174 RADIATION BIOLOGY 



Medium- to high-pressure lamps operate at pressures from a half to 

 several hundred atmospheres. They are high-intensity sources whose 

 spectral energy distribution is shifted toward the visible; the Hnes are 

 broadened, and a continuous spectrum is superposed on the line spectrum. 

 The various types of mercury arcs have been discussed in detail by several 

 authors (Barnes and Forsythe, 1936; Bourne, 1948; Forsythe and Adams, 

 1948; Forsythe et al., 1942; Illuminating Engineering Society, 1952; 

 Roller, 1952; Weitz, 1950). 



Low-pressure Discharge. At very low vapor pressure the mean free 

 path between the atoms is sufficiently large so that there is a high prob- 

 abiUty that most of the energy will be radiated in the resonance line at 

 253.7 mil, which represents the transition from the lowest excited state 

 to the ground state in the mercury atom. The efficiency of production 

 of the resonance radiation is highest when the pressure is between 0.008 

 and 0.10 mm Hg, which corresponds to an envelope temperature of about 

 45°C. Under optimum conditions as much as 60 per cent of the electrical 

 input energy may appear as resonance radiation at 253.7 m/x, an amaz- 

 ingly high efficiency for conversion of electrical to radiant energy. Very 

 little of the radiated energy (less than 3 per cent) appears in the other 

 mercury lines of the near ultraviolet and visible. As the pressure is 

 increased to several millimeters, a larger proportion of the energy appears 

 in the near-ultraviolet and visible lines, but the lines are still sharp, and 

 there is almost no continuum (Forsythe and Adams, 1948). As is com- 

 mon with all low-pressure low-current-density discharges, the intensity is 

 low and the energy is diffusely distributed. Consequently, high power 

 involves large distributed sources. 



Medium-pressure Arc. As the vapor pressure is increased by a higher 

 operating temperature, the discharge becomes increasingly concentrated, 

 the current density increases, and an increasing proportion of the radi- 

 ated energy appears in the visible and near ultraviolet as a continuum. 

 Many of the medium-pressure lamps operating at 0.5-10 atm have a 

 small arc tube surrounded by a second envelope. The space between 

 the two envelopes is evacuated so as to reduce heat losses from the arc 

 tube. A Hmited amount of mercury is introduced which is just sufficient 

 to maintain the proper operating pressure when completely vaporized. 



High-pressure Capillanj Arc. The highest intensities are produced by 

 the high-pressure capillary arc in which the pressure is from 30 to several 

 hundred atmospheres and the temperature so high that even with fused 

 quartz the tube must be cooled by an air blast or by flowing water 

 (Bourne, 1948; Weitz, 1950). In Fig. 3-14 are given the spectral-energy- 

 distribution curves of a series of capillary arcs operating at pressures of 

 31-285 atm. It will be noted that at the highest pressure the continuum 

 between the lines has become the predominant feature of the arc, whereas 



