Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 89

the item is located in a position that projects onto the periphery of the retina.
In this way the movement or flashing gets detected by the rods in the eye. There is
a negative implication here too: if your user is carrying out an important task that
requires high levels of concentration using the display screen, you should try to
avoid having items on the screen move or flash, particularly if they would project
onto the periphery of the user's eye.
If you are designing a system that has to be operated in light sensitive condi-
tions, such as a photography dark room or a radar operations room, you need to
consider how you can help the users see so that they can carry out their tasks in the
dimly lit conditions. Usually in these situations, red light is used to illuminate
the rooms, albeit at quite low levels. The rods are relatively less sensitive to light at
the red end of the visible spectrum, which means that the rods start dark adapting
even though there is still some light available.
In most cases people using technology work at arm's length from their display
screen. This means that the blind spot is usually not a problem, because the brain
processes the images from both eyes together to perceive what is on the screen. If
you have a system which requires the user to be much closer to the screen, and
they are not detecting some items that are important, you should consider inves-
tigating whether those items are in fact located in their blind spot.
4.4 Low Level Visual Perception
Here we consider the low level details of vision. These range from how light is
detected, through various aspects associated with color, to flicker and pop-out effects.
Several of these aspects of low-level vision have implications for system design.
4.4.1 Vision and the Measurement of Light
There are two important ways in which light gets from an object to the eye:
incident light (light falling on an object) and reflected light (light reflected from an
object). Incident light is referred to as illuminance whereas reflected light is termed
luminance. White surfaces typically have reflectances of 80% and black surfaces
around 10%.
Luminance is measured in candelas per square meter (cd/m 2 ). As the luminance
of an object increases, so does the eye's visual acuity or ability to discern small
details. The pupil's diameter decreases and therefore increases the depth of focus in
the same way as a standard camera lens when the aperture is adjusted. An increase in
luminance of an object or display will also make the eye more sensitive to flicker.
Contrast describes the relationship between light emitted from an object and
light emitted from the surrounding background. It is defined as the difference