Stereoscopic displays: Shutter process
2023-02-18
Overview
The so-called shutter process is also a color-capable stereoscopic process that, unlike the polarization filter process, uses active glasses with liquid crystal shutters (formerly also mechanical shutters 22 / 23 ) in front of the eyes that can be made electrically opaque in synchronization with the image reproduction 21 / 22 (see Fig. 16 ). However, it has largely failed commercially due to technical challenges as well as lower profit margins for cinemas due to the high price of glasses as a consumable associated with each ticket sold 5 .
Functioning
With shutter glasses, only one eye at a time is alternately presented with the image on the display, while it is darkened for the other eye with the help of electronically controlled liquid crystal shutters 21 (see Fig. 17 ). Thus, normal displays can be used, which, however, must support a frame rate of at least 48 Hz, which is twice as high as the 24 Hz playback frequency required for smooth motion perception, since the effectively usable frame rate is halved by the alternating monocular images 5 / 21 / 22 and the stereoscopic effect would otherwise be at the expense of smooth playback.
In practice, however, much higher playback frequencies of up to 144 Hz are usually used, in which the partial images intended for one eye only alternate several times per effective frame of visual material in a process known as strobing in order to reduce the perception of flicker for particularly sensitive people 5 . With an actual playback frequency of 144 Hz and an effective playback frequency of 24 Hz, 144 Hz ÷ (24 Hz • 2 eyes) = 3 strobed images per eye per effective frame of visual material get displayed.
The liquid crystal shutters (also LC shutters) consist of two linear polarization filters twisted by 90 degrees to each other, between which there is a layer of liquid crystals which, depending on the voltage applied, influences the polarization of the light and thus also its blocking by the shutter 5 / 22 / 29 (see Fig. 18 ). If no voltage is applied, the crystals naturally form a spiral twisted by 90 degrees, through which the polarization direction of the passing light is also rotated by 90 degrees and this can pass the second polarization filter of the arrangement and thus the shutter as a whole 5 / 22 / 29 (see Fig. 18 upper part). If, on the other hand, a sufficient voltage is applied, the liquid crystals align themselves parallel to each other in such a way that they do not influence the polarization of the passing light and this is hence absorbed by the second polarization filter and thus the shutter as a whole 5 / 22 / 29 (see Fig. 18 lower part). Depending on the applied voltage, the shutter can thus be electronically switched to be transparent or opaque 5 / 29 .
The synchronization of the shutter with the display is usually done via a infrared or radio connection of each pair of glasses with the playback system, but sometimes also so-called white pulses are used 5 / 21 . These are very briefly displayed, completely white images that do not affect the image perception of humans, but can be used by the glasses via photosensors for timing coordination with the display 5 / 21 .
Benefits and drawbacks
Besides the mere possibility of color reproduction without significant color distortions or viewing angle restrictions, the shutter process allows the use of a single projector, which significantly reduces acquisition costs, or the use of the full resolution of a display. Also, the problem of poor optical channel separation can be circumvented by selectively displaying the individual partial images 5 / 22 .
However, there is the additional challenge of exact synchronization of glasses and display, the failure of which can also lead to the typical symptoms of poor channel separation, namely confusion and headaches 5 / 6 / 7 / 21 / 22 . Moreover, the costly and thus comparatively expensive glasses as consumables make this method unprofitable for most commercial purposes, which is why it has not been able to establish itself widely 5 .
Appendix
Fig. 16: Shutter glasses
Fig. 17: Alternating partial images in the shutter process
Fig. 18: Structure and function of liquid crystal shutters
Sources
Text
- Grasnick, Armin: Grundlagen der virtuellen Realität. Von der Entdeckung der Perspektive bis zur VR-Brille, Berlin 2020, p. 189 – 195.
Appendix
- Based on: Amidror1973: Xpand LCD shutter glasses.jpg, https://commons.wikimedia.org/wiki/File:Xpand_LCD_shutter_glasses.jpg , 14.02.2023.
- Based on: Locafox: Active-3d-shutter-technology.gif, https://commons.wikimedia.org/wiki/File:Active-3d-shutter-technology.gif , 14.02.2023.
- Based on: Kebes: LiquidCrystalDisplay-field off.jpg, https://commons.wikimedia.org/wiki/File:LiquidCrystalDisplay-field_off.jpg , 14.02.2023, and: Kebes: LiquidCrystalDisplay-field on.jpg, https://commons.wikimedia.org/wiki/File:LiquidCrystalDisplay-field_on.jpg' , 14.02.2023.
