256 



VISUAL COMMUNICATION 



require such a tremendously complex series 

 of visual symbols, each with its own mean- 

 ing, that only with extreme difficulty could 

 sender and receiver learn them. Planning 

 and content analysis of communications, di- 

 rected at reducing the symbols necessary to 

 a minimum, is a necessary starting point for 

 the further development of visual communi- 

 cation. 



Vakiations in Visual Stimuli which 



MAY Serve as the Basis of 



Communication Systems 



Before examining the present status or 

 the potentialities of various types of visual 

 communication systems, let us inquire into 

 some of the stimulus variables which might 

 serve as symbols in the transmission of mes- 

 sages. Since these dimensions are treated 

 in detail elsewhere in this book, they will 

 be considered only for their probable utility 

 in communication. Except where otherwise 

 specified, the discussion will relate to the 

 properties of a single visual stimulus. 



Brightness 



The human being is capable of making 

 numerous discriminations with respect to 

 the brightness of a visual stimulus. How- 

 ever, such discriminations are unreliable, 

 since they occur in the absence of a uniformly 

 illuminated background, with which the 

 stimulus can be compared, and depend great- 

 ly upon the level of adaptation of the eye. 

 Brightness is, moreover, a significant var- 

 iable only at night, when the background 

 illumination is negligible. In this situation, 

 it is a remarkably untrustworthy basis for 

 establishing discriminations, since it will de- 

 pend, not only on the brightness of the 

 stimulating source, but also on the distance 

 of the source from the receiver, and on the 

 atmospheric attenuation of the light. 



Brightness Contrast 



When a visual stimulus is presented 

 against a background of determined bright- 



ness, its brightness contrast may serve as a 

 basis for discrimination. Provided that the 

 background illumination is stable (as it is 

 in the interior compartments of a sub- 

 marine), and that the eye of the receiver 

 is adapted to that background, contrast dis- 

 criminations are stable and precise. It is 

 well-established that, within limits, the 

 greater the difference in brightness between 

 a stimulus and its background, the shorter 

 the reaction time to the stimulus when it 

 is first presented. It is probable that at 

 least four values of contrast may be em- 

 ployed in visual communication systems 

 without sacrifice of accuracy or speed of 

 discrimination. As with brightness, con- 

 trast is reduced over long ranges. 



Color 



Color suggests itself as a basis for dis- 

 crimination among stimuli for the purpose 

 of communication. Provided that the colors 

 employed are selected so that they are not 

 easily confused, either by the color-blind 

 or color-weak, and that they can be dis- 

 criminated rapidly (11), color can be em- 

 ployed as a basis for interior communica- 

 tions. 



Very serious difficulties arise in the use of 

 color discrimination for visual communica- 

 tions where any great distance may inter- 

 vene between the receiver and the stimulus 

 he is responding to. 



There are decided differences in the abil- 

 ities of humans to differentiate among colors. 

 These differences are exaggerated when the 

 discrimination is made more difficult, e.g., 

 when a colored light must be discriminated 

 at small subtense (as in blinker signalling), 

 or where it must be discriminated through 

 a column of air, which has the inevitable 

 effect of reducing the brightness and con- 

 trast of the source. Lights of easily dis- 

 criminable hue, when viewed through a great 

 space of atmosphere, are not accurately and 

 quickly discriminable by a person with nor- 

 mal vision, to say nothing of the color-blind 

 and color-weak. 



