 
horopter


last updated: 20041112 
Given a binocular system, to be seen as a model of the human eyes. The horopter
^{1)} then, is a 3D curve that
can be defined as the set of points for which the light falls on corresponding areas in
the two retinas ^{2)}. This definition could be
equivalent to the intersection of a cylinder and a hyperbolic paraboloid
(exercise is to be done).
The curve is a kind of oneturn helix around a cylinder. In the special situation that he
horopter is in the horizontal plane through the eyes which contains the center of the
retinas ^{3)}, the horopter takes the form
of a circle, named the ViethMuller
circle, plus a line along the cylinder: the vertical
horopter.
The ViethMuller circle is the theoretical horopter, the more general case is
found in experiment.
The first to mention the horopter curve was Aquilonius (1613), but he
gave the word a different meaning. He thought that monocular objects were
perceived to lie in a plane, parallel to the face, containing the fixation
point. It was this plane he called horopter.
The modern definition of the horopter is from Vieth, in his "Uber
die Richting der Augen" (1818).
notes
1) From the Greek words horos (= boundary) and opter (observer).
2) This is called the condition of zero retinal disparity: one object seems to lay in the
same direction for both eyes.
There can be confusion about the definition of this term. When when you define
'zero retinal disparity' as that objects appear to lie in the same direction, or
at the same distance, you get a similar, but different curve.
3) The center region of the retina, where the light falls on when you look straight to an
object, is called the fovea.
With thanks to Edgar Erwin for his remarks about the curve. 