Nelson & Associates :: Human Factors :: Visual Acuity & Line of Sight
The capabilities and limitations of human vision is a prime subject matter of human factors engineering as applied to the design of walking-working surfaces.
The Visual Field.
During the walking process, it is not only normal but also critical for one's safety to pay attention to a host of objects in one's visual field. One's "visual field" is not limited to any specific line of sight, but rather, it includes everything that is available to be seen, both near and far, both conspicuous and inconspicuous. However, whether or not individual objects in one’s visual field are actually seen or perceived depends on specific dynamic factors involving issues related to line of sight, visual acuity, the limitations of peripheral vision, expectancy, the task being performed, and the character of surrounding activity and other visual distractions.
Due to limitations of time and visual acuity, awareness of one's surrounding environment is conducted by a process of sampling -- that is, by focusing on a few but not all features within one's field of view. This is very important to understand. Given natural and ordinary human factor limitations, and especially those of human physiology, just because an object is "available" to be seen in one's field of view does not mean that it "will" be seen by ordinary people behaving themselves in an ordinary prudent manner.
Normal Line of Sight.
"Looking where you are going" or "paying attention to what you are doing" during the walking process is not synonymous with "looking at your feet" or "paying exclusive attention to your feet." Not only is "looking at their feet" while walking an unnatural practice, in most circumstances, it can actually be a dangerous practice. Normal line of sight during the walking process is about 10 degrees downward from horizontal. (See Figure 1) This means that objects at or near one's feet, to the far left or right, or overhead, must be seen in peripheral vision, a relatively poor means of perceiving detail.
Figure 1. Normal Standing Line of Sight.
Regarding visual acuity (the ability to see detail), it is important to realize and take into account that within one's forward visual field, only those objects that are in the center of the eye's focus are "in focus" and "seen" in any detail. This is called "foveal vision." Simply stated, objects seen in one’s foveal vision (direct line of sight) are seen in detail, while objects that are not in one’s foveal vision are not seen in detail. For example, if one stares at the first word in the headline of a newspaper, one cannot read the entire headline without moving the eyes across the page; or, if one is looking at the face of someone standing next to them, and that person is wearing a nametag, one cannot read the nametag while continuing to look at that person's face. This inability to see detail without looking directly at an object can be illustrated through many other common experiences.
Since the human eye can only see the detail of objects specifically focused on, and since this focus activity is effected by normal body movements (such as turning maneuvers) and the available time (or more accurately, the unavailable time) needed for eye movement and focus from one specific point to another, most of what is actually seen during most human activity, especially in a novel or relatively rich visual environment, is perceived with peripheral vision (that portion of the visual field that is located outside the central area of focus), an excellent detector of motion, but a poor detector of color, distance, and detail. Naturally limited as such, as persons glance at eye level in the direction they are walking (or intend to walk), if no specifically threatening (high visual contrast) hazard is immediately perceived, and trusting that others having control over the walking surface have exercised ordinary care for their safety, little further attention is given to the surface. That is, in order to become aware of potential hazards in the walking surface, if persons are to be given a chance of their detection, such hazards must be conspicuous enough, unusual enough, and threatening enough to attract special attention from a distance.
All peripheral vision does not provide the same degree of vision in terms of object detection and perception. Human factors references indicate that the placement of objects in one’s visual field for best recognition should take into account that optimum eye rotation left and right from one’s normal line of sight is 15 degrees, and the maximum angle of eye rotation to the left and right is 35 degrees. Optimal eye rotation upward and downward is also 15 degrees, while the maximum upward eye rotation is 25 degrees, and the maximum downward eye rotation is 30 degrees.
Two different regions of one's peripheral field of view might be described as one’s near peripheral field, the region of maximum eye rotation away from one’s direct line of sight (within 25 degrees upward, 35 degrees left and right, and 30 degrees downward), and one’s far peripheral field, the region of one’s visual field where eye rotation plus head rotation must be used to achieve a direct line of sight. Obviously, all other things equal, objects in one’s near peripheral field will be more readily detected that objects in one’s far peripheral field.
