UV (0.1 m m) to the near UV (0.4 m m). A useful breakdown of the UV region is as follows: UV region g-range (±m) (eV) Vacuum Ͻ0.60 Ͼ7.7 Far 0.16–0.28 7.7 4.4 Middle 0.28–0.32 4.4 3.9 Near 0.32 0.4 3.9 3.1 The photon energy range for wavelengths between 0.1 and 0.4 m m is 12.4–3.1 eV, respectively. C014_004_r03.indd 779C014_004_r03.indd 779 11/18/2005 3:09:21 PM11/18/2005 3:09:21 PM © 2006 by Taylor & Francis Group, LLC 780 NON-IONIZING RADIATIONS common knowledge that signifi cant numbers of workers who routinely expose themselves to coal tar products while working outdoors experience a photosensitization of the skin. Abiotic effects from exposure to UV radiation occurs in the spectral range of 0.24–0.31 m m. In this part of the spec- trum, most of the incident energy is absorbed by the corneal epithelium at the surface of the eye. Hence, although the lens is capable of absorbing 99% of the energy below 0.35 m m only a small portion of the radiation reaches the anterior len- ticular surface. Photon-energies of about 3.5 eV (0.36 m m) may excite the lens of the eye or cause the aqueous or vitreous humor to fl uoresce thus producing a diffuse haziness inside the eye that can interfere with visual acuity or produce eye fatigue. The phenomenon of fl uorescence in the ocular media is not of concern from the bioeffects standpoint; the condition is strictly temporary and without detrimental effect. The development of photokeratitis usually has a latency period varying from 30 min to as long as 24 hrs depend- ing on the severity of the exposure. A sensation of “sand in the eyes” accompanied by varying degrees of photo- phobia, lacrimination and blepharospasm is the usual result. Blepharospasm is a refl ex protective mechanisms character- ized by an involuntary tight closing of the lids, usually over a damaged cornea. Exposure Criteria The biological action spectrum for keratitis peaks at 0.28 m m. At this wavelength, the threshold for injury has been deter- mined to be approximately 0.15 ϫ 10 6 ergs. It has been sug- gested that the corneal reaction in due primarily to selective absorption of UV by specifi c cell constituents, for example, globulin. Verhoeff and Bell (1916) gave the fi rst quantitative mea- surement of the UV energy necessary for threshold damage as 2 ϫ 10 6 ergs/cm 2 for the whole UV spectrum. More recent data by Pitts, using 10 nm bands of radiation produced a threshold of approximately 0.5 ϫ 10 5 ergs/cm 2 in rabbit eyes. The exposure criteria adopted by the American Medical Association based on erythemal thresholds at 0.2537 m m radiation are as follows: 0.5 ϫ 10 Ϫ6 W/cm 2 for exposure up to 7 hr; 0.1 ϫ 10 Ϫ6 W/cm 2 for exposure periods up to and exceeding 24 hr. Although these criteria are generally thought to be very conservative, i.e. stringent, they are nev- ertheless in common use. The American Conference of Governmental Industrial Hygienists (1982) recommend threshold limit values (TLV) for UV irradiation of unprotected skin and eyes for active wave- lengths between 0.2 and 0.315 m m (200 and 315 nm) 37 . Typical values are: for 200 nm, a TLV of 100 mJ/cm 2 ; for 240 nm, a TLV of 10 mJ/cm 2 ; for 280 nm, a TLV of 3.4 mJ/cm 2 ; and for 315 nm, a TLV of 1 J/cm 2 . Measurement of Ultraviolet Radiation Various devices have been used to measure UV radiation, e.g. photoelectric cells, photoconductive cells, photovoltaic Representative Sources of Ultraviolet Radiation The manor source of UV radiation is the sun, although absorption by the ozone layer permits only wavelengths greater than 0.29 m m to reach the surface of the earth. Low and high pressure mercury discharge lamps constitute sig- nifi cant manmade sources. In low pressure mercury vapor discharge lamps over 85% of the radiation is usually emit- ted at 0.2537 m m, viz. at germicidal wavelengths. At the lower pressures (fractions of an atmosphere) the charac- teristic mercury lines predominate whereas at higher pres- sures (up to 100 atmos.) the lines broaden to produce a radiation continuum. In typical quartz lamps the amount of energy at wavelengths below 0.38 m m may be 50% greater than the radiated visible energy, depending on the mercury pressure. Other manmade sources include xenon discharge lamps, lasers, and relatively new types of fl uo- rescent tubes, which emit radiation at wavelengths above 0.315 m m reportedly at an irradiance less than that mea- sured outdoors on a sunny day. Biological Effects of Ultraviolet Radiation The biological action spectrum for erythema (reddening) produced by UV radiation of the skin has been the subject of investigation for many years. The most recent data show that a maximum erythemal effect is produced at 0.260 m m with the secondary peak at approximately 0.290 m m. Erythemal response to wavelengths above 0.32 m m is predictably poor. The greatly increased air absorption of wavelengths below 0.25 m m and diffi culty in obtaining monochromatic radia- tions in this region probably account for the lack of defi nitive bioeffects data. This may change with the increase in the number of UV lasers. Wavelengths between 0.28 and 0.32 m m penetrate appreciably into the corium of the epidermis; those between 0.32 and 0.38 m m are absorbed in the epidermis, while those below 0.28 m m appear to be absorbed almost completely in the stratum corneum of the epidermis. Depending on the total UV dose, the latent periods for erythema may range from 2 to several hours; the severity may vary from simple erythema to blistering and desquama- tion with severe secondary effects. A migration of melanin granules from the basal cells to the maphigian cell layers of the epidermis may cause a thickening of the horny layers of the skin. The possible long-term effects of the repeated process of melanin migration is not completely understood. The available data seem to support the contention that some regions of the UV may produce or initiate carcinogenesis in the human skin. The experiments which have supported this contention indicate that the biological action spectrum for carcinogenesis is the same as that for erythema. Cases of skin cancer have been reported in workers whose occupation requires them to be exposed to sunlight for long periods of time. The reportedly high incidence of skin cancer in outdoor workers who are simultaneously exposed to chemicals such as coal tar derivatives, benzpyrene, methyl cholanthrene, and other anthracene compounds raises the question as to the role played by UV radiation in these cases. It is a matter of C014_004_r03.indd 780C014_004_r03.indd 780 11/18/2005 3:09:23 PM11/18/2005 3:09:23 PM © 2006 by Taylor & Francis Group, LLC NON-IONIZING RADIATIONS 781 cells, and photochemical detectors. It is common practice to employ the use of selective fi lters in front of the detecting device in order to isolate that portion