IMAGE TRANSMISSION SYSTEAI 465 



In addition, pure neon has an after-glow; the gas continues to glow 

 for an appreciable time after current ceases to flow. This after-glow 

 casts spurious bands of illumination out to one side of the brighter 

 image details. 



A small amount of hydrogen in the neon prevents such an afterglow; 

 and at the same time improves the circuit characteristics of the lamp. 

 The total impedance of the lamp is lower, making it a less influential 

 part of the lamp circuit; and the resistance and reactance vary in 

 such a manner that the phase shift is more nearly proportional to 

 frequency (a phase shift proportional to frequency causes no distortion 

 in the reproduction of an image). Other active gasses may be used 

 with the neon to improve the operation of a television lamp, but one 

 or two per cent of hydrogen is most satisfactory. 



Since hydrogen is absorbed by the electrodes in a glow discharge, it 

 slowly disappears from the neon during operation of the lamp. For 

 this reason the lamp is provided with a small side reservoir of hydrogen. 

 The lamp and the reservoir carry porous plugs immersed in a pool of 

 mercury; and a flexible rubber connection permits the two plugs to be 

 brought into contact at will. Minute quantities of hydrogen may be 

 introduced into the lamp by simply bringing the two plugs into con- 

 tact for a short time. 



Even with this improvement the circuit characteristics of a lamp are 

 not ideal. W'ith power tubes it is usually desirable to include a 

 fixed resistance in series with the lamp to prevent semi-arcing con- 

 ditions. Such a resistance also makes the lamp a less influential 

 fraction of the total circuit impedance. 



Optical Monitoring System 



In order to insure that the incoming and outgoing images are prop- 

 erly positioned, no matter what the stature of the person sitting in the 

 booth, and that the images shall be of proper quality, it is essential to 

 have some means for the operator to observe and adjust these images. 

 The optical monitoring system provided consists of an outgoing mon- 

 itor and means for adjusting the scanning beam, and an incoming 

 monitor and means for adjusting the position of the viewing lens to 

 suit the height of the sitter. 



The outgoing monitoring system is the same as that used in the one- 

 way television apparatus which has already been described. A small 

 neon lamp (Fig. 14, at bottom of top disc) is placed behind the sending 

 disc but displaced several frames from the aperture through which the 

 arc lamp beam passes. By continuing the spiral of holes part way 

 around it is possible to see the complete image from the auxiliary neon 



