CenterVue
COMPASS Operating Manual
Pag. 22 di 53
9.11 Retinal tracking
Eye tracking is a fundamental component of Fundus Perimetry.
Infrared retinal images
, acquired at the rate of 25 images per second, allow for continuous,
automated, tracking of
eye movements
. Determination of eye movements allows, in turn,
active
compensation of fixation losses
, with perimetric stimuli being automatically re-positioned prior to
and during projection based on the current eye position. This mechanism reduces test-retest
variability and ensures accurate correlation between function (i.e. retinal threshold values) and
structure (retinal appearance). Compensation of eye movements takes place before and during
projection of a certain stimulus. In absence of this mechanism shifts in eye position occurring at the
time of projection of a certain stimulus even in healthy subjects easily produce artifacts in VF
results.
Regular operation of the eye tracking is indicated by a GREEN frame surrounding the retinal image
(see Fig. 17).
A RED frame indicates that eye tracking is NOT operational
: in such case stimuli
cannot be projected and
the test is paused until the retina can be tracked again
. Typical
reasons for tracking failure include: blinking, small pupil (requires dilation), closed eye lids,
incorrect subject position (e.g. far from forehead rest).
Fig. 17 - Tracking active (left) and on hold (right)