of the UV spectrum of interest to the investigator. A commonly used detector is the barrier or photo- voltaic cell. Certain semiconductors such as selenium or copper oxide deposited on a selected metal develop a potential barrier between the layer and the metal. Light falling upon the surface of the cell causes the fl ow of electrons from the semi- conductor to the metal. A sensitive meter placed in such a cir- cuit will record the intensity of radiation falling on the cell. Ultraviolet photocells take advantage of the fact that cer- tain metals have quantitative photoelectric responses to spe- cifi c bands in the UV spectrum. Therefore a photocell may be equipped with metal cathode surfaces which are sensitive to certain UV wavelengths of interest. One of the drawbacks of photocells is solarization or deterioration of the envelope, especially with long usage or following measurement of high intensity UV radiation. This condition requires frequent reca- libration of the cell. The readings obtained with these instru- ments are valid only when measuring monochromic radiation, or when the relationship between the response of the instrument and the spectral distribution of the source is known. A desirable design characteristic of UV detectors is to have the spectral response of the instrument closely approxi- mate that of the biological action spectrum under consid- eration. However, such an instrument is unavailable at this time. Since available photocells and fi lter combinations do not closely approximate the UV biological action spec- tra it is necessary to standardize (calibrate) each photocell and meter. Such calibrations are generally made at a great enough distance from a standard source that the measuring device is in the “far fi eld” of the course. Special care must be taken to control the temperature of so-called standard mer- cury lamps because the spectral distribution of the radiation from the lamps is dependent upon the pressure of the vapor- ized mercury. A particularly useful device for measuring UV is the thermopile. Coatings on the receiver elements of the ther- mopile are generally lamp black or gold black to simu- late black body radiation devices. Appropriate thermopile window material should be selected to minimize the effects of air convection, the more common windows being crystal quartz, lithium chloride, calcium fl uoride, sodium chloride, and potassium bromide. Low intensity calibration may be made by exposing the thermopile to a secondary standard (carbon fi lament) fur- nished by the National Bureau of Standards. Other UV detection devices include (1) photodiodes, e.g. silver, gallium arsenide, silver zinc sulfi de, and gold zinc sulfi de. Peak sensitivity of these diodes is at wavelengths below 0.36 ␮ m; the peak effi ciency or responsivity is of the order of 50–70%; (2) thermocouples, e.g. Chromel-Alumel; (3) Golay cells; (4) superconducting bolometers, and (5) zinc sulfi de Schottky barrier detectors. Care must be taken to use detection devices having the proper rise time characteristics (some devices respond much too slowly to obtain meaningful measurements). Also, when measurements are being made special attention should be given to the possibility of UV absorption by many materi- als in the environment, e.g. ozone or mercury vapor, thus adversely affecting the readings. The possibility of photo- chemical reactions between UV radiation and a variety of chemicals also exists in the industrial environment. Control of Exposure Because UV radiations are so easily absorbed by a wide variety of materials appropriate attenuation is accomplished in a straightforward manner. In the case of UV lasers no fi rm bioeffects criteria are available. However the data of Pitts may be used because of the narrow band UV source used in his experiments to determine thresholds of injury to rabbit eyes. LASER RADIATION Sources and Uses of Laser Radiation The rate of development and manufacture of devices and systems based on stimulated emission of radiation has been truly phenomenal. Lasers are now being used for a wide vari- ety of purposes including micromachining, welding, cutting, sealing, holography, optical alignment, interferometry, spec- troscopy, surgery and as communications media. Generally speaking lasing action has been obtained in gases, crystalline materials, semiconductors and liquids. Stimulated emission in gaseous systems was fi rst reported in a helium-neon mix- ture in 1961. Since that time lasing action has been reported at hundreds of wavelengths from the UV to the far IR (several hundred micrometers). Helium–neon (He–Ne) lasers are typ- ical of gas systems where stable single frequency operation is important. He–Ne systems can operate in a pulsed mode or continuous wave (CW) at wavelengths of 0.6328, 1.15, or 3.39 m m depending upon resonator design. Typical power for He–Ne systems is of the order of 1–500 mW. The carbon dioxide gas laser system operates at a wavelength of 10.6 m m in either the continuous wave, pulsed, or Q-switched modes. The power output of CO 2 –N 2 systems may range from sev- eral watts to greater than 10 kW. The CO 2 laser is attractive for terrestrial and extra-terrestrial communications because of the low absorption window in the atmosphere between 8 and 14 qm. Of major signifi cance from the personal hazard standpoint is the fact that enormous power may be radiated at wavelength which is invisible to the human eye. The argon ion gas system operates predominantly at wavelengths of 0.488 and 0.515 m m in either a continu ous wave or pulsed mode. Power generation is greatest at 0.488 m m, typically at less than 10 W. Of the many ions in which laser action has been pro- duced in solid state crystalline materials, perhaps neo- dymium (Nd 3 ϩ ) in garnet or glass and chromium (Cr 3 ϩ ) in aluminum oxide are most noteworthy. Garnet (yttrium alu- minum garnet) or YAG is an attractive host for the trivalent neodymium ion because the 1.06 m m laser transition line is C014_004_r03.indd 781C014_004_r03.indd 781 11/18/2005 3:09:23 PM11/18/2005 3:09:23 PM © 2006 by Taylor & Francis Group, LLC 782 NON-IONIZING RADIATIONS sharper than in other host crystals. Frequency doubling to 0.530 m m using lithium niobate crystals may produce power approaching that available in the fundamental mode at 1.06 m m. also through the use of electro-optic materials such as KDP, barium–sodium niobate or lithium tantalite, “tuning” or scanning of laser frequencies over wide ranges may be accomplished. The ability to scan rapidly through wide frequency ranges requires special consideration in the design of protective measures. Perhaps the best known example of a semi-conductor laser is the gallium arsenide types operating at 0.840 m m; however, semiconductor materials have already operated over a range of