Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 88

Blind
spot

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Fig. 4.6 Cover your right eye and focus directly on a digit with your left eye. Then move the
page away from your head. At about a foot the word blind spot will disappear. The location of the
blind spot and screen sizes will vary, so you may have to try different focus points

When you switch from looking at colored objects in bright light to looking at
the same objects in dim light, all other things being equal you will notice that one
object now seems brighter than the other. If you look at a blue flower and a red
flower, for example, first in daylight and then at dusk, you will notice that under
low illumination both appear faded, but the blue seems brighter than the red.
Likewise, a piece of green paper and a piece of red paper which are matched for
brightness in good light will not be matched in dim light. This effect is called the
Purkinje Shift, and is based on the fact that long wavelength colors such as red
appear duller under low illumination than shorter wavelength colors (such as blue).
The effect occurs because of the shift from high illumination vision (cones) to low
illumination vision (rods) under different light conditions. The rods are relatively
more sensitive to light in the blue region than the cones, hence the apparent greater
brightness of the blue flower in dim light. These relationships can be seen in
spectral sensitivity curves that illustrate that maximum sensitivity goes from red to
blue green (i.e., to shorter wavelengths) when we shift from bright to dim light and
from the cones to the rods.
One final point about rods and cones: there is one part of the retina where there
are no receptors (rods or cones) present. This is where the optic nerve leaves the
retina. When the image of an object falls on this blind spot, nothing is seen. You
can use Fig. 4.6 to help you find your own blind spot. The blind spot is in a
different place for each eye.

4.3.5 Implications for System Design

If your system is going to be deployed in an area where users experience large
step changes in lighting conditions on entering that area (from light to dark or vice
versa) you will need to take account of the fact that their eyes will need time to
adapt to the new lighting conditions. You could consider having gradual changes
in lighting levels as they enter the area, for example.
People will often use a relatively large dialog box positioned in the center of the
screen to get the user's attention for something that is important. This dialog box is
usually large enough to project an image onto the retina that fills a significant
portion of the fovea. It would be equally possible to capture the user's attention by
make an item on the display screen move, or make it flash or blink, as long as