LIGHT AND INFRARED RADIATION
David H. Sliney
Light and infrared (IR) radiant energy are two forms of optical radiation, and together with ultraviolet radiation, they form the optical spectrum. Within the optical spectrum, different wavelengths have considerably different potentials for causing biological effects, and for this reason the optical spectrum may be further subdivided.
The term light should be reserved for wavelengths of radiant energy between 400 and 760 nm, which evoke a visual response at the retina (CIE 1987). Light is the essential component of the output of illuminating lamps, visual displays and a wide variety of illuminators. Aside from the importance of illumination for seeing, some light sources may, however, pose unwanted physiological reactions such as disability and discomfort glare, flicker and other forms of eye stress due to poor ergonomic design of workplace tasks. The emission of intense light is also a potentially hazardous side-effect of some industrial processes, such as arc welding.
Infrared radiation (IRR, wavelengths 760 nm to 1 mm) may also be referred to quite commonly as thermal radiation (or radiant heat), and is emitted from any warm object (hot engines, molten metals and other foundry sources, heat-treated surfaces, incandescent electric lamps, radiant heating systems, etc.). Infrared radiation is also emitted from a large variety of electrical equipment such as electric motors, generators, transformers and various electronic equipment.
Infrared radiation is a contributory factor in heat stress. High ambient air temperature and humidity and a low degree of air circulation can combine with radiant heat to produce heat stress with the potential for heat injuries. In cooler environments, unwelcome or poorly designed sources of radiant heat can also produce discomfort-an ergonomic consideration.
Occupational hazards presented to the eye and skin by visible and infrared forms of radiation are limited by the eye’s aversion to bright light and the pain sensation in the skin resulting from intense radiant heating. The eye is well-adapted to protect itself against acute optical radiation injury (due to ultraviolet, visible or infrared radiant energy) from ambient sunlight. It is protected by a natural aversion response to viewing bright light sources that normally protects it against injury arising from exposure to sources such as the sun, arc lamps and welding arcs, since this aversion limits the duration of exposure to a fraction (about two-tenths) of a second. However, sources rich in IRR without a strong visual stimulus can be hazardous to the lens of the eye in the case of chronic exposure. One can also force oneself to stare at the sun, a welding arc or a snow field and thereby suffer a temporary (and sometimes a permanent) loss of vision. In an industrial setting in which bright lights appear low in the field of view, the eye’s protective mechanisms are less effective, and hazard precautions are particularly important.
There are at least five separate types of hazards to the eye and skin from intense light and IRR sources, and protective measures must be chosen with an understanding of each. In addition to the potential hazards presented by ultraviolet radiation (UVR) from some intense light sources, one should consider the following hazards (Sliney and Wolbarsht 1980; WHO 1982):
1. Thermal injury to the retina, which can occur at wavelengths from 400 nm to 1,400 nm. Normally the danger of this type of injury is posed only by lasers, a very intense xenon-arc source or a nuclear fireball. The local burning of the retina results in a blind spot (scotoma).
2. Blue-light photochemical injury to the retina (a hazard principally associated with blue light of wavelengths from 400 nm to 550 nm) (Ham 1989). The injury is commonly termed “blue light” photoretinitis; a particular form of this injury is named, according to its source, solar retinitis. Solar retinitis was once referred to as “eclipse blindness” and associated “retinal burn”. Only in recent years has it become clear that photoretinitis results from a photochemical injury mechanism following exposure of the retina to shorter wavelengths in the visible spectrum, namely, violet and blue light. Until the 1970s, it was thought to be the result of a thermal injury mechanism. In contrast to blue light, IRA radiation is very ineffective in producing retinal injuries. (Ham 1989; Sliney and Wolbarsht 1980).
3. Near-infrared thermal hazards to the lens (associated with wavelengths of approximately 800 nm to 3,000 nm) with potential for industrial heat cataract. The average corneal exposure to infrared radiation in sunlight is of the order of 10 W/. By comparison, glass and steel workers exposed to infrared irradiances of the order of 0.8 to 4 kW/daily for 10 to 15 years have reportedly developed lenticular opacities (Sliney and Wolbarsht 1980). These spectral bands include IRA and IRB (see figure 49.1). The American Conference of Governmental Industrial Hygienists (ACGIH) guideline for IRA exposure of the anterior of the eye is a time-weighted total irradiance of for exposure durations exceeding 1,000 s (16.7 min) (ACGIH 1992 and 1995).
4. Thermal injury of the cornea and conjunctiva (at wavelengths of approximately 1,400 nm to 1 mm). This type of injury is almost exclusively limited to exposure to laser radiation.
