KINDS OF FACTOES INFLUENCING VISIBILITY 



107 



the glass tube in the direction of the ob- 

 server. Some is reflected back into the bulb 

 and finally ends up at points on the screen 

 remote from the spot. Law (28) lists the 

 followang as factors unfavorable to good con- 

 trast: (a) bulb wall reflections, (b) screen 

 curvature, and (c) halation, i.e., internal re- 

 flections in the glass adjacent to the spot 

 which give rise to concentric rings or "halos" 

 of light whose separation is proportional to 

 the thickness of the glass. He compared 

 these factors with two others, viz., room il- 

 lumination and stray electrons, and con- 

 cluded that halation was much the most 

 important factor in CRT design. Stray 

 electrons are no longer bothersome because 

 of improved electron guns. Bulb wall reflec- 

 tions apparently are minimized by their 

 being diffused, a consequence of proper bulb 

 shape and internal coatings. (A comment 

 on this is of interest, because the cone shape 

 of American tubes may or may not be the 

 best possible from the diffusion standpoint 

 alone and one wonders why the German 

 tubes are so nearly cyhndrical. One would 

 think that a more nearly spherical bulb 

 would be preferable, provided other design 

 considerations do not outweigh it.) Screen 

 curvature is somewhat more important than 

 bulb wall reflections but is still not a limiting 

 factor in contrast, except insofar as it may 

 enhance halation. This matter apparently 

 has not been studied for its relevance to 

 radar. Halation alone. Law found, could 

 limit contrast to a ratio of six to one; but it 

 can be materially reduced by the introduc- 

 tion of light-absorbing material into either 

 the glass face or the binder of the phosphor. 

 In amounts of 10 to 20 percent, this absorb- 

 ing action will have a negligible effect on the 

 rays directly transmitted through the glass 

 to the eye, but will largely eliminate the 

 weaker halation reflections. This seems to 

 be a good enough idea when considering tele- 

 vision's high-brightness CRT's, as Law was, 

 for these tubes produce more than enough 

 hght for good vision anyway, but whether 



the principle should be apphed to low lumi- 

 nosity radar scopes is another matter. The 

 writer has been able to find no evaluation of 

 the question. 



An additional factor has been studied by 

 Bachman (2), namely, the effect of non-re- 

 flecting glass surfaces. Tests made with 

 non-reflecting lead glass yielded contrasts 

 superior to those of good-quahty untreated 

 glass. Whether the same would hold for the 

 several other types of non-reflectance treat- 

 ments is not known. In fact, we have not 

 been able to ascertain whether the commer- 

 cial CRT faces are actually treated. 



The new metal-backed screens may as well 

 be mentioned here, because they represent 

 one attempt to save some of the light ordi- 

 narily lost in internal reflections. Bramley 

 (7) has confirmed the work of Bachman (2) 

 and of Epstein and Pensak (12) which 

 showed that a thin metalhc coating on the 

 gun side of the phosphor screen actually re- 

 flected back toward the observer's side a 

 sizeable fraction of this hght, approximately 

 doubling the light output of the tube. The 

 metal backing acts like a mirror, but one, 

 fortunately, which is pervious to electrons. 

 Total light increase is greater than contrast 

 improvement, but both together ought to be 

 beneficial for detecting weak radar signals. 

 No known experimental test has been made. 

 In addition, the luminous efficiency is said to 

 be increased, at least at higher anode volt- 

 ages, because the metal is an aid in reducing 

 the space-charge on the screen which nor- 

 mally prevents faithful following of the 

 anode potential. If true, the metal backing 

 deserves a real try-out on radar CRT's. At 

 this date it is only just beginning to be used 

 in the television tubes. 



Phosphor Screens 



The color and decay characteristics of 

 commercial screens allow a narrow selection 

 for the researcher. If the psychology or 

 physiology of phosphors is to be knowTi, it 

 may eventually be necessary to carry out 