A practical application of these human visual capabilities and limitations is illustrated in regard to the marketing strategies used by retail establishments to display primary merchandise where customers will most naturally see it.
Visual Acuity vs. Perception.
It is important to understand the difference between visual acuity and perception. Visual acuity is the ability of the eye to clearly perceive spatial detail. It is the ability to detect and truly "see" the fine detail of objects in one's visual field. Perception, on the other hand, involves the process of not only detecting an object in a general sense, but also comprehension of the object's significance.
Perception must occur before reaction can take place, and the analysis of what persons perceive or do not perceive in their surrounding environment must take into account that most objects in our visual field are perceived below the conscious level. That is, just because objects are within our immediate visual field, does not mean that they are perceived at a conscious level.
Just as visual acuity depends on the fixed characteristics of human physiology (that is, on characteristics of human vision that have specific capabilities an limitations) as well as the complexity of the external environment, perception also involves certain relatively fixed characteristics of the human thought process, which among other things, is strongly influenced by elements of the prevailing task and surrounding environmental complexities that tend to distract the mind from comprehending the significance or meaning of every object that is otherwise available to be seen and understood.
Perception (again, object detection and comprehension of the object's significance) is further affected by the quality (strength) of sensory conditions such as lighting, color, color contrast or camouflage, as well as the degree of expectancy related to the object to be seen.
Perception vs. Time to Perceive
and the Process of Visual Sampling
Another aspect of visual acuity that should specifically be taken into account is that the process of focusing one's line of sight on various objects, these being the only objects in one's visual field that are seen with clarity, takes precious time. That is, the time necessary for eye movement and focus will significantly limit the number of specific objects that can be truly seen and understood as one moves through space during the walking process. In real-world tasks, such as during the walking process, or while engaged in ordinary (non-emergency) activity involving casual searches of the visual field for specific objects (such a searching for one’s favorite soup or cereal at a grocery store), individuals seldom make more than two or three eye movements per second (eye movements can typically take 0.48 seconds each), and accurate perception is possible only within an area of about 2 degrees of arc around the point of fixation (try reading a newspaper headline while focusing only on the first word).
Vision shifts involving differences in focal distance, from near focus to distant focus, or from distant focus to near focus, take additional time. Even for highly trained and "visually fit" airplane pilots, the time to shift focus from a relatively near object to a distant object, and then return to focus on a near object have been measured as taking 2.39 and 1.5 seconds respectively to accomplish.
The Concept of "Expectancy" in Human Vision
Perception is affected by expectancy related to one's subconscious awareness or reliance on the surrounding environment as being normal in the sense that its physical condition conforms to what is most ordinarily encountered in terms of design. Such trust in the prudence of those who have designed or have control over the designed environment, while often misplaced, is common and strongly influences one's initial perception, thwarting the detection and understanding of an unanticipated danger.
These natural visual characteristics (normal lines of sight, random foveal focus points, peripheral vision, etc.) and their limitations explain why materials that cause slipping and objects that cause tripping are often not seen by reasonably prudent people until a slip or trip initiates a potential fall.Technical/scientific references and authorities supporting Normal Human Lines of Sight: • Evans, Henry, Traffic Engineering Handbook, Second Edition, 1950. • Joint Army-Navy-Air Force Steering Committee, Human Engineering Guide to Equipment Design, 1963. • Panero, Julius and Martin Zelnik, Human Dimension & Interior Space, 1979. • Marshall, Gilbert, Safety Engineering, 1982. • Miller, Barrett C., "Falls, A Cast of Thousands Cost of Millions," Safety & Health, February 1988.
© Nelson & Associates
- Evans, Henry, Traffic Engineering Handbook, Second Edition, 1950.
- Joint Army-Navy-Air Force Steering Committee, Human Engineering Guide to Equipment Design, 1963.
- Panero, Julius and Martin Zelnik, Human Dimension & Interior Space, 1979.
- Marshall, Gilbert, Safety Engineering, 1982.
- Miller, Barrett C., "Falls, A Cast of Thousands Cost of Millions," Safety & Health, February 1988.