approximately 0.4–5.1 m m. Generally speaking, the semiconductor laser is a moderately low-powered (mil- liwatts to several watts) CW device having relatively broad beam divergence thus tending to reduce its hazard poten- tial. On the other hand, certain semiconductor lasers may be pumped by multi-kV electron beams thus introducing a potential ionizing radiation hazard. Through the use of carefully selected dyes, it is possible to tune through broad wavelength ranges. Biological Effects of Laser Radiation The body organ most susceptible to laser radiation appears to be the eye; the skin is also susceptible but of lesser impor- tance. The degree of risk to the eye depends upon the type of laser beams used, notably the wavelength, output power, beam divergence, and pulse repetition frequency. The ability of the eye to refract long UV, visible, and near IR wave- lengths is an additional factor to be considered in assessing the potential radiation hazard. In the UV case of UV wavelengths (0.2–0.4 m m) pro- duced by lasers the expected response is similar to that produced by non-coherent sources, e.g. photophobia accom- panied by erythema, exfoliation of surface tissues and possible stromal haze. Absorption of UV takes place at or near the surface of tissues. The damage to epithelium results from the photochemical denaturization of proteins. In the case of IR laser radiation damage results exclu- sively from surface heating of the cornea subsequently to absorption of the incident energy by tissue water in the cornea. Simple heat fl ow models appear to be suffi ciently accurate to explain the surface absorption and damage to tissue. In the case of the visible laser wavelengths (0.4–0.75 m m) the organ at risk is the retina and more particularly the pigment epithelium of the retina. The cornea and lens of the eye focus the incident radiant energy so that the radi- ant exposure at the retina is at least several orders of mag- nitude greater than that received by the cornea. Radiant exposures which are markedly above the threshold for producing minimal visions on the retina may cause physi- cal disruption of retinal tissue by steam formation or by projectile-like motion of the pigment granules. In the case of short transient pulses such as those produced by Q-switched systems, acoustical phenomena may also be present. There are two transition zones in the electromagnetic spectrum where bio-effects may change from one of a corneal hazard to one of a retinal hazard. These are located at the interface of the UV-visible region and the visible–near IR region. It is possible that both corneal and retinal damage as well as damage to intermediate structures such as the lens and iris could be caused by devices emitting radiation in these transitional regions. Several investigators noticed irreversible changes in electroretinograms with attendant degeneration of visual cells and pigment epithelium, when albino and pig- mented rats were exposed to high illumination environments. The chronic and long term effects of laser radiation have not been fully explored. The biological signifi cance of irradiating the skin with lasers is considered to be less than that caused by exposure of the eye since skin damage is usually repairable or reversible. The most common effects on the skin range from erythema to blistering and charring dependent upon the wavelength, power, and time of exposure to the radiation. Depigmentation of the skin and damage to underlying organs may occur from exposure to extremely high powered laser radiation, particu- larly Q-switched pulses. In order that the relative eye-skin hazard potential be kept in perspective, one must not over- look possible photosensitization of the skin caused by injec- tion of drugs or use of cosmetic materials. In such cases the maximum permissible exposure (MPE) levels for skin might be considerably below currently recommended values. The thresholds for producing retinal lesions at all visible wavelengths were considered to be approximately the same i.e., 5 to 10 W/cm 2 , until more recent investigations discovered a much greater sensitivity of the eye to blue wavelengths. The mechanism for this enhanced sensitivity is explained on the basis of photochemical, rather than thermal effects. Exposure Criteria Permissible levels of laser radiation impinging upon the eye have been derived from short term exposure and an exami- nation of damage to eye structures as observed through an ophthalmoscope. Some investigators have observed irre- versible visual performance changes at exposure levels as low as 10% of the threshold determined by observation through an ophthalmoscope. McNeer and Jones found that at 50% of the ophthalmoscopically determined threshold the ERG B wave amplitude was irreversibly reduced. Mautner has reported severe changes in the visually evoked cortical potential at 25% of the ophthalmoscopically determined threshold. Since most, if not all, of the so-called laser cri- teria have been based on ophthalmoscopically-determined lesions on the retina, the fi ndings of irreversible functional changes at lower levels causes one to ponder the exact magnitude of an appropriate safety factor which should be applied to the ophthalmoscope data in order to derive a rea- sonable exposure criterion. There is unanimous agreement that any proposed maxi- mum permissible exposure (MPE) or threshold limits value (TLV) does not sharply divide what is hazardous from what is safe. Usually any proposed values take on fi rm meaning only C014_004_r03.indd 782C014_004_r03.indd 782 11/18/2005 3:09:23 PM11/18/2005 3:09:23 PM © 2006 by Taylor & Francis Group, LLC NON-IONIZING RADIATIONS 783 after years of practical use. However, it has become general practice in defi ning laser exposure criteria to: 1) Measure the radiant exposure (J/cm 2 ) or irradi- ance (W/cm 2 ) in the plane of the cornea rather than making an attempt to calculate the values at the retina. This simplifies the measurements and cal- culations for the industrial hygienists and radiation protection officers. 2) Use a 7 mm dia. limiting aperture (pupil) in the calculations. This assumes that the largest amount of laser radiation may enter the eye. 3) Make a distinction between the viewing of colimated sources, for example lasers and extended sources, such as fluorescent tubes or incandescent lamps. The MPE for extended source viewing takes into account the solid angle subtended at the eyes in view- ing the light source; therefore the unit is W/cm 2 ·sr (Watts per square centimeter and steradian). 4) Derive permissible levels on the basis of the wavelength of the laser radiation, e.g. the MPE for neodymium wavelength (1.06 m m) should be increased, i.e. made less stringent by a factor of approximately five than the MPE for visible wavelengths. 