In a global context, the International Electrotechnical Commission's IEC 62471:2006 standard, which was based on the ANSI/IES RP‐27 standards, has been adopted as a regulatory requirement by the European Union (EU) and other countries and has also been widely implemented on a voluntary basis by manufacturers of non‐laser light sources.
5 OPTICAL RADIATION CONTROL PRINCIPLES
Traditionally, radiation control principles have been summarized under the rubric of “time, distance, and shielding.” “Time” as a control principle refers to reducing risk by limiting the duration of exposure to the radiation. “Distance” refers to the reduction in risk associated with increasing the distance from the source. “Shielding” refers to the presence of an opaque or filtering medium between the source and the worker.
An additional control principle to consider is source optimization. Optimizing a source would include operating a source at the lowest power necessary to do the job, and, if possible, selecting sources with reduced spectral output in the blue‐light, UV‐A, UV‐B, and/or UV‐C regions if these wavelengths are not needed for the practical application.
5.1 Exposure Duration
For actinic UV radiation, the maximum permissible exposure time is (15)
where Eeff (in W m−2) is defined in Eq. (10). The maximum permissible exposure time for blue‐light sources that subtend an angle greater than 0.011 rad is (20):
(25)
and the maximum permissible exposure time for blue‐light sources that subtend an angle less than 0.011 rad is (20):
(26)
Unlike the photochemical effects represented by the actinic UV hazard and the blue‐light hazard, thermal effects do not show a strictly reciprocal relationship between spectrally weighted irradiance or radiance and permissible exposure time. Viewing durations may be limited involuntarily to a few seconds by the aversion response or the sensation of pain. A worker performing a visual task might, however, be motivated to persist in viewing an excessively bright source despite eye discomfort.
5.2 Exposure Geometry
5.2.1 Direction of Irradiation
When possible, potentially hazardous optical radiation sources should be located out of the line of sight of workers. UV sources, which may pose a hazard to the skin as well as the eyes, should be oriented to avoid direct irradiation of exposed skin. An example is the use of upper room germicidal UV radiation, where germicidal lamps are positioned and baffled so as to flood the unoccupied upper space of a room with UV‐C radiation while limiting the amount of reflected or stray radiation that reaches the lowest 2 m (6.5 ft) of the room (45).
If a surface directly irradiated by a hazardous optical radiation source is capable of reflecting radiation into a worker's eye, or onto exposed skin in the case of UV radiation, that surface should be treated with a matte or absorptive finish.
5.2.2 Distance from Source
When the source directly irradiates a worker, increasing the distance of the worker from the source is an effective control measure for optical radiation hazards generally.
UV hazards and IR hazards to the cornea and lens, as well as the blue‐light hazard for small sources subtending less than 0.011 rad, depend on the irradiance received at the eye. If the distance from a source is more than five times the longest dimension of the source, the irradiance received at the surface of the skin or cornea decreases inversely with the square of the distance from source, as stated in Eq. (23). If the exposure geometry does not meet the “five times” rule, the irradiance decreases less rapidly with distance. For example, for a uniform line source of length l and radiant power Φ, the irradiance at a distance r is given by
(27)
where α is the angle in radians subtended by the source. The angle subtended by a source of length l when its midline is viewed straight on from a distance r is
(28)
For retinal thermal hazards, the maximum allowable thermal‐hazard‐weighted radiance is inversely proportional to the angular subtense of the source (Eq. (16)). Increasing the distance from the source reduces the angular subtense, potentially increasing the allowable limit for the retinal‐thermal‐hazard‐weighted radiance.
The allowable limit for the blue‐light hazard for sources subtending an angle greater than 0.011 rad is a blue‐light‐weighted radiance of 100 J cm−2 sr−1. This limit is not altered by changing the distance from the source unless the change reduces the angular subtense of the source below 0.011 rad. The size of the area on the retina affected by an overexposure would, however, be reduced if the angular subtense were reduced by any amount. Increasing the viewing distance r from an excessive blue‐light source could, therefore, limit the areal extent of potential damage to the retina, even if the risk of incurring some photochemical damage remained unchanged.
5.2.3 Proximity to IR Sources
Erythema ab igne is a reddening of the skin caused by proximity to a radiant heat source. Traditionally caused by prolonged sitting close to a fire (ab igne is Latin for “from fire”) or by close work with hot objects in foundries, glassworks, and bakeries, this condition has also been reported as a result of exposure to electric space heaters (46) and a laptop computer (47). Workers should avoid direct skin contact with laptops and the common practice of locating space heaters very close to their legs and feet.
5.3 Shielding
Shielding for optical radiation includes enclosures around sources, protective clothing and eyewear, and barrier creams (such as sunscreens) applied to exposed skin. Shields attenuate radiation by absorbing or reflecting it. Because absorption and reflection tend to be wavelength dependent, shields should be selected based on their ability to attenuate radiation in the spectral range(s) of concern. It should be noted that shields attenuate radiation but might not entirely block it. The transmittance of a shield in the spectral region of concern should be small enough that the transmitted radiation reaching the skin or eye will be below the applicable exposure limit.
5.3.1 UV Shielding
5.3.1.1 UV‐Protective Properties of Shielding Materials
Rigid