The effect is caused by the brightness-dependent delay in the perception of optical stimuli. This delay time is around 250 to 300 milliseconds near the visual threshold and is reduced by 3 to 4 milliseconds in full sunlight.
The effect is based on the fact that light optical stimuli are perceived by the eye more quickly than dark stimuli. The optical information of an object that is darker in the field of vision thus reaches the brain later than that of a light object.
If an object moved sideways is viewed with both eyes, with one eye being darkened, the image of the object of the darkened eye is perceived a little later . This small time difference in perception leads to a sideways moving object that it appears to be in slightly different places for each eye. This apparent parallax is interpreted as depth information. This creates the distinctive "3D effect".
In the example, this effect can be experienced well with a pendulum swinging back and forth and a "darkened" eye. The pendulum swinging in its plane (i.e. two-dimensional) seems to rotate on a circular path (three-dimensional). The apparent direction of rotation changes with the change in the darkening of the left or right eye.
The effect is also visible when observing the moon at night when its light is scattered on clouds. When the axes of the eyes move, the clouds jump in front of the moon because they are darker and they are perceived with a delay. In the case of total lunar eclipses, this effect is even more pronounced due to the lower brightness and the greater perceived delays.
Use of the effect
A better effect is obtained when using a film camera that is moved to the side, usually from left to right. The recorded film can be as pseudo stereoscopic be considered. For real stereoscopic recordings, a separate image is required for each eye. So there must be two images recorded at eye distance that can be viewed with real 3D glasses .
Commercial application was the pulfrich effect the so-called Nuoptix method that with the RTL - TV show Tutti Frutti became known. Here, however, glasses with yellow and violet color filters are used, whereby the violet color filter darkened the image considerably. In addition, various animal programs were presented at ProSieben or kabel eins that used the process. An indirect use of the effect is used in the development of 3D television .
There is a similar effect with a comparable cause with time-of-flight cameras . These, too, register darker picture elements later than lighter ones, since in addition to the "flight time" it takes longer before a first photon is registered. This must be taken into account when evaluating the image.
- C. Pulfrich: Stereoscopy in the service of isochromic and heterochromic photometry . In: Die Naturwissenschaften , 10, 1922, in the June – September issues (Part I = Booklet 25, pp. 553-564; Part II = Booklet 26, pp. 569-574; Part III = Booklet 27, pp. 596– 601; Part IV = Booklet 33, pp. 714–722; Part V = Booklet 34, pp. 735–743; Part VI = Booklet 35, pp. 751–761)
- 3D video with "pulfrich effect" - video on YouTube