5) Urge caution in the use of laser systems that emit multiple pulses. A conservative approach would be to limit the power of energy in any single pulse in the train to the MPE specified for direct irra- diation at the cornea. Similarly the average power for a pulse train could be limited to the MPE of a single pulse of the same duration as the pulse train. More research is needed to precisely define the MPE for multiple pulses. Typical exposure criteria for the eye proposed by several organizations are shown in Wilkening (1978). These data do not apply to permissible levels at UV wavelengths or to the skin. A few supplementary comments on these factors are in order: There appears to be general agreement on maximum permissible exposure levels of radiation for the skin, e.g. the MPE values are approximately as follows for exposure times greater than 1 sec, an MPE of 0.1 W/cm 2 ; exposure times 10 Ϫ1 Ϫ1 sec, 1.0 W/cm 2 ; for 10 Ϫ4 Ϫ10 sec, 0.1 J/cm 2 , and for exposure times less than 10 Ϫ4 sec, 0.01 J/cm 2 . The MPE values apply to visible and IR wavelengths. For UV radiations the more conservative approach is to use the stan dards established by the American Medical Association. These exposure limits (for germicidal wavelengths viz. 0.2537 m m) should not exceed 0.1 ϫ 10 Ϫ6 W/cm 2 for con- tinuous exposure. If an estimate is to be made of UV laser thresholds then it suggested that the more recent work of Pitts be consulted. Major works to be consulted on hazard evaluation and classifi cation, control measures, measurement, safety and training programs, medical surveillance and criteria for exposure of the eye and skin to laser radiation are the American National Standards Institute (ANSI) and Bureau of Radiological Health (BRH) documents. Also see the ACGIH document for additional laser, microwave and ultraviolet exposure criteria. A major work on laser safety, soon to be released, is the laser radiation standard of the International Electrotechnical Commission (IEC). Measurement of Laser Radiation The complexity of radiometric measurement techniques, the relatively high cost of available detectors and the fact that calculations of radiant exposure levels based on man- ufacturers’ specifi cations of laser performance have been found to be suffi ciently accurate for protection purposes, have all combined to minimize the number of measure- ments needed in a protective program. In the author’s experience, the output power of commonly used laser systems, as specifi ed by the manufacturers, has never been at vari- ance with precision calibration data by more than a factor of two. All measurement systems are equipped with detection and readout devices. A general description of several devices and their application to laser measurements follow. Because laser radiation is monochromatic, certain sim- plifi cations can be made in equipment design. For example, it may be possible to use narrow band fi lters with an appro- priate type of detector thereby reducing sources of error. On the other hand, special care must be taken with high powered beams to prevent detector saturation or damage. Extremely short Q-switched pulses require the use of ultrafast detec- tors and short time-constant instrumentation to measure instantaneously power. Photoelectric detectors and radiation thermopiles are designed to measure instantaneous power, but they can also be used to measure total energy in a pulse by integration, provided the instrumental timeconstants are much shorter than the pulse lengths of the laser radiation. High current vacuum photo-diodes are useful for measur- ing the output of Q-switched systems and can operate with a linear response over a wide range. Average power measurements of cw lasers systems are usually made with a conventional thermopile or photovoltaic cells. A typical thermopile will detect signals in the power range from 10 m W to about 100 mW. Because thermopiles are composed of many junctions the response of these instru- ments may be non-uniform. The correct measure of average power is therefore not obtained unless the entire surface of the thermopile is exposed to the laser beam. Measurements of the cw power output of gas lasers may also be made with semiconductor photocells. The effective aperture or aperture stop of any measure- ment device used for determining the radiant expose (J/cm 2 ) or irradiance (W/cm 2 ) should closely approximate, if not be identical to, the papillary aperture. For purposes of safety the diameter should correspond to that of the normal dark- adapted eye, i.e. 7 mm. The response time of measurement system should be such that the accuracy of the measurement is not affected especially when measuring short pulse durations or instantaneous peak power. C014_004_r03.indd 783C014_004_r03.indd 783 11/18/2005 3:09:23 PM11/18/2005 3:09:23 PM © 2006 by Taylor & Francis Group, LLC 784 NON-IONIZING RADIATIONS Many calorimeters and virtually all photographic meth- ods measure total energy, but they can also be used for mea- suring power if the time history of the radiation is known. Care should be taken to insure that photographic processes are used within the linear portion of the fi lm density vs. log radiant exposure (gamma) curve. Microammeters and voltmeters may be used as read out devices for cw systems; microvoltmeters or electrometers coupled to oscilloscopes may be used for pulsed laser systems. These devices may be connected in turn to panel displays or recorders, as required. Calibration is required for all wavelengths at which the instrument is to be used. It should be noted that Tungsten Ribbon fi lament lamps are available from the National Bureau of Standards as secondary standards of spectral radiance over the wavelength region from approximately 0.2–2.6 m m. The calibration procedures using these devices permit comparisons within about 1% in the near UV and about 0.5% in the visible. All radiometric standards are based on the Stefan–Boltzmann and Planck laws of blackbody radiation. The spectral response of measurement devices should always be specifi ed since the ultimate use of the measure- ments is a correlation with the spectral response of the bio- logical tissue receiving the radiation insult. Control of Exposure In defi ning a laser hazard control program, some attempt should be made to classify the lasers or laser system accord- ing to their potential hazard. For example, one may wish to classify the lasers in terms of their potential for exceeding the Maximum Permissible Exposure (MPE) level or Threshold Limit Values (TLV). This could mean that a classifi cation of “low powered,” “exempt” or special “protected” lasers could evolve. “Exempt” may apply to lasers and laser systems which cannot, because of inherent design parameters, emit radiation levels in excess of the MPE; “low powered” could refer to systems