5. Thermal injury of the skin. This is rare from conventional sources but can occur across the entire optical spectrum.
Figure 49.1 The electromagnetic spectrum
The importance of wavelength and time of exposure
Thermal injuries (1) and (4) above are generally limited to very brief exposure durations, and eye protection is designed to prevent these acute injuries. However, photochemical injuries, such as are mentioned in (2) above, can result from low dose rates spread over the entire workday. The product of the dose rate and the exposure duration always results in the dose (it is the dose that governs the degree of photochemical hazard). As with any photochemical injury mechanism, one must consider the action spectrum which describes the relative effectiveness of different wavelengths in causing a photobiological effect. For example, the action spectrum for photochemical retinal injury peaks at approximately 440 nm (Ham 1989). Most photochemical effects are limited to a very narrow range of wavelengths; whereas a thermal effect can occur at any wavelength in the spectrum. Hence, eye protection for these specific effects need block only a relatively narrow spectral band in order to be effective. Normally, more than one spectral band must be filtered in eye protection for a broad-band source.
Sources of Optical Radiation
The greatest occupational exposure to optical radiation results from exposure of outdoor workers to the sun’s rays. The solar spectrum extends from the stratospheric ozone-layer cut-off of about of 290-295 nm in the ultraviolet band to at least 5,000 nm (5 ) in the infrared band. Solar radiation can attain a level as high as during the summer months. It can result in heat stress, depending upon ambient air temperature and humidity.
The most significant artificial sources of human exposure to optical radiation include the following:
1. Welding and cutting. Welders and their co-workers are typically exposed not only to intense UV radiation, but also to intense visible and IR radiation emitted from the arc. Under rare instances, these sources have produced acute injury to the retina of the eye. Eye protection is mandatory for these environments.
2. Metals industries and foundries. The most significant source of visible and infrared exposure are from molten and hot metal surfaces in the steel and aluminium industries and in foundries. Worker exposure typically ranges from 0.5 to .
3. Arc lamps. Many industrial and commercial processes, such as those involving photochemical curing lamps, emit intense, short-wave visible (blue) light as well as UV and IR radiation. While the likelihood of harmful exposure is low due to shielding, in some cases accidental exposure can occur.
4. Infrared lamps. These lamps emit predominantly in the IRA range and are generally used for heat treatment, paint drying and related applications. These lamps do not pose any significant exposure hazard to humans since the discomfort produced upon exposure will limit exposure to a safe level.
5. Medical treatment. Infrared lamps are used in physical medicine for a variety of diagnostic and therapeutic purposes. Exposures to the patient vary considerably according to the type of treatment, and IR lamps require careful use by staff members.
6. General lighting. Fluorescent lamps emit very little infrared and are generally not bright enough to pose a potential hazard to the eye. Tungsten and tungsten-halogen incandescent lamps emit a large fraction of their radiant energy in the infrared. Additionally, the blue light emitted by tungsten-halogen lamps can pose a retinal hazard if a person stares at the filament. Fortunately, the eye’s aversion response to bright light prevents acute injury even at short distances. Placing glass “heat” filters over these lamps should minimize/eliminate this hazard.
7. Optical projectors and other devices. Intense light sources are used in searchlights, film projectors and other light-beam collimating devices. These may pose a retinal hazard with the direct beam at very close distances.
Measurement of Source Properties
The most important characteristic of any optical source is its spectral power distribution. This is measured using a spectroradiometer, which consists of suitable input optics, a monochromator and a photodetector.
In many practical situations, a broad-band optical radiometer is used to select a given spectral region. For both visible illumination and safety purposes, the spectral response of the instrument will be tailored to follow a biological spectral response; for example, lux-meters are geared to the photopic (visual) response of the eye. Normally, aside from UVR hazard meters, the measurement and hazard analysis of intense light sources and infrared sources is too complex for routine occupational health and safety specialists. Progress is being made in standardizations of safety categories of lamps, so that measurements by the user will not be required in order to determine potential hazards.
Human Exposure Limits
From knowledge of the optical parameters of the human eye and the radiance of a light source, it is possible to calculate irradiances (dose rates) at the retina. Exposure of the anterior structures of the human eye to infrared radiation may also be of interest, and it should be further borne in mind that the relative position of the light source and the degree of lid closure can greatly affect the proper calculation of an ocular exposure dose. For ultraviolet and short-wavelength light exposures, the spectral distribution of the light source is also important.
A number of national and international groups have recommended occupational exposure limits (ELs) for optical radiation (ACGIH 1992 and 1994; Sliney 1992). Although most such groups have recommended ELs for UV and laser radiation, only one group has recommended ELs for visible radiation (i.e., light), namely, the ACGIH, an agency well-known in the field of occupational health. The ACGIH refers to its ELs as threshold limit values, or TLVs, and as these are issued yearly, there is an opportunity for a yearly revision (ACGIH 1992 and 1995). They are based in large part on ocular injury data from animal studies and from data from human retinal injuries resulting from viewing the sun and welding arcs. TLVs are furthermore based on the underlying assumption that outdoor environmental exposures to visible radiant energy are normally not hazardous to the eye except in very unusual environments, such as snow fields and deserts, or when one actually fixes the eyes on the sun.
