Patty's Industrial Hygiene, Physical and Biological Agents. Группа авторов
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Название: Patty's Industrial Hygiene, Physical and Biological Agents
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119816225
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FIGURE 4 Spectral radiant exitance (W m−2 μm−1) in the optical radiation region (100 nm–1000 μm) calculated using Planck's formula for blackbody radiance at different absolute temperatures: 300 K ∼room temperature, 1000 K ∼red‐hot object, 3000 K ∼incandescent lamp filament, 5780 K ∼effective temperature of the sun. The visible radiation band (0.4–0.78 μm) lies between the vertical dotted lines.
3.2 Solar Radiation
The solar radiation spectrum incident on the outermost part of the earth's atmosphere is fairly close to the theoretical blackbody radiation curve for an object at 5780 K, with peak emission around 500 nm in the visible region and significant emission throughout the UV region. Oxygen (O2) strongly absorbs UV at wavelengths shorter than 200 nm, with weaker absorption out to 245 nm, and ozone (O3) in the stratosphere absorbs UV over the range 230–300 nm. Due to atmospheric absorption by O2 and O3, practically no solar UV of wavelength less than 290 nm reaches the earth at sea level. Selected wavelength bands in the visible and IR regions are attenuated by O2, O3, water vapor, and carbon dioxide (11). The spectral distribution of direct sunlight reaching the earth's surface is further altered by Rayleigh scattering, which takes some light out of the direct path from the sun and disperses it around the upper atmosphere. Short wavelengths, including UV and blue light, are more strongly scattered than longer wavelengths. The blue color of the sky comes from this selective scattering of short‐wavelength sunlight. Change in the path length of the sun's rays through the atmosphere throughout the day, as well as variation in path length and ozone content by latitude and season, lead to temporal and geographical variation in the solar spectrum at the earth's surface.
3.3 Gas Discharge Lamps and Arc Lamps
In gas discharge lamps or arc lamps, an electric current is carried across a gap by ionized gas or vapor in a sealed tube. The ionized gas or vapor emits narrow spectral peaks, which may be superimposed on a continuum. Deuterium and hydrogen arcs emit over a continuum in the UV region, where the spectral intensity increases with decreasing wavelength, as well as emitting various spectral peaks in the visible region. Xenon arcs emit radiation at high intensity over a broad continuum from the UV‐C through the IR‐B, similar to a 6000 K blackbody emission spectrum, with some small spectral peaks around 500 nm and higher spectral peaks in the IR region. Low‐pressure mercury‐vapor arcs emit most of their radiant power in a spectral peak at 254 nm, which can be useful for germicidal applications, with other major peaks at 185, 285, 297, 313, 365, 405, 436, 546, 615, and 1013 nm, and numerous smaller peaks (12). In medium‐pressure and high‐pressure mercury‐vapor arcs, relatively more power is emitted in spectral peaks in the UV‐B, UV‐A, and visible regions, and there may be broadening and shifting of peaks as well as emission of a continuum. Examples of emission spectra from a 150 W xenon lamp and a 200 W mercury lamp are shown in Figures 5 and 6, respectively (13).
FIGURE 5 Spectral irradiance from a 150 W xenon lamp (13). The irradiance is plotted on a logarithmic scale to show the continuum radiation. The peaks would appear higher and sharper on a linear scale.
Source: From Ref. (13). Reproduced by permission of Newport Corporation, Oriel Product Line.
FIGURE 6 Spectral irradiance from a 200 W mercury lamp (13). The irradiance is plotted on a logarithmic scale to show the continuum radiation. The peaks would appear higher and sharper on a linear scale.
Source: Adapted from Newport Corporation, Oriel Product Line.
High‐intensity discharge (HID) lamps are defined as electrical discharge lamps in which the arc is stabilized by wall temperature and the arc tube has a wall loading greater than 3 W cm−2 (14). HID lamps include some mercury lamps, as well as metal halide lamps, high‐pressure sodium vapor lamps, and xenon arc lamps. Metal halide lamps contain mercury in the arc tube and are capable of emitting significant amounts of UV radiation.
The emission spectrum of a gas discharge lamp depends not only on the gas or vapor contained in the arc tube and the operating conditions of the arc but also on the composition of the arc tube and of any outer envelope. Most common types of glass attenuate UV‐B and UV‐C. Lamps intended for use as sources of UV radiation have arc tubes and outer envelopes made of fused silica, sometimes called “quartz,” which is transparent to UV. Even when UV transmission is not desired, as in HID lamps intended for illumination, the arc tube may be made of quartz to withstand the high operating temperature of the arc, with an outer envelope of glass to attenuate unwanted UV radiation. Lamp emissions may also be filtered by chemical coatings or dopants that absorb undesired wavelengths.
Chemicals known as phosphors absorb short‐wavelength optical radiation and fluoresce radiation of longer wavelengths, usually in a broad band. Fluorescent lamps are low‐pressure mercury‐vapor tubes with a coating of phosphors on the inside of the tube. Depending on the intended application, phosphors may be selected that fluoresce broadly in the visible region (“fluorescent lights”), the UV‐A region (“black lights” and phototherapy lamps), or the UV‐A and UV‐B regions (sunlamps for tanning).
3.4 Electrical Discharges
Electrical discharges used in arc welding and plasma arc cutting are a common source of potentially hazardous visible, UV‐A, UV‐B, and UV‐C radiations. Emission spectra from welding and cutting arcs consist of numerous spectral peaks that may be superimposed on a continuum. The spectral distribution of the radiation depends on the shielding gas for the arc, the composition of the electrodes and the base metal, and the welding current.
3.5 Light‐Emitting Diodes
LEDs are solid‐state electronic devices that emit noncoherent optical radiation, generally over a moderately narrow wavelength band several tens of nanometers wide. LEDs are increasingly being used for illumination because they are relatively efficient at converting electrical power into visible or UV radiation; some LEDs now on the market have efficiencies of 40–50%. UV and blue LEDs may emit potentially hazardous levels of radiation.
3.6 Excimer Lamps
Excimer lamps are being used increasingly as sources of noncoherent UV radiation. An excimer is a diatomic molecule, typically a homonuclear noble gas or noble gas–halogen complex, in an excited electronic state that is more stable than its ground state, such that the molecule breaks apart when the excitation energy is released in the form of a UV photon. Depending on the excimer, the radiation is emitted in one or more narrow wavelength bands in the UV‐C region. Phosphors may be used to shift and broaden the emission spectrum of the lamp for various applications.
4 ASSESSMENT OF OPTICAL RADIATION HAZARDS
4.1 СКАЧАТЬ