Positioning the Monitor
Why should we worry about positioning the monitor?
Postural discomfort and the consequent aches and pains result from the effort to view the monitor when it is set at the wrong place in relation to the operator's position. Two factors come into play: viewing angle and viewing distance. Viewing angle refers to the degree above or below an imaginary horizontal line at the level of the viewer's eyes and the centre of the object being looked at; in case of a computer work it is the centre of the screen. - See Figure 1. Viewing distance refers to the space between the operator's eyes and the screen. - See Figure 1. Clearly these factors are critical for correct placement of a computer monitor. A poor angle leads to postural (neck and shoulders) discomfort, while the wrong distance can contribute to eyestrain.
The existing guidelines and recommendations on both viewing angle and particularly on viewing distance differ, sometimes significantly from one another. For this reason they should not be taken as a commandment "carved in stone" but merely as guidelines. As such, they can be used as a starting point for tailoring any given situation.
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 15 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.
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).
What should you know while setting up 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 40 cm to 70 cm (about 15 - 27 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.
Farther is better
If we just consider viewing distance, farther is better. So where do recommendations for maximum viewing distance come from? The arm's-length limitation most likely came from recommendations on monitor placement in cockpits.
Some try to justify a limit to how far away the monitor can be placed with the argument that if the screen is beyond a certain distance, you might not be able to read the letters.
It's clear that if you can't read the characters, the viewing distance is goo great. Or is it? Instead of moving the monitor closer, why not make the characters larger? In fact, guidelines recommending close viewing distances can only encourage the computer industry to maintain relatively small characters. Those in turns forces closer viewing distances and can perpetuate eyestrain.
How close is too close?
It is difficult to set an exact limit for a minimum viewing distance. If sustained viewing closer than the resting point of vergence contributes to eyestrain, perhaps we should say that eye-screen distance should not be closer than the resting point of vergence. (On average, about 45 inches away at horizontal eye level and 35 inches away with a 30-degree downward gaze angle.)
But there are no cliffs in ergonomics (one inch closer and you fall): If your RPV at a 30- degree downward gaze angle is 35 inches, your eyes are not going to fall out with a viewing distance of 30 inches.
Somewhere between your resting point of vergence and 6 inches in front of your nose you are going to experience discomfort. That distance is a combination of gaze angle, how long you've been working at the computer, your individual visual system's capabilities, and a number of other factors.
Does this help to put an absolute number on how close is too close? Recommendation is at least 25 inches. But closer-viewing distances do not bother some people.
How Far is Too Far?
The reality is that there is no limit, based on visual fatigue considerations, to maximum viewing distance at computer workstations. From what we know about visual strain, farther viewing distances are better, at least up to the RPV. For example, if the RPV is 35 inches, an eye-to-screen distance of 25 inches is preferred to 20 inches. Thirty-five inches is better than 25 inches. Viewing distances beyond 35 inches (the RPV in this case) should neither increase nor decrease eyestrain.To allow for greater eye-to-screen distances, we need software programs and monitors that allow font sizes to be increased easily. We need guidelines that don't force people to sit closer to their monitors than the distances at which they are comfortable.
· Fisher, R.F. (1977). The force of contraction of the human ciliary muscle during accommodation. Journal of Physiology, London.
· Jaschinski-Kruza, W. (1988). Visual strain during VDU work: the effect of viewing distance and dark focus. Ergonomics.
· Collins, C., O'Meara, D., and Scott, A.B. (1975). Muscle strain during unrestrained human eye movements. Journal of Physiology, London.