Optical Radiation Safety Evaluation
Since a comprehensive hazard evaluation requires complex measurements of spectral irradiance and radiance of the source, and sometimes very specialized instruments and calculations as well, it is rarely carried out onsite by industrial hygienists and safety engineers. Instead, the eye protective equipment to be deployed is mandated by safety regulations in hazardous environments. Research studies evaluated a wide range of arcs, lasers and thermal sources in order to develop broad recommendations for practical, easier-to-apply safety standards.
Occupational exposure to visible and IR radiation is seldom hazardous and is usually beneficial. However, some sources emit a considerable amount of visible radiation, and in this case, the natural aversion response is evoked, so there is little chance of accidental overexposure of the eyes. On the other hand, accidental exposure is quite likely in the case of artificial sources emitting only near-IR radiation. Measures which can be taken to minimize the unnecessary exposure of staff to IR radiation include proper engineering design of the optical system in use, wearing appropriate goggles or face visors, limiting access to persons directly concerned with the work, and ensuring that workers are aware of the potential hazards associated with exposure to intense visible and IR radiation sources. Maintainance staff who replace arc lamps must have adequate training so as to preclude hazardous exposure. It is unacceptable for workers to experience either skin erythema or photokeratitis. If these conditions do occur, working practices should be examined and steps taken to ensure that overexposure is made unlikely in the future. Pregnant operators are at no specific risk to optical radiation as regards the integrity of their pregnancy.
Eye protector design and standards
The design of eye protectors for welding and other operations presenting sources of industrial optical radiation (e.g., foundry work, steel and glass manufacture) started at the beginning of this century with the development of Crooke’s glass. Eye protector standards which evolved later followed the general principle that since infrared and ultraviolet radiation are not needed for vision, those spectral bands should be blocked as best as possible by currently available glass materials.
The empirical standards for eye protective equipment were tested in the 1970s and were shown to have included large safety factors for infrared and ultraviolet radiation when the transmission factors were tested against current occupational exposure limits, whereas the protection factors for blue light were just sufficient. Some standards’ requirements were therefore adjusted.
Ultraviolet and infrared radiation protection
A number of specialized UV lamps are used in industry for fluorescence detection and for photocuring of inks, plastic resins, dental polymers and so on. Although UVA sources normally pose little risk, these sources may either contain trace amounts of hazardous UVB or pose a disability glare problem (from fluorescence of the eye’s crystalline lens). UV filter lenses, glass or plastic, with very high attenuation factors are widely available to protect against the entire UV spectrum. A slight yellowish tint may be detectable if protection is afforded to 400 nm. It is of paramount importance for this type of eyewear (and for industrial sunglasses) to provide protection for the peripheral field of vision. Side shields or wraparound designs are important to protect against the focusing of temporal, oblique rays into the nasal equatorial area of the lens, where cortical cataract frequently originates.
Almost all glass and plastic lens materials block ultraviolet radiation below 300 nm and infrared radiation at wavelengths greater than 3,000 nm (3 ), and for a few lasers and optical sources, ordinary impact-resistant clear safety eyewear will provide good protection (e.g., clear polycarbonate lenses effectively block wavelengths greater than 3 ). However, absorbers such as metal oxides in glass or organic dyes in plastics must be added to eliminate UV up to about 380-400 nm, and infrared beyond 780 nm to 3 . Depending upon the material, this may be either easy or very difficult or expensive, and the stability of the absorber may vary somewhat. Filters that meet the American National Standards Institute’s ANSI Z87.1 standard must have the appropriate attenuation factors in each critical spectral band.
Protection in various industries
Fire-fighters may be exposed to intense near-infrared radiation, and aside from the crucially important head and face protection, IRR attenuating filters are frequently prescribed. Here, impact protection is also important.
Foundry and glass industry eyewear
Spectacles and goggles designed for ocular protection against infrared radiation generally have a light greenish tint, although the tint may be darker if some comfort against visible radiation is desired. Such eye protectors should not be confused with the blue lenses used with steel and foundry operations, where the objective is to check the temperature of the melt visually; these blue spectacles do not provide protection, and should be worn only briefly.
Infrared and ultraviolet filtration properties can be readily imparted to glass filters by means of additives such as iron oxide, but the degree of strictly visible attenuation determines the shade number, which is a logarithmic expression of attenuation. Normally a shade number of 3 to 4 is used for gas welding (which calls for goggles), and a shade number of 10 to 14 for arc welding and plasma arc operations (here, helmet protection is required). The rule of thumb is that if the welder finds the arc comfortable to view, adequate attenuation is provided against ocular hazards. Supervisors, welder’s helpers and other persons in the work area may require filters with a relatively low shade number (e.g., 3 to 4) to protect against photokeratitis (“arc eye” or “welder’s flash”). In recent years a new type of welding filter, the autodarkening filter has appeared on the scene. Regardless of the type of filter, it should meet ANSI Z87.1 and Z49.1 standards for fixed welding filters specified for dark shade (Buhr and Sutter 1989; CIE 1987).
