Pulfrich Effect: Pendulum and polaroid experiment.

Pendulum and polaroid experiment. See figure 1a and 1b. In 1992, Pulfrich noted that a pendulum swinging in one plane 'appeared to have' an elliptical path when viewed with a neutral density filter in front of one eye. This startling illusion is due to perceptual mechanisms in the eye and circuits in the brain. See figure 2a. The pendulum ball is swinging left to right. The left eye looking through the neutral density filter sees a dimmer image of the pendulum ball. The rods and cones in the retina of this eye take longer to register and send their image to the brain. This is called a 'latency period'. 1 is the brightly illuminated image seen by the right eye. 2 is the delayed image seen by the left eye. The brain combines these images and the ball appears to be at position 3, further away than expected. In figure 2b, the pendulum is swinging right to left. In this case, applying the same reasoning shows that the image seems closer than expected. Looking at figure 3, one sees how the path of the pendulum appears to be elliptical. Another less obvious effect is that the ball appears larger when farther away, and seems smaller when closer. The perception of the speed of the ball is also distorted. In fact, the greater the speed, the greater the distortion. Thus, the use of a ND filter (1.0-2.5) in front of one eye creates distortions int he perception of depth, size, velocity and position. The Pulfrich effect has been implicated in vehicular crashes when eyes experience conditions of unequal illumination.