What should you know while setting up an appropriate viewing angle?Researchers agree that at rest, the eyes naturally assume a straightforward (see Figure 2) and downward cast (see Figure 1) - a normal line of sight. How far downwards, however, is not clear. Experimental findings range from about 10 degrees to almost 30 degrees. People engaged in visually demanding tasks limit their downward eye movements to about half of the whole available range of 60 degrees.
Figure 2
Therefore, for comfortable viewing of images on a computer screen it is probably reasonable to place the monitor at about 15 degrees (or slightly lower) below the horizontal line. Such a location creates a preferable visual zone of 30 degrees (+ 15 degrees to -15 degrees from the normal line of sight).
Numerous field studies among people doing intense visual work indicate that looking upwards (above the horizontal) is tiring. On the other hand, looking downwards, that is, lower than 15 degrees below the horizontal, was not reported as particularly fatiguing. This finding allows one to extend the visual zone downward by another 15 degrees (an acceptable visual zone) for a total of 45 degrees. See Figure 3.
Figure 3
In many, if not the majority of offices, owing to real or imaginary space constraints, it is common practice to place the monitor on top of the computing module commonly called CPU. A monitor located that high is a source of discomfort and, in the long run, can cause musculoskeletal problems in the neck and shoulder area. At a workstation where the desk and chair heights are properly adjusted, the monitor should be placed at the same level as the keyboard. The fact that discomfort caused by a monitor which is too high (above the horizontal) is worse than one which is slightly too low (below an acceptable visual zone) should be kept in mind while arranging a monitor at any workstation.
Warning two: When using a larger monitor (17", 19" or larger) or one that is oriented to the "portrait" position, make sure that the top of the screen is not at a level higher than the operator's eye.
What should you know while setting up (selecting) a proper viewing distance?Looking at far distances does not cause eyestrain - it is the muscular effort required to focus on objects at close distances that strains the eyes. The distinction between "far distance" and "close distance" is never "razor sharp". It is not only individual but it also can change over time, for example, due to age.
Accommodation* and convergence** are the two main functions, which govern the viewing of objects at closer distances. The shorter the viewing distance, the greater the muscular effort required for accommodation and convergence, and which, in turn, increases the risk for eye discomfort. A greater viewing distance, on the other hand, eliminates the risk for eyestrain but can make the resolving of the finer images or characters displayed on the computer screen more difficult. Consequently, the right viewing distance is the one at which the computer operator can easily read the screen without experiencing eyestrain.
The eyes have a default accommodation distance, called the Resting Point of Accommodation (RPA) and a default Resting Point of Vergence (RPV). In practical terms, at distances greater than the RPA and RPV, there is no need for either accommodation or convergence. Under these conditions, individuals with perfect (20/20) vision or those with properly corrected vision are not likely to experience eyestrain (assuming they take appropriate eye "rest breaks" from focussing on the screen).
Numerical values of RPA -- around 75 cm (about 30 in.)-- and RPV -- around 80 cm (about 32 in.)-- are close to each other which makes setting the right viewing distance simple. The viewing range 60 cm to 80 cm (about 24 - 32 in.) provides visual comfort for majority of computer users. In the situation where the recommended viewing distance is too great for the operator to see images clearly it is better to increase the font size (images) than to force a shorter viewing distance.
* Accommodation is the automatic adjustment of the eye for focusing on near objects. The eyes achieve this by changing the convexity (or shape) of the lenses.
** Convergence is the automatic inward (toward the nose) turn of the eyes when viewing close objects.
Document last updated on August 21, 2001
Copyright ©1997-2008 Canadian Centre for Occupational Health & Safety
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