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program, and last 20-200 ms, which is just about the interval of the persistence
of vision.
The pupil of the eye reacts to the amount of light falling on the retina by
expanding and contracting in such a way as to keep that amount approximately
constant. This feedback mechanism operates within a certain fairly narrow range
of illumination (about 16-1), but the enormously greater variation in retinal illu-
mination (about 10 9 -1) demands an intermediate mechanism to allow the eye to
function over the whole range of illumination. This mechanism is called adap-
tation. Adaptation is one of the most profound and pervasive sensory phenomena.
Our eyes are prevented from adapting to what they are seeing by continually
moving. These normal movements, which are normally unnoticeable, are faster
and smaller than saccades, and described as micro-saccades.
You may have had the experience of entering a theatre from the bright outdoors
and stumbling to your seat in the darkness, tripping over the feet of people already
seated. In a few moments you will have adapted to the darkness, that is, you have
become more accustomed to the lack of light. This mechanism applies to the sense
of smell too: when walking into a kitchen where someone is cooking you will
notice the odors as very strong, but after a few minutes they become less
noticeable. These examples illustrate that after exposure to a stimulus the sensi-
tivity to that stimulus reduces; after removal of the stimulus, the sensitivity returns.
4.3.3 Using Eye-Tracking to Measure Eye Movements
A lot has been learned about how people read and use interfaces by using eye
trackers (Duchowski 2007 ; Holmqvist et al. 2011 ). Eye trackers are electro-optical
systems (typically cameras that can be automatically focused and adjusted) that
record where a user has focused their eyes. Simple systems measure where a single
eye is looking. Better versions of this simple system can do this without the user
having their head fixed, and many can now measure this non-intrusively.
Figure 4.4 shows a head-mounted eye-tracker and two example summaries of eye-
tracking data. More complex systems can measure both eyes and note how the
eyes work together (e.g., strabismus, a measure of how much the eyes have
focused onto something near to the head by rotating).
Simple eye-trackers can generally tell which line the user is looking at (to an
accuracy of typically 0.5-1 of angular measure, which is about a line of 12 point
text on a display at 12 in.). Better eye trackers can tell which letter or which part of
the letter the user is focusing on (down to 0.25).
A useful exercise that will help you to appreciate the accuracy of eye-trackers is
to compute the size of a 10 point (1/7 in.) letter at 12 in. using trigonometry.
When you have done this, you should see that the tolerance in eye-tracker accuracy
is less than the size of the fovea. So although we may know where the center of the
user's fovea is focused, there could be several other items that are also projecting
onto it and to which the user is attending.
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