GENERATION, CONTROL, AND MEASUREMENT 183 



ation, where it is desired to obtain uniform flux distribution over large 

 areas. Any attempt to increase the brightness greatly by increasing the 

 current density within the lamp is defeated in part by a marked drop in 

 efficiency of ultraviolet production by the arc discharge. 



CATHODES 



Commercial fluorescent lamps are available with cold cathodes, as used 

 in neon sign lighting, and with hot cathodes of the preheat and instant- 

 or quick-starting types. In general, the instant-starting hot-cathode 

 lamps are to be preferred to the preheat lamps. The life of the lamp is 

 determined chiefly by the rate of depreciation of cathode emission and is 

 therefore a complex function of the frequency of starting and the time of 

 operation. 



ELECTRICAL CHARACTERISTICS 



Like all gaseous discharge lamps, the fluorescent lamp has a negative 

 resistance characteristic and requires a current-limiting ballast. In addi- 

 tion, the ballast must supply sufficient open-circuit voltage to initiate the 

 discharge, which is usually more than twice that required for its oper- 

 ation. For lamps under 30 in. in length, the ballast for alternating cur- 

 rent is an inductance and for direct current is a resistance, since the 

 standard 118-v service is adequate to initiate the discharge. For tubes 

 longer than 30 in., the lamp must be operated on a higher voltage, and 

 the ballast may be a combination transformer and limiting impedance. 

 The use of an inductive ballast on alternating current results in a low 

 power factor, which may be less than 50 per cent. For installations 

 involving many lamps, the current demand on the power system may 

 become excessive, and high-power-factor ballasts are usually employed. 

 The two-lamp ballast corrects the power factor and greatly reduces flicker 

 by shifting the current in the two lamps out of phase. The current of 

 one lamp leads the line voltage while the other lags; thus the radiation 

 of one lamp is at a maximum when the other is approaching a minimum. 

 In the four-lamp ballast, two lamps are in series on each phase. 



There are four general methods of powering large banks of fluorescent 

 lamps for the irradiation of plants. Conventional two-lamp power- 

 factor-corrected ballasts may be used. The principal disadvantages are 

 that, if the ballasts are mounted in close proximity to the lamps and in 

 the same room, provision must be made for cooling the assembly, since 

 even the "tulamp" ballasts consume 15-25 per cent of the total power. 

 Placing the ballasts in a separate room seriously complicates the wiring, 

 since at least one wire is required per lamp. 



A second method consists in using an incandescent lamp both as a 

 resistive ballast and to supplement the fluorescent-lamp radiation (Parker 

 and Borthwick, 1950). Such operation has low power efficiency and 



