3D With Half a Pair


An avionics designer wearing dark glasses and watching an in‑flight movie (during a scene with significant motion) suddenly realizes that he is seeing the scene in 3D.  This occurs beginning a moment after the left lens of his glasses pops out.  Has he discovered a new approach for 3D navigation displays?  The 3D effect can be explained as (select the best answer):

a.     The subtle change in the refraction of the light path as seen in the left eye (when compared to the right eye) causes the customary two dimensional movie image to appear to have gained a third dimension as the brain interprets the resulting images created in the retina.

b.     The brain reacts to the delayed light path seen by the right eye and compensates for this out-of-phase condition by creating the added third dimension for each instant of motion.

c.     Motion parallax is induced as the view compensates for the disturbed light path seen by the right eye which is interpreted by the brain as an added dimension.

d.     Both a. and c.

The answer is:

The image seen by the right eye is delayed by the presence of the dark lens.  The image seen by the left eye is not delayed.  The brain composes a scene based on the interaction of the nondelayed and delayed sources of light striking the retina which effectively results in a displacement of the scene either in front or behind the screen depending upon the direction of motion and which eye’s light path is delayed by the darkened lens.  As seen in the diagram, the right and left eye view the same scene at slightly different times.  A slight delay is introduced into the light path of the right eye (by the dark glass lens) and the image on the retina is interpreted by the brain as one compensated by the intersection of the two light rays at a displaced point from the plane of view.  Tracing the locus of these successive intersection points reveals an elliptical path as seen in the diagram.  The reader can demonstrate this phenomenon by fashioning a pendulum with a light weight hanging on the end of a string from the top of an open doorway and imparting a swing to the weight so that it oscillates in a plane.  The observer, covering one eye with a dark lens, directing his line of sight perpendicular to the plane of the oscillating pendulum will see an elliptical pattern described by the weight during its swinging motion.

Note:  This 3D effect was first noted by a German physicist by the name of Pulfrich (earlier in the century) and is known as the Pulfrich effect.  In recent years, the effect has been used for commercial and research purposes.

Pendulum Experiment (top view) 

The pendulum swings along the major diameter of the ellipse.  The observer views the scene with the right eye line-of-sight interrupted.  Examine the mid point of the pendulum swing and assume the right center block is the leading image as the pendulum’s motion is from left to right.  The left eye sees the right block and a fraction of a second later the right eye sees the left center block.  The two lines-of-sight intersect at the center point of the lower limb of the ellipse.  Conversely, as the pendulum swings from right to left, the left center block repre-sents the pendulum as initially seen by the left eye and the right center block represents the delayed image of the pendulum as seen by the right eye.  The lines-of-sight of the two eyes inter-sect at the midpoint of the ellipse’s upper limb (Figure 37 ).

The ellipse is the path of intercepts of the successive images.  This experiment illustrates the 3D illusion and demonstrates how two dimensional motion in a plane can be interpreted by the brain as three dimensional.

Figure 37.  Pulfrich Effect