Ординатура / Офтальмология / Английские материалы / Progress in Lens and Cataract Research_Hockwin_2002

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Yasuo Sakamoto, Department of Ophthalmology, Kanazawa Medical University, 1-1 Daigaku, Uchinada-machi, Kahoku-gun, Ishikawa-ken 920-0293 (Japan)

Tel. 81 76 286 2211, Fax 81 76 286 1010, E-Mail yassan-s@kanazawa-med.ac.jp

Wavelength (nm)

Fig. 1. Blue-light hazard function [35], showing different degrees of hazard for different wavelengths of light.

Light has varying degrees of hazard, depending on wavelength, with shorter wavelengths of approximately 400–500 nm being particularly hazardous (fig. 1). Because this wavelength range appears blue to the human eye and is mainly responsible for the occurrence of photoretinitis in normal situations, photoretinitis is referred to as blue-light hazard.

Light comes from various sources around us, and evaluating the potential blue-light hazard of each light source is a first step in preventing photoretinitis. However, only a few reports on such evaluation have been published [28–34], in part because the evaluation usually requires special equipment and techniques.

In this study, various light sources were evaluated for blue-light hazard. In particular, the sun and CO2 arc welding were examined in detail. The hazard criteria used are from the ACGIH (American Conference of Governmental Industrial Hygienists) standard [35], which is basically the same as the ICNIRP (International Commission on Non-Ionizing Radiation Protection) guidelines [36]. The spectral radiance of each light source was measured, and blue-light effective radiance and the corresponding permissible exposure time per day were calculated.

Methods

The measuring instrument used was a spectroradiometer (Photo Research PR-705). This instrument measures spectral radiance in the wavelength range of 380–780 nm at intervals of 2 nm, with a circular measuring field of 1/8° (2.2 mrad) or 1° (17.4 mrad) in diameter. For the majority of measurements, neutral density filters (Photo Research ND-10

and ND-100, Kenko PRO ND2, PRO ND4 and PRO ND8) were attached in an appropriate combination to the aperture of the instrument so as to reduce the intensity of incident light, because the light source was too bright for direct measurement. Corrections for spectral transmittance of accessory filters (ND-10 and ND-100) were made automatically by the instrument, while corrections for Kenko filters were made by multiplying the measured spectral radiance by spectral transmittance after measurement. When measuring a small light source such as the filament of an incandescent lamp, a welding arc, the arc of a discharge lamp, or a light-emitting diode (LED), a conversion lens (Kenko KR2005) was attached to the spectroradiometer so as to reduce the measuring field to 1/16° (1.1 mrad) in diameter. Corrections for spectral transmittance of the conversion lens were made after measurement.

The spectroradiometer was aimed at the light source to be measured in such a manner that, as observed through the viewfinder, the black spot that represents the measuring field fell within the source. Thus, the mean spectral radiance over this area was measured.

The measured spectral radiance was weighted against the blue-light hazard function (fig. 1) [35] and integrated over the wavelength range of 380–780 nm in order to obtain bluelight effective radiance. In this case, the wavelength range of 305–380 nm, where the blue-light hazard function has small values, was ignored, and this is acceptable for white-light sources, because the relative contribution of this range to effective radiance is negligible. For example, a trial calculation showed that the value obtained in this manner would be only 1% lower than the exact effective radiance for completely flat spectral radiance.

For an effective radiance exceeding 10 mW/cm2 . sr, the permissible exposure time per day was obtained by dividing 100 J/cm2 . sr by effective radiance, in accordance with the base criteria [35]. In the case of the sun, however, criteria relaxed for small light sources subtending an angle less than 11 mrad [35] should be applied, because the sun always subtends an angle of approximately 9.3 mrad when viewed from the earth. Thus, for the sun the permissible exposure time per day was obtained by dividing 100 (11/9.3)2 J/cm2 . sr by effective radiance.

The spectroradiometer was used in measurements within a half year of calibration by the manufacturer, as recommended.

The effective radiance of the sun was measured on 12 sunny or partly sunny days during the period from November 25, 2000 to January 12, 2001 in Machida, Japan, which is located at approximately 35° 30 N and 139° 30 E. Measurement was conducted at halfhourly intervals from sunrise to sunset, except when the sun was occulted by clouds, for a total of 187 measurements. The spectroradiometer to which three neutral density filters (ND-10, ND-100 and PRO ND4) were attached was aimed at the approximate center of the sun. The diameter of the measuring field was set to 1/8°. Exposure time was set automatically by the instrument in response to the brightness of the sun. The sun’s altitude was calculated for each measurement by use of PC software (MyPlanet Mitsunori Asami), and the relation between effective radiance and the sun’s altitude was examined.

The effective radiance of the arc associated with CO2 arc welding of mild steel was measured experimentally. CO2 arc welding is a type of metal active gas welding using CO2 gas to shield the weld and is currently the most commonly employed welding method in Japan. Measurement was conducted for welding currents ranging from 120 to 400 A. The welding wire employed was a solid wire (Kobe Steel MG-50T). Welding speed was set at 30 or 45 cm/min. Neutral density filters (4 PRO ND8 filters and, if necessary, 1 PRO ND4 filter) were attached to the spectroradiometer. The conversion lens was also attached so as to reduce the measuring field to 1/16° in diameter, which corresponds to 1 mm at the position of the arc. Exposure time was set to 100 ms. Effective radiance was measured 50 times for each

Table 1. Light sources in the workplace

Table 3. Halogen lamps and incandescent lamps

condition, but only the 25 highest values were used for calculation of the mean and standard deviation, in order to exclude inappropriate measurements where the measuring field failed to fall precisely within the arc.

