Vision Science III  Ocular Motility & Binocular Vision
Lab 2  The Apparent FrontoParallel Plane (AFPP) Horopter
THE THEORETICAL HOROPTER
A
horopter
is a set of points in space where an object must be placed if it is to stimulate exactly
corresponding points
on the two retinas. The horopter also represents a set of points that cause
zero
disparity
on the retinas. If a visual line is projected out of one eye, and the corresponding visual line is
projected out of the other eye, the intersection of these is a point on the horopter.
In theory, the horopter is an arc that lies on a circle that goes through the fixation point and the nodal points of
the two eyes. This theoretical horopter is known as the
ViethMüller Circle
; each different fixation distance
has its own its own ViethMüller Circle.
The ViethMüller Circle (theoretical horopter) is based on certain geometric assumptions about the eyes.
These are:
•
Each retina may be represented by a perfect circle.
•
Corresponding points are evenly space across the nasal and temporal retinas of each eye.
•
Both retinas are the same size and corresponding points are perfectly matched for their locations in
the two eyes.
These are reasonable assumption for model eyes. Based on these assumptions, if you project corresponding
visual lines out of the two eyes and plot their intersection points, they should form a circle—the ViethMüller
Circle.
This geometry has some interesting characteristics, as illustrated in Figure 1. Since all points on this circle
are projections from a common chord (PD), the binocular convergence angle between the eyes to any point
on the circle is always the same (Angle
θ
). The alternate angles at Point O formed by the crossing of the
visual axes are equal, and two similar triangles are created—triangles FON
1
and PON
2
. Then Angle
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 Spring '08
 Bill
 Vieth Müller Circle

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