In almost every man - machine system there has to be some kind of display some where if the operator is to know what is going on. In achieving this man needs and uses his eyes as his major source of contact with his environment. The eye furnishes him with his primary means of knowing things and finding his way about in life. And if man gets himself involved as a functioning part of a man-machine system, ones effectiveness is very often determined entirely by the acuity and efficiency with which one can use their eyes. The word display applied to man - machine systems, means some way of providing information which a operator cannot or does not get directly from his or her senses. A fuel gage is a display because it gives information about the status of the gas tank. If one have a curve pipe leading into the tank on a car, one cannot see how much gas is in it, even if one wanted to. Therefore, one must depends on the information provided by a display.

In technological language, physiologist call problems of visual displays "perceptual problems" because they demand more than just seeing. The eye and the brain are involved. In perception, the observer must not only see things but also interpret what he or she sees. We must relate what he or she sees to what he or she already knows and make judgments and decisions on the basis of what he or she sees.

This student will discuss two important visual functions: visibility and visual acuity. Visibility means recognizing the presence of a light or object without having to recognize its shape or form. One might also call it detectability. Visual acuity means that one also have to recognize its form or shape. Most all of us are familiar with the physician eye chart. This measuring devise compares our visual acuity with what is suppose to be that of the average person. When one looks up at the sky and can just barely see a blob which one thinks is an airplane way off in the distance that is a case of visibility or detectability. When it gets close enough so that one can see that it has two engines, that is visual acuity. Visual acuity and visibility are not two entirely separate visual functions. One must have visibility before one must have visual acuity; the later is just a higher order of the former. One could see something long before one can recognize it. In general, therefore, one should expect curves for visibility to be somewhat lower than those for visual acuity. In addition, problems involving the recognition of light or objects are almost entirely visibility problems. For example, radar is the eyes of the fleet. But radar never sees. It is people who sees. If their eyes fail at a critical time, because they either are inherently poor or are placed at an undue disadvantage by poorly designed equipment, then radar fails, the ship fails, the mission fails.

Somewhere in the middle of the spectrum, between 16 and 32 millionth of an inch, are the radiations that we can see. These visible radiant energy waves. Isaac Newton discovered in 1666 that if you pass a beam of sunlight through a triangular glass prism you get a wide band of different colors. If we look at radiation that has only one wavelength in it, we will see a color. One of the 150 different identifiable colors in the spectrum. We do not have names for all of these different colors (i.e. violet, blue, green, yellow, red) can be seen in the spectrum. Light waves are commonly measured in millimicrons, or angstroms units.

When one talks about the sensitivity of the eye, it usually refers to the fact that the size of the light influences visibility. The point sources of light are much more efficient light signals than are large sources. The visibility of the point source is also a function of the brightness of its background. At star light levels, a point source can be seen if it is very dim, but at moon levels a point source must be 10 to 15 times more intense in order to be seen. In daylight, a point source must be extremely intense in order to be seen. Brightness contrast is how much lighter or darker an object is than its background. This difference is expressed as a ratio of the background brightness to a measure called percent brightness contrast.

Higher levels of contrast are needed by older persons to identify and to discriminate real world targets, including signs (Owsley and Sloane, 1987). The visibility distance of text and symbolic signs among young observers. Researchers have found that symbolic signs could be identified at about twice the distance of their text versions (Jacobs, Johnston, & Cole, 1975). The advantage of the symbolic signs are even more apparent under conditions of blur induced reductions and acuity. Researchers have also found that the visibility of symbolic signs was greater than that of text signs for all age groups.

Although the magnitude of the difference varies with the individual sign and the observer age, the visibility of the standard and the improved symbolic signs was, on average, greater than that of text signs. The visibility distance of standard symbolic signs are about double those of corresponding text signs under daylight conditions among all age groups ( Jacobs 1975). The visibility distance of standard symbolic signs exceeded that of their text equivalents. Because of their highly restrictive effects on message length, however, increased letter size and separation are not viable means for enhancing the visibility of most text signs. In the contrast, a comparable low pass format would be effective most, perhaps, for all symbolic signs. Presently, the visibility distances of symbolic signs that were improved using an optical filtering approach to reduce their dependance on high spatial frequency information. The visibility of the improved signs, which exceeded that of the text versions, was also about 50% greater than that of the standard symbolic signs. Most importantly, this improvement was realized with little adverse effect on the signs comprehensibility.

The differences in the relationship between acuity and visibility distance for the sign types show the utility of acuity for predicting the legibility of text signs as well as it limitations for assessing the visibility of symbolic signs. The researchers observed a strong relationship between visual acuity and the visibility of text signs is probably attributable to their mutual dependence on the discrimination of fine and high contrast details (Kline 1990). In fact, there was a very close correspondence between the gap size of text sign letters and the mean best eye acuities. By contrast, the relationship between visual acuity and visibility of the improved symbolic signs may reflect the difference in the spatial frequency domain of the two tasks. Researchers have found that age and contrast sensitivity at low and intermediate spatial frequencies were better predictors than was acuity of the contrast threshold for the detection of the identification of faces, every day objects, and signs.

Literature Cited 


Jacobs, R. J., Johnstson, A. W., and Cole, B. L. 1975. The visibility of 
     alphabetic and symbolic traffic signs. Australian Road Research, 5, 
     68-86 

Kline, T. J. B., Ghali, L. M., Kline, D. W. 1990. Visibility distance of 
     highway signs among young, middle-aged, and elderly observers. Human 
     Factors, 32, 609-619 

Owsley, C. E., and Sloane, M. E. 1987. Contrast sensitivity, acuity, and 
     the perception of "real-world" targets. British Journal of 
     Ophthalmology, 71, 791- 796.