162 RADIATION BIOLOGY 



factor during the daylight period (Withrow and Withrow, 1947). In 

 small compartments where plants are grown under high irradiances of 

 total artificial radiant energy, several internal-reflector flood lamps allow 

 better control of the irradiance than is possible where a single high- 

 wattage lamp with a large reflector is used. 



The projection lamp is designed for use with optical systems that 

 require small sources of the highest and most uniform intensity (Bourne, 

 1948; Weitz, 1950). To achieve this, the filament is a short rod (Beutler 

 and Metropolis, 1940) or is arranged in tight closely spaced coils and 

 operated at the highest temperature consistent with a reasonable life. 

 The coils are usually arranged parallel to one another and as close as 

 possible without touching. When arranged in a single plane (monoplane 

 filament), the brightness varies from a maximum at the coil to zero 

 between the coils. If the parallel coils are arranged in two planes so that 

 the coils of one plane are behind and between those of the other, the 

 brightness becomes more uniform. Another modification is the "solid- 

 source" design of Aldington (1945), in which the coils are arranged elec- 

 trically in parallel on a low-voltage circuit. Since corresponding points 

 on adjacent coils are at the same potential, the coils can be arranged so 

 close together that they actually touch. The average intensity and 

 brightness resulting from the biplane and " sohd-source " filament con- 

 structions closely approach those of the filament wire. 



The internal-reflector spot, or concentrating, lamp is one of the most 

 efficient sources for the intense irradiation of small areas. A 300-w 

 reflector spot lamp can produce irradiances of over 10,000 ft-c. Such 

 high values are difficult to obtain with a 1000-w projection lamp and con- 

 denser system. The high efficiency of the concentrating reflector lamp is 

 due to its relatively high effective-aperture ratio. The radiant energy 

 from the filament is collected by the parabohc reflector through a very 

 large angle. Unfortunately the optics of the molded bulb are not perfect 

 enough for use in refined optical systems, but for the production of high- 

 beam intensities with small, compact, and inexpensive equipment, the 

 concentrating reflector lamp is unsurpassed. 



The principal hmitation on the use of the incandescent lamp for the 

 irradiation of biological materials with visible irradiances of over a few 

 hundred foot-candles is the high proportion of the total energy which is 

 radiated. It is evident that all the thermal sources radiate most of their 

 energy in the infrared (see Table 3-13). For the general-service incan- 

 descent lamp, 83-94 per cent of the input electrical energy is radiated. 

 Of the total radiated energy, only about 10 per cent is in the visible. 



If a water filter is used to absorb the infrared, the resulting transmitted 

 energy has a higher luminous efficiency (Gordon, 1930). For a 1-cm layer 

 of water in a glass cell with parallel faces, the visible energy is reduced 

 about 10 per cent, largely by reflection from the air-glass and water-glass 