Autodarkening welding filters
The autodarkening welding filter, whose shade number increases with the intensity of the optical radiation impinging upon it, represents an important advance in the ability of welders to produce consistently high-quality welds more efficiently and ergonomically. Formerly, the welder had to lower and raise the helmet or filter each time an arc was started and quenched. The welder had to work “blind” just prior to striking the arc. Furthermore, the helmet is commonly lowered and raised with a sharp snap of the neck and head, which can lead to neck strain or more serious injuries. Faced with this uncomfortable and cumbersome procedure, some welders frequently initiate the arc with a conventional helmet in the raised position-leading to photokeratitis. Under normal ambient lighting conditions, a welder wearing a helmet fitted with an autodarkening filter can see well enough with the eye protection in place to perform tasks such as aligning the parts to be welded, precisely positioning the welding equipment and striking the arc. In the most typical helmet designs, light sensors then detect the arc flash virtually as soon as it appears and direct an electronic drive unit to switch a liquid crystal filter from a light shade to a preselected dark shade, eliminating the need for the clumsy and hazardous manoeuvres practised with fixed-shade filters.
The question has frequently been raised whether hidden safety problems may develop with autodarkening filters. For example, can afterimages (“flash blindness”) experienced in the workplace result in permanently impaired vision? Do the new types of filter really offer a degree of protection that is equivalent or better than that which conventional fixed filters can provide? Although one can answer the second question in the affirmative, it must be understood that not all autodarkening filters are equivalent. Filter reaction speeds, the values of the light and dark shades achieved under a given intensity of illumination, and the weight of each unit may vary from one pattern of equipment to another. The temperature dependence of the unit’s performance, the variation in the degree of shade with electrical battery degradation, the “resting state shade” and other technical factors vary depending upon each manufacturer’s design. These considerations are being addressed in new standards.
Since adequate filter attenuation is afforded by all systems, the single most important attribute specified by the manufacturers of autodarkening filters is the speed of filter switching. Current autodarkening filters vary in switching speed from one tenth of a second to faster than 1/10,000th of a second. Buhr and Sutter (1989) have indicated a means of specifying the maximum switching time, but their formulation varies relative to the time-course of switching. Switching speed is crucial, since it gives the best clue to the all-important (but unspecified) measure of how much light will enter the eye when the arc is struck as compared with the light admitted by a fixed filter of the same working shade number. If too much light enters the eye for each switching during the day, the accumulated light-energy dose produces “transient adaptation” and complaints about “eye strain” and other problems. (Transient adaptation is the visual experience caused by sudden changes in one’s light environment, which may be characterized by discomfort, a sensation of having been exposed to glare and temporary loss of detailed vision.) Current products with switching speeds of the order of ten milliseconds will better provide adequate protection against photoretinitis. However, the shortest switching time-of the order of 0.1 ms-has the advantage of reducing transient adaptation effects (Eriksen 1985; Sliney 1992).
Simple check tests are available to the welder short of extensive laboratory testing. One might suggest to the welder that he or she simply look at a page of detailed print through a number of autodarkening filters. This will give an indication of each filter’s optical quality. Next, the welder may be asked to try striking an arc while observing it through each filter being considered for purchase. Fortunately, one can rely on the fact that light levels which are comfortable for viewing purposes will not be hazardous. The effectiveness of UV and IR filtration should be checked in the manufacturer’s specification sheet to make sure that unnecessary bands are filtered out. A few repeated arc strikings should give the welder a sense of whether discomfort will be experienced from transient adaptation, although a one-day trial would be best.
The resting or failure state shade number of an autodarkening filter (a failure state occurs when the battery fails) should provide 100% protection for the welder’s eyes for at least one to several seconds. Some manufacturers use a dark state as the “off” position and others use an intermediate shade between the dark and the light shade states. In either case, the resting state transmittance for the filter should be appreciably lower than the light shade transmittance in order to preclude a retinal hazard. In any case, the device should provide a clear and obvious indicator to the user as to when the filter is switched off or when a system failure occurs. This will ensure that the welder is warned in advance in case the filter is not switched on or is not operating properly before welding is begun. Other features, such as battery life or performance under extreme temperature conditions may be of importance to certain users.
Although technical specifications can appear to be somewhat complex for devices that protect the eye from optical radiation sources, safety standards exist which specify shade numbers, and these standards provide a conservative safety factor for the wearer.