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).
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.
Warning one
Occasionally monitors are placed
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 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.
REFERENCES
·
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.
·
www.ccohs.ca/oshanswers/ergonomics/offie/monitor_positioning.
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