emitting levels greater than the MPE for direct exposure to collimated beams but less than the MPE for extended sources; “high powered” could refer to systems that emit levels greater than the MPE for direct exposure to collimated laser beams as well as the MPE for extended sources; a “protected” laser system could be one where by virtue of appropriate engineering controls the emitted levels of radiation are less than any MPE value. Other variations are possible. Once a classifi cation scheme has been established it is possible to devise engineering measures and operating procedures to maintain all radiation at or below the desired levels, the stringency of the controls being directly related to the degree of risk to personnel in each category. It stands to reason that certain basic control principles apply to many laser systems: the need to inform appropri- ate persons as to the potential hazard, particularly with the discharge of capacitor banks associated with solid state Q-switched systems, the need to rely primarily on engineer- ing controls rather than procedures, e.g. enclosures, beam stops, beam enlarging systems, shutters, interlocks and iso- lation of laser systems, rather than sole reliance on memory or safety goggles. The “exempt” laser system is an exception to these measures. In all cases, particular attention must be given to the safety of unsuspecting visitors or spectators in laser areas. “High powered” systems deserve the ultimate in pro- tective design: enclosures should be equipped with inter- locks. Care should be taken to prevent accidental fi ring of the system and where possible, the system should be fi red from a remote position. Controls on the high powered sys- tems should go beyond the usual warning labels by installing an integral warning system such as a “power on” audible signal or fl ashing light which is visible through protective eye wear. Infrared laser systems should be shielded with fi reproof materials having an appropriate optical density (O.D.) to reduce the irradiance below MPE values. The main hazard of these systems is absorption of excessive amounts of IR energy by human tissue or by fl ammable or explosive chemicals. Before protective eye wear is chosen, one must deter- mine as a minimum the radiant exposure or irradiance levels produced by the laser at the distance where the beam or refl ected beam is to be viewed, one must know the appro- priate MPE value for the laser wavelength and fi nally one must determine the proper O.D. of protective eyewear in order to reduce levels below the MPE. Likewise, the visible light transmission characteristics should be known because suffi cient transmission is necessary for the person using the device to be able to detect ordinary objects in the immediate fi eld of vision. MICROWAVE RADIATION Physical Characteristics of Microwave Radiation Microwave wavelengths vary from about 10 m to about 1 mm; the respective frequencies range from 30 MHz–300 GHz. Certain reference documents, however, defi ne the microwave frequency range as 10 MHz–100 GHz. The region between 10 MHz and the IR is generally referred to as the RF or radiofrequency region. Certain bands of microwave frequencies have been assigned letter designations by industry; others, notably the ISM (Industrial, Scientifi c, Medical) frequencies have been assigned by the Federal Communications Commission for industrial, scientifi c and medical applications. Source of Microwave Radiation Microwave radiation is no longer of special interest only to those in communications and navigational technology. Because of the growing number of commercial applications of microwaves, e.g. microwave ovens, diathermy, materials drying equipment, there is widespread interest in the pos- sible new applications as well as an increased awareness of potential hazards. Typical sources of microwave energy are klystrons, magnetrons, backward wave oscillators and semiconductor transmit time devices (impatt diodes). Such C014_004_r03.indd 784C014_004_r03.indd 784 11/18/2005 3:09:24 PM11/18/2005 3:09:24 PM © 2006 by Taylor & Francis Group, LLC NON-IONIZING RADIATIONS 785 sources may operate continuously as in the case of some communications systems or intermittently, e.g. in microwave ovens, induction heating equipment and diathermy equip- ment or in the pulsed mode in radar systems. Natural sources of RF and microwave energy also exist. For example, peak fi eld intensities of over 100 V/m are produced at ground level by the movement of cold fronts. Solar radiation intensi- ties range from 10 Ϫ18 to 10 Ϫ17 watts per square meter per Hz (Wm Ϫ2 Hz Ϫ1 ) however, the integrated intensity at the earth’s surface for the frequency range of 0.2–10 GHz is approxi- mately 10 Ϫ8 mW/cm 2 . This value is to be compared with an average of 10 2 mW/cm 2 on the earth’s surface attributable to the entire (UV, visible IR and microwave) solar spectrum. Biological Effects of Microwave Radiation The photon energy in RF and microwave radiation is con- sidered to be too low to produce photochemical reactions in biological matter. However, microwave radiation is absorbed in biological systems and ultimately dissipated in tissue as heat. Irradiation of the human body with a power density of 10 mW/cm 2 will result in the absorption of approximately 58 W with a resultant body temperature elevation of 1ЊC, a value which is considered acceptable from a personal hazard standpoint. By way of comparison, the human basal meta- bolic rate is approximately 80 W for a person at rest; 290 for a person engaged in moderate work. Microwave wavelengths less than 3 cm are absorbed in the outer skin surface, 3–10 cm wavelengths penetrate more deeply (1 mm–1 cm) into the skin and at wavelengths from 25–200 cm penetration is greatest with the potential of causing damage to internal body organs. The human body is thought to be essentially transparent to wavelengths greater than about 200 cm. Above 300 MHz the depth of penetration changes rap- idly with frequency, declining to millimeter depths at frequen- cies above 3000 MHz. Above 10 GHz the surface absorption of energy begins to approach that of the IR radiation. The observed effects of radiofrequency radiation on bio- logical systems seem to depend more on a differential rate of energy deposition than in the case with ionizing radiation where biological effects seem to be related more to energy and integral (time independent) quantities, such as absorbed dose. The National Council on Radiation Protection and Measurements (NCRP) has attempted to consolidate the many quantities and units used to describe absorption of radio fre- quency electromagnetic energy by introducing the term “spe- cifi c absorption rate” (SAR). The specifi c absorption rate is the rate at which electromagnetic energy is absorbed at a point in a