Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 90

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Fig. 4.7 A Hermann grid on the far left. In the center a similar effect in a file system. Adding
shading and color removes the effect on the right

between the luminance of the object and its background divided by the luminance
of the background, as shown in Eq. ( 4.1 ).

ð Object Luminance Background luminance Þ= Background Luminance ð 4 : 1 Þ

The contrast will be positive if the object is emitting more light than the
background and negative if the background is emitting more light than the object.
Objects can therefore be described as having positive or negative contrast.
Brightness is a subjective response to light. There is no real means of measuring
absolute levels of brightness but, in general, a high luminance from an object
implies a high brightness. It is possible to experience odd effects at high-to-low
brightness boundaries, as shown in the left part of Fig. 4.7 , which is called a
Hermann grid. Designers should be wary of creating effects like the Hermann grid
(in the center of Fig. 4.7 ) because they can be distracting. Adding color or shading
can remove this effect. Indeed, the design of folders on most modern desktops
avoids this by increasing the spacing of the icons both vertically and horizontally,
but this choice costs display space.
Related to the Hermann grid are the concepts of figure and ground: figure refers
to the objects that are to be attended to; ground refers to the background objects. In
Fig. 4.7 the boxes and files are the figure and the white background is the ground.
When the objects are placed too close together, however, the perceived gray fill
that results can become more prominent and appear to be part of the figure. It is
important to consider keeping objects that are figure prominent, and maintaining a
useful ground.
The interpretation of Fig. 4.8 is based on figure and ground. If the white is
perceived as the figure, it appears to be a vase or a goblet; if the black is the figure,
it appears to be two heads.
The objects that you want the users to see or distinguish need to be appropri-
ately sized. In good viewing conditions a minimal perceptible visual angle of about
15 min of arc should be maintained and in poor viewing conditions this should be
increased to 21 min. These correspond to a 4.3-mm object and a 6.1-mm object,
respectively, viewed from 1 m.
The full field of view for a stationary forward looking eye covers about 208
horizontally (although it is blocked by the head and the nose at certain points), and
about 120 vertically. This only refers to light falling on the eye, and does not