MOTION-PICTURE PHOTOGRAPHY 



689 



lens, which must resist considerable heat, and to its long focal length. The reflecting 

 spotlight is efficient with a sharply concentrated beam, but as the beam is widened, it 



44' 



Fig. 34. — Schematic drawing of " Solarspot" incandescent, showing angular Ught collection. 



becomes a ring of intense light surrounding a shadowed center, necessitating diffusion 

 and overlapping of a number of beams to build up the required intensity over a suffi- 

 cient area. This type of lamp is normally used with 



a "spill ring" to reduce forward radiation from the 

 source in directions other than that parallel to the 

 axis of the beam. The spill ring is merely a spiral of 

 sheet metal, positioned with its axis coinciding with 

 the beam axis, so that from the front of the unit the 

 spiral is seen edgewise and stary light is trapped by 

 the turns. 



A widely used spherical mirror-condensing lens 

 type of spotlight is shown schematically in Fig. 34. 

 The lens, constructed of heat-resisting glass, is of the 

 Fresnel or echelon type — a compound lens present- 

 ing a corrugated surface, built up of a succession of 

 fitted annular lenses about a central lens, all having 

 a common focus. It is the equivalent of a plano- 

 convex condensing lens of short focal length and 

 large aperture but considerably thinner and more 

 efficient. This lens is used in conjunction with a 

 spherical mirror, which collects rear light, with 

 both incandescent and arc sources. An incan- 

 descent lamp type is illustrated in the photograph 

 of Fig. 35, and Fig. 36 shows the light distribution 

 at various spot and flood divergences. In Fig. 37 a 

 high-intensity arc with its associated resistor is 

 shown; its distribution curves appear in Fig. 38. 

 In both types the lamp house may be lifted from the pedestal and mounted on 

 an elevated parallel or catwalk; in the case of arcs the resistance unit is also 

 detachable. 



Fig. 35. 



-5,000-watt 

 cent lamp. 



incandes- 