medium per unit mass of the medium, and is expressed in W/kg. Energy absorption is a continuous and differentiable function of space and time and one may speak of its gradient and its rate, hence the time derivative of the incremental energy (d W ) absorbed in an incremental mass (d m ) contained in a volume element (d V ) of a given density ( r ) may be expressed: SAR d d d d d d d d ϭϭ t W mt w V ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ r . Carpenter and Van Ummersen (1968) investigated the effects of microwave radiation on the production of cataracts in rabbit eyes. Exposures to 2.45 GHz radiation were made at power densities ranging from 80–400 mW/cm 2 for different exposure times. They found that repeated doses of 67 J/cm 2 spaced a day, a week, or 2 weeks apart produced lens opaci- ties even though the single threshold exposure dose at that power density (280 mW/cm 2 ) was 84 J/cm 2 . When the single exposure dose was reduced to 50 J/cm 2 opacities were pro- duced when the doses were administered 1 or 4 days apart, but when the interval between exposures was increased to 7 days no opacifi cation was noted even after 5 such weekly exposures. At the low power density of 80 mW/cm 2 (dose of 29 J/cm 2 ) no effect developed but when administered daily for 10 or 15 days cataracts did develop. The conclusion is that microwaves may exert a cumulative effect on the lens of the eye if the exposures are repeated suffi ciently often. The interval between exposures is an important factor in that a repair mechanism seems to act to limit lens damage if ade- quate time has elapsed between exposures. Certain other biological effects of microwave radiation have been noted in literature. One of these is the so-called “pearl chain effect” where particles align themselves in chains when subjected to an electric fi eld. There is considerable dis- agreement as to the signifi cance of the pearl chain effect. Investigators at the Johns Hopkins University have sug- gested a possible relationship between mongolism (Down’s Syndrome) in offspring and previous exposure of the male parent to radar. This suggested relationship was based on the fi nding that of 216 cases of mongolism, 8.7% of the fathers having mongol offspring vs. 3.3% of the control fathers (no mongol offspring) had contact with radar while in military service. This possible association must be regarded with extreme caution because of many unknown factors includ- ing the probability of a variety of exposures to environmental agents (including ionizing radiation) while in military service. Soviet investigators claim that microwave radiation pro- duces a variety of effects on the central nervous system and without a temperature rise in the organism. Claims are also made for biochemical changes, specifi cally a decrease in cholinesterase and changes in RNA at power density levels of approximately 10 mW/cm 2 . The reported microwave effects on the central nervous system usually describe ini- tial excitatory action, e.g. high blood pressure followed by inhibitory action and low blood pressure over the long term. Electroencephalographic data have been interpreted as indi- cating the presence of epileptiform patterns in exposed sub- jects. Other reported effects ranged from disturbances of the menstrual cycle to changes in isolated nerve preparations. Field interactions with brain tissue in cats have been assessed by effects on calcium ion fl uxes. Increases in cal- cium effl ux of the order of 20% have been reported under conditions of direct stimulation of synaptic terminals. Moreover, exposure of intact animals (cats) to a 450 MHz 0.375 mW/cm 2 fi eld, amplitude modulated at 16 Hz pro- duced a sharp rise in calcium effl ux, with a response curve identical to that obtained by direct electrical stimulation of brain tissue at the same intensity. C014_004_r03.indd 785C014_004_r03.indd 785 11/18/2005 3:09:24 PM11/18/2005 3:09:24 PM © 2006 by Taylor & Francis Group, LLC 786 NON-IONIZING RADIATIONS In addition, power and frequency “windows” have been reported, that is enhanced biological responses have been elicited within narrow bands of incident power and radiation frequency. What is often overlooked in any description of the bio- logical effects of microwave radiation is that such radiations have produced benefi cial effects. Controlled or judicious exposure of humans to diathermy or microthermy is widely practiced. The localized exposure level in diathermy may be as high as 100 mW/cm 2 . Exposure Criteria Schwan in 1953 examined the threshold for thermal damage to tissue, notably cataractogenesis. The power density nec- essary for producing such changes was approximately 100 mW/cm 2 to which he applied a safety factor of 10 to obtain a maximum permissible exposure level of 10 mW/ cm 2 . This number has been subsequently incorporated into many offi cial standards. The current American National Standards Institute C95 standard requires a limiting power density of 10 mW/cm 2 for exposure periods of 0.1 hr or more; also an energy density of 1 milliwatt-hour per square centimeter (1 mWh/cm 2 ) during any 0.1 hr period is permitted. The latter criterion allows for intermittency of exposure at levels above 10 mW/cm 2 , on the basis that such intermittency does not produce a temperature rise in human tissue greater than 1ЊC. More recently, Schwan has suggested that the permissible exposure levels be expressed in terms of current density, especially when dealing with measurements in the near or reactive fi eld where the con- cept of power density loses its meaning. He suggests that a permissible current density of approximately 3 mA/cm 2 be accepted since this value is comparable to a far fi eld value of 10 mW/cm 2 . At frequencies below 10 100 KHz this value should be somewhat lower and for frequencies above 1 GHz it can be somewhat higher. The most recent proposal of the American National Standards Institute (ANSI) specifi es a frequency dependent criterion, with a minimal elvel of 1 mW/cm 2 in the so-called resonant frequency range of the human body (approximately tens of MHz to several hundred MHz) and higher permis- sible levels at lower and higher frequencies. The performance standard for microwave oven specifi es a level of 1 mW/cm 2 at any point 5 cm or more from the external oven surfaces at the time the oven is fabricated by manufacturer. 