In addition, blue-light effective radiance for various other light sources was surveyed (tables 1–3). The arc associated with shielded metal arc welding was measured experimentally under four sets of conditions in a laboratory. The arc of plasma cutting, molten steel, molten iron (in two factories), the interior of a converter, molten glass, and the interior of a glass furnace (in two factories) were measured in workplaces. The effective radiance of the arc was measured for a xenon lamp, a metal halide arc lamp, and a high-pressure mercury lamp, and the effective radiance on the surface of the envelope was measured for a high-pressure sodium arc lamp (inner envelope), a metal halide fluorescent lamp, a high-pressure mercury fluorescent lamp, and ordinary fluorescent lamps. Three LEDs of different colors were measured, the measuring field being 0.2 mm in diameter. The effective radiance of the filament was measured for halogen lamps and ordinary incandescent lamps of the clear-envelope type, and the effective radiance on the surface of the envelope was measured for ordinary incandescent lamps of the frosted-envelope type.

Results and Discussion

Figures 2 and 3 show results for the sun and CO2 arc welding, and tables 1–3 summarize those for other light sources.

Fig. 2. Effective radiance of the sun versus altitude.

The highest effective radiance measured was obtained in measurement of the sun (fig. 2). The effective radiance of the sun under the study conditions ranged from 0.0229 to 191 W/cm2 . sr, with a mean of 89.1 W/cm2 . sr. Permissible exposure times corresponding to the maximum and mean effective radiance are only 0.73 and 1.6 s, suggesting that viewing the sun is very hazardous to the retina.

The effective radiance of the sun varied considerably with cloud conditions, but tended to do so even when no clouds were in the sky, probably because of changes in the conditions of the unseen moisture or dust in the atmosphere. However, despite these variations, higher effective radiances tend to be measured when the sun is at higher altitudes. When the sun is at a higher altitude, its light travels a shorter distance through the atmosphere to reach the earth’s surface, and, as a result, the sun’s light is less absorbed by the atmosphere. In summer or in lower latitudes, therefore, theoretically the sun can be more hazardous than was measured in this study, because it can be at higher altitudes.

High effective radiances were also obtained by measurements of welding arcs. These fell within the range of 37.5–158 W/cm2 . sr for CO2 arc welding (fig. 3) and 19.0–34.2 W/cm2·sr for shielded metal arc welding (table 1). The corresponding permissible exposure times are only 0.63–2.7 and 2.9–4.7 s, suggesting that viewing welding arcs is also very hazardous to the retina. For both welding methods studied, effective radiance tends to increase with welding current. As the current increases, more energy is put into the arc, and a portion of this energy is emitted as light. Thus, theoretically, welding arcs produced

Welding current (A)

Fig. 3. Effective radiance (mean standard deviation) of the arc associated with CO2 arc welding versus welding current.

with larger currents in the workplace are more hazardous than those measured in this study and will become increasingly hazardous in the future, as increasingly larger welding currents are being used, for the sake of increased efficiency.

High effective radiances were also measured for arc lamps that allow the arc to be seen directly, i.e., the xenon lamp, the metal halide lamp, and the highpressure mercury lamp (table 2). In particular, the xenon lamp had an effective radiance as high as 110 W/cm2 . sr, with a corresponding permissible exposure time per day of only 0.91 s. Thus, xenon lamps may be as hazardous as the sun or welding arcs, although they have not been reported to have actually caused photoretinitis.

The arc of plasma cutting was also found to have a high effective radiance of 9.85 W/cm2 . sr, with a corresponding permissible exposure time per day of only 10 s (table 1).

Other light sources were found to have low effective radiances under the study conditions and would pose no hazard, at least for short exposure times.

In this study, except for the sun, evaluation of blue-light hazards was made on the worst-case criteria, which assume that the light source subtends an angle greater than 11 mrad [35]. Thus, the permissible exposure times obtained for light sources other than the sun are on the conservative side and would become longer in situations where the criteria relaxed for small light sources subtending an angle less than 11 mrad can be applied. In such cases, however, obtaining the exact permissible exposure time is difficult, because information on the size

of the light source and the distance at which it is viewed is required, or a completely different kind of measurement, i.e. that of irradiance, is required.

Only a few sets of data found in the literature can be compared directly with the present results. Hietanen [31] measured the blue-light effective irradiance of sunlight in a suburb of Helsinki at noon on three sunny days, in summer, winter and spring, and obtained a comparable range of permissible exposure times of 1–6 s. For CO2 arc welding, Sliney and Wolbarsht [28] reported effective radiances of 17.5 and 53.7 W/cm2 . sr at welding currents of 90 and 150 A. The latter value differs only slightly from that obtained under comparable conditions in this study. Also, Okuno [29] measured blue-light effective irradiance for CO2 arc welding and shielded metal arc welding under several sets of conditions and, using the size of the arc, converted the results into blue-light effective radiance. However, the obtained effective radiances are considerably lower than those obtained under comparable conditions in this study, because of Okuno’s overestimation of arc size.

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Tsutomu Okuno, National Institute of Industrial Health, 21-1,

Nagao 6, Tama-ku, Kawasaki 214-8585 (Japan)

Tel. 81 44 865 6111, Fax 81 44 865 6124, E-Mail okuno@niih.go.jp