260 



VISUAL COMMUNICATION 



Searchlight Blinker Signalling 



Searchlight bhnker signalHng is perhaps 

 the most conunon of the supplemented visual 

 communication systems. Here, the sender 

 modulates in a temporal pattern the in- 

 tensity of a light directed at the receiver, 

 to whom the signal appears as a flashing 

 point of light. The pattern of long and 

 short flashes follows the same alphabetical 

 code that is used in auditory code com- 

 munication (13, 14). Like all other direct 

 visual communication, its utility is restricted 

 by the variability of the meteorological visi- 

 bility range. From the data available, it 

 would appear that all such modulated light 

 beam communication systems have a prac- 

 tical upper limit on speed of communication 

 of some 15 words per minute. The average 

 speed with which messages may be received 

 is approximately 12 words per minute, which 

 is strikingly slower than the speeds obtained 

 with verbal communication, but only slight- 

 ly slower than that of auditory code. 



In blinker signalling a problem arises 

 which is also pertinent to mediated visual 

 communication systems which employ in- 

 tensity modulation of a stimulating light 

 source: does the present maximal sending 

 and receiving rate represent the highest 

 obtainable maximum? There is reason to 

 believe that this is not the case, but that suit- 

 able research procedures might reveal tech- 

 niques whereby the transmission rate can 

 be increased. First, studies (3) of critical 

 flicker frequencies (the rate of flashing which 

 is just too high for the retina to be able to 

 respond to the separate flashes, with the 

 result that the flashing source is seen as a 

 dimmer but steady light) show that the ret- 

 ina, at least, is able to keep up with a much 

 higher rate than that equivalent to the pres- 

 ent spacing of dots and dashes in blinker 

 signalling. There are also data which in- 

 dicate that the critical flicker frequency is 

 not the same for light of different wave- 

 lengths, but is higher for the shorter wave- 

 lengths, and also that this difference is de- 

 pendent on the light-dark ratio. It is not 



unlikely that if suitable investigations were 

 made with the view of relating blinker- 

 reading to critical flicker frequency func- 

 tions, and the nature of the relationship, 

 if any, were established, our knowledge of 

 the flicker function might be employed to 

 increase both the speed and accuracy of 

 blinker communication. 



A second possibility for improving the 

 efficiency of blinker signalling suggests itself 

 in that blinker signalling now makes no use 

 of the position variable in visual stimulation. 

 The use, for example, of four blinking 

 sources, arranged at the corners of a square 

 whose size is determined by the distance 

 of the receiver should make possible the use 

 of simpler codes, requiring less transmission 

 time per letter. The position and number 

 of the points of light, and perhaps illusory 

 movement, in the code, should improve the 

 speed and accuracy of communication with- 

 out unduly complicating the apparatus. 



Blinker has been widely used in recogni- 

 tion signalling. It has proven reasonably 

 effective. However, when it fails, it fails 

 with a vengeance. The flash of the blinker 

 searchlight or Aldis lamp is all too readily 

 mistaken for the fire of light arms at short 

 ranges, and for anti-aircraft fire when seen 

 from great distances. Well-trained pilots, 

 alert to attack, all too often have fallen 

 victims to this easy misinterpretation of a 

 flash, with at least uncomfortable, if not 

 disastrous, consequences for the vessel em- 

 ploying blinker in this way. In Naval war- 

 fare, blinker is dangerous as a recognition 

 method. 



Signal Flags 



Signal flags represent the visual code par 

 excellence. Flags may be selected to sym- 

 bolize not only letters of the alphabet, but 

 also complete messages. The difficulties en- 

 countered are, however, numerous. Not 

 only are flags too slow for most transient 

 communication, but wind often renders con- 

 trol of their mode of presentation impossible. 

 Finally, the demands made by them on hue 