5 mW is permitted throughout the useful life of the oven. Because Soviet investigators believe that effects on the central nervous system are more appropriate measure of the possibly detrimental effects of microwave radiation than are thermally induced responses, their studies have reported “thresholds” which are lower than those reported in Western countries. Soviet permissible exposure levels are several orders of magnitude below those in Western countries. The Soviet Standards for whole body radiation are as fol- lows: 0.1 mW/cm 2 for 2 hr exposure per day and 1 mW/cm 2 for a 15–20 min exposure provided protective goggles are used. These standards apply to frequencies above 300 MHz. Recent reports indicate that the Soviet Union has raised the above mentioned value of 0.01 mW/cm 2 to 0.025 mW/cm 2 ; also, the Soviet value of 0.001 mW/cm 2 for continuous exposure of the general population has been raised to 0.005 mW/cm 2 . There appears to be no serious controversy about the power density levels necessary to produce thermal effects in biological tissue. The nonthermal CNS effects reported by the Soviets are not so much controversial as they are a refl ection of the fact that Western investigators have not used the conditioned refl ex as an end point in their inves- tigations. Measurement of Microwave Radiation Perhaps the most important factor underlying some of the controversy over biological effects is the lack of standard- ization of the measurement techniques used to quantify results. To date, unfortunately, there seems to be little promise that such standardization will be realized in the near future. The basic vector components in any electromagnetic wave are the electric fi eld ( E ) and the magnetic fi eld ( H ). The simplest type of microwave propagation consists of a plane wave moving in an unbounded isotropic medium, where the electric and magnetic fi eld vectors are mutu- ally perpendicular to each other and both are perpendicu- lar to the direction of wave propagation. Unfortunately the simple proportionality between the E and H fi elds is valid only in free space, or in the so-called “far fi eld” of the radiating device. The far fi eld is the region which is suf- fi ciently removed from the source to eliminate any inter- action between the propagated wave and the source. The energy or power density in the far fi eld is inversely pro- portional to the square of the distance from the source and in this particular case the measurement of either E of H suffi ces for their determination. Plane-wave detection in the far fi eld is well understood and easily obtained with equipment which has been cali- brated for use in the frequency range of interest. Most hazard survey instruments have been calibrated in the far fi eld to read in power density (mW/cm 2 ) units. The simplest type of device uses a horn antenna of appropriate size coupled to a power meter. To estimate the power density levels in the near fi eld of large aperture circular antennas one can use the following simplifi ed relationship W A ϭϭ 16 4 2 P D P p near field ( ) , where P is the average power output, D is the diameter of the antenna, A is the effective area of the antenna and W is power density. If this computation reveals a power density which is less than a specifi ed limit, e.g. 10 mW/cm 2 , then no further calculation is necessary because the equation give the C014_004_r03.indd 786C014_004_r03.indd 786 11/18/2005 3:09:24 PM11/18/2005 3:09:24 PM © 2006 by Taylor & Francis Group, LLC NON-IONIZING RADIATIONS 787 maximum power density on the microwave beam axis. If the computed value exceeds the exposure criterion then one assume that the calculated power density exists through-out the near fi eld. The far fi eld power densities are then com- puted from the Friis free space transmission formula W GP r AP r ϭϭ 4 222 pl far field ( ) , where λ is the wavelength, r is the distance from the antenna and G is the far fi eld antenna gain. The distance from the antenna to the intersection of the near and far fi elds is given by r DA 1 2 82 ϭϭ p ll . These simplifi ed equations do not account for refl ections from ground structures or surfaces; the power density may be four times greater than the free space value under such circumstances. Special note should be made of the fact that microwave hazard assessments are made on the basis of average, not peak power of the radiation. In the case of radar generators, however, the ratio of peak to average power may be as high as 10 5 . Most microwave measuring devices are based on bolometry, calometry, voltage and resistance changes in detectors and the measurement of radiation pressure on a refl ecting surface. The latter three methods are self- explanatory. Bolometry measurements are based upon the absorption of power in a temperature sensitive resistive element, usually a thermistor, the change in resistance being proportional to absorbed power. This method is one of the most widely used in commercially available power meters. Low frequency radiation of less than 300 MHz may be measured with loop or short ship antenna. Because of the larger wavelengths in the low frequency region, the fi eld strength in volts per meter (V/m) is usually deter- mined rather than power density. One troublesome fact in the measurement of micro- wave radiation is that the near fi eld (reactive fi eld) of many sources may produce unpredictable radiative pat- terns. Energy density rather than power density may be a more appropriate means of expressing hazard potential in the near fi eld. In the measurement of the near fi eld of microwave ovens it is desirable that the instrument have certain characteristics, e.g. the antenna probe should be electrically small to minimize perturbation of the fi eld, the impedance should be matched so that there is no back- scatter from the probe to the source, the antenna probe should behave as an isotropic receiver, the probe should be sensitive to all polarizations, the response time should be adequate for handling the peak to average power of the radiation and the response of the instrument should be fl at over a broad band of frequencies. In terms of desirable broad band characteristics of instruments it is interesting to note that one manufac- turer has set target specifi cations for the development of a microwave measurement and monitoring device as fol- lows: frequency range 20 KHz–12.4 GHz and a power density range of 0.02–200 mW/cm 2 Ϯ 1 dB. Reportedly two models of this device will be available: one a hand held version complete with meter readout, the other a lapel model equipped with audible warning signals if excessive power density levels develop . Useful radiometric and related units Term Symbol Description Unit and abbreviation Radiant energy O Capacity of electromagnetic wages to perform work Joule (J) Radiant power P Time rate at which energy is emitted Watt (W) Irradiance or radiant flux density (dose rate in photobiology) E Radiant flux density Watt per square meter (W · m Ϫ2 ) Radiant intensity I` Radiant flux of power emitted per solid angle (steradian) Watt per steradian (W · sr Ϫ1 ) Radiant exposure (dose in photobiology) H Total energy incident on unit area in a given time interval Joule per square meter (J · m Ϫ2 ) Beam divergence f Unit of angular measure. One radian Ϸ 57.3Њ 2p radians ϭ 360Њ Radian APPENDIX A C014_004_r03.indd 787C014_004_r03.indd 787 11/18/2005 3:09:24 PM11/18/2005 3:09:24 PM © 2006 by Taylor & Francis Group, LLC 788 NON-IONIZING RADIATIONS APPENDIX B that they can adequately withstand power densities of at least 10 mW/cm 2 without interference with their function.” PREFERRED READING 1. Clarke, A.M. (1970), “Ocular Hazards from Lazers and other Optical Sources,” CRC Critical Reviews in Environmental Control, 1 , 307. 