Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 106

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).