2. Cleary, S.F. (1970), “The Biological Effects of Microwave and Radio- frequency,” CRC Critical Reviews in Environmental Control, 1 , 257. REFERENCES 1. Matelsky, I., The non-ionizing radiations, Industrial Hygiene High- lights 1 , Indus, Hygiene Foundation of America Inc. Pittsburgh, Pa., 1968. 2. Ibid. p. 145. 3. Ibid. p. 149. 4. Cogan, D.G. and V.E. Kinsey (1946), Action spectrum of keratitis pro- duced by ultraviolet radiation, Arch. Ophthal. , 35, 670. 5. Verhoeffr, F.H. and L. Bell (1916), Pathological Effects of Radiant Energy on the Eye, Proc. Amer. Acad. Arts and Sci. , 51, 630. 6. Pitts, D.G., J.E. Prince, W.I. Butcher, K.R. Kay, R.W. Bowman, H.W. Casey, D.G. Richey, L.H. Mori, J.E. Strong, and T.J. Tredici, The effects of ultraviolet radiation on the eye, Report SAM-TR -69-10, USAF School of Aerospace Medicine, Brooks AFB, Texas, Feb., 1969. 7. Pitts, D.G. and K.R. Kay (1969), The photophthalmic threshold for the rabbit, Amer. J. Optom. , 46, 561. 8. Permissible limit for continuous ultraviolet exposure, Council on Physical Therapy, Amer. Med. Assn., Chicago, 1948. Conversion factors AϪradiant energy units erg joule W/sec Ϯ W/sec g-cal erg ϭ 110 Ϫ7 10 Ϫ7 0.1 2.39 ϫ 10 Ϫ8 10 joule ϭ 1110 6 0.239 W sec ϭ 107 1 1 10 4 0.239 ϮW sec ϭ 10 10 Ϫ6 10 Ϫ6 1 2.39 ϫ 10 Ϫ7 g-cal ϭ 4.19 ϫ 10 7 4.19 4.19 4.19 ϫ 10 6 1 BϪradiant exposure (dose) units erg/cm 2 joule/cm 2 W/sec cm 2 ϮW/sec cm 2 g-cal/cm 2 erg cm 2 ϭ 10 Ϫ7 10 Ϫ7 0.1 2.39 ϫ 10 Ϫ8 joule cm 2 ϭ 10 7 1110 6 0.239 W sec cm 2 ϭ 10 7 1110 6 0.239 ϮW sec cm 2 ϭ 10 10 Ϫ6 10 Ϫ6 2.39 ϫ 10 Ϫ7 gϪcal cm 2 ϭ 4.19 ϫ 10 7 4.19 4.19 4.19 ϫ 10 6 1 C-irradiance (dose rate) units erg/cm 2 · sec joule/cm 2 · sec W/cm 2 ϮW/cm 2 gϪcal/cm 2 · sec erg/cm 2 · sec ϭ 110 Ϫ7 10 Ϫ7 0.1 2.39 ϫ 10 Ϫ6 joule cm 2 · sec ϭ 10 7 1110 6 0.239 W/cm 2 ϭ 10 7 1110 6 0.239 ϮW/cm 2 ϭ 10 10 Ϫ6 10 Ϫ6 1 2.39 ϫ 10 Ϫ7 g-cal/cm 2 · sec ϭ 4.19 ϫ 10 7 4.19 4.19 4.19 ϫ 10 6 1 A tabular summary of typical characteristics of instru- mentation used for electromagnetic fi eld measurements is available in an NCRP report. Control Measures The control of excessive exposures to microwave radiation is basically an engineering matter. The engineering measures may range from the restriction of azimuth and elevation settings on radar antennas to complete enclosures of mag- netrons in microwave ovens. The use of personnel protec- tive devices have their place but are of much lower priority importance to engineering controls. Various types of micro- wave protective suits, goggles and mesh have been used for special problems. It has been shown that cardiac pacemakers, particularly those of the demand type, may have their function compro- mised by microwave radiation. Furthermore, the radiation levels which cause interference with the pacemaker may be orders of magnitude below levels which cause detrimental bio- logical effects. The most effective method of reducing the sus- ceptibility of these devices to microwave interference seems to be improved shielding. Manufacturers of cardiac pacemak- ers “ . . . have successfully redesigned and shielded the units so C014_004_r03.indd 788C014_004_r03.indd 788 11/18/2005 3:09:24 PM11/18/2005 3:09:24 PM © 2006 by Taylor & Francis Group, LLC [...]... Development Command, Wash., DC 18 McNeer, K.W., M Ghosh, W.J Geeraets, and D Guerry (1963), Erg after light coagulation, Acta Ophthal Suppl 76, 94 19 Jons, A.E., D.D Fairchild, and P Spyropoulos (1968), Laser radiation effects on the morphology and function of ocular tissue, Second Annual Report, Contr No DADA-17–67-C-0019, US Army Medical Research and Development Command, Wash., DC 20 Safety level of microwave... 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(IEC), Geneva, Switzerland Wilkening, G.M et al (1981), Radiofrequency Electromagnetic Fields, Properties, Quantities and Units, Biophysical Interaction and Measurements National Council on Radiation Protection and Measurements, Report No 67, Washington, DC, 20014 Adey, W.R (1979), Neurophysiologic Effects on Radiofrequency and Microwave Radiation, Bulletin of the New York Academy of Medicine 55, 1079.. .NON-IONIZING RADIATIONS 9 Bulletin No 3, The Eppley Laboratory Inc., Newport, Rhode Island, 1963 10 Richardson, J.R and R.D Baertsch (1969), Zinc sulfide schottky barrier ultraviolet detectors, Solid State Electronics 12, 393 11 Javan, A., W.R Bennett, and D.R Herriott (1961), Population inversion and continuous optical maser oscillation in a gas discharge containing a He-Ne mixture, Phys... Kang, and S Berman (1966), Retinal damage by light in rats, Invest Ophthal., 5, 450 16 Kotiaho, A., I Resnick, J Newton, and H Schwell (1966), Temperatures rise and photocoagulation of rabbit retinas exposed to the CW Laser, Amer J Ophthal., 62, 644 17 Davis, T.P., and W.J Mautner (1969), Helium–neon laser effects on the eye, Annual Report Contract No DADA 17–69-C-9013, US Army Medical Research and Development... Specifications for Techniques and Instrumentation 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 789 for Evaluating Radio Frequency Hazards to Personnel, New York, NY., 1968 Wacker, P (1970), Biol Effects and Health Implication of Microwave Radiation, US Govt Printing Office Bowman, R., Ibid King, G.R., A.C Hamburger, F Parsa, S.J Heller, and R.A Carleton (1970), Effect of microwave oven on implanted... Rev Lett., 6, 106 12 Miller, R.C and W.A Nordland (1967), Tunable Lithium Niobate Optical Oscillator with external mirrors, Appl Phys Lett., 10, 53 13 Ham, W.T., R.C Williams, H.A Muller, D Guerry, A.M Clarke, and W.J Geeraets (1965), Effects of laser radiation on the mammalian eye, Trans N.Y Acad Sci., 28, 517 14 Clarke, A.M., W.T Ham, W.J Geeraets, R.C Williams, and H.A Mueller (1969), Laser Effects . development and administration of performance standards to control the emission of electronic product radiation. The most outstanding feature of the Act is its omnibus cover- age of all types of electromagnetic. handling the peak to average power of the radiation and the response of the instrument should be fl at over a broad band of frequencies. In terms of desirable broad band characteristics of. approxi- mately 0.7–0.4 m m, UV from approximately 0.4–0.1 m m and g - and X-radiation, below 0.1 m m. The photon energies of electromagnetic radiations are proportional to the frequency of the