In this method, a visual illusion of volume is created with the help of two simultaneous views showing slightly different angles or perspectives of the displayed scene and destined respectively for the left and right eyes of the observer. Special glasses worn by the observer are used in order to make only one of these views visible to the corresponding eye. These glasses use either polarising lenses or shutters synchronised with the display unit to make visible to each eye only its corresponding view.
The first of these configurations, also called passive, relies upon a special polarising mask placed over the display panel, which orthogonally polarises the light emitted by different pixels. Half of the pixels are dedicated to each of the views displayed at the same time. The lenses of the viewing glasses also have corresponding polarisers that filter out one of these polarisations, thus separating the views for the observer.
The other approach, also known as active, uses frame-sequential display of different views, in which a complete stereoscopic frame consists of two sub-frames displayed one after the other and destined to the right and the left eyes respectively. This does not require any special design of the display panel, but in order to separate the left and right views it is necessary to use headgear with shutters synchronised with the frame display. One of these shutters becomes transparent, while the other one turns opaque with each successive sub-frame, making the corresponding view visible to one of the observer’s eyes only.
Most so-called 3D TV’s (notably Sony, Samsung, and Panasonic) are frame-sequential and use active shutter glasses. In a television screen or a panel display, the refresh rate is defined by the speed and the frequency at which the image displayed on the screen is updated. The higher the refresh rate (measured in Hz, i.e. cycles per second), the smoother the observer’s viewing perception of the image is. An image updated every 17 milliseconds approximately corresponds to a refresh rate of 60 Hz. This rate can produce a satisfactory 2D image with little perceived flicker. However, when producing two separate views, the output refresh rate is effectively divided by two, so that 60Hz total becomes only 30 Hz for each eye, which may cause flicker in the image and lead to viewer discomfort. To reduce flicker, display manufacturers have increased the refresh rate to a minimum of 120 Hz so that each eye experiences a refresh rate of at least 60 Hz. Such systems also require rather complicated, and therefore relatively cumbersome and expensive viewing glasses for their operation.
Some of the newer glasses-based 3D TV’s are based on the passive stereoscopic technology (e.g. LG displays). They do not require active synchronised shutters in the viewing headgear, using cheap and very light plastic polarisers in their lenses. Unfortunately, this approach effectively halves the resolution seen by each eye.
Glasses-based systems have gained some popularity because of their relative simplicity, affordability, and the possibility of spectacular “depth” effects combined with comparatively low band-width requirements and an easy way to complete stereo-TV (usually referred to as “3D”) systems.
Apart from the most obvious drawback of obligatory special eyewear, all glasses-based systems (with the possible exception of head-tracking head-mounted devices) suffer from the absence of motion parallax and the occlusion effect. Motion parallax is the apparent difference in the direction of movement or speed produced when the subject moves relative to his environment. Without motion parallax, everyone viewing a display (or a theatre screen) sees the identical image no matter where they are positioned, or how much they move their heads. In a similar way, the occlusion effect is the blocking of one object by another opaque (non-transparent) object located in front of it, but where the hidden object can still be seen if viewed from a different angle, for example in side view. The absence of both motion parallax and the occlusion effect can create a sense of imbalance and dizziness, sensations commonly experienced when only two views are presented (and which are contributing to concerns over health and safety of 3D viewing, in particular by children during their early development).
For detailed information about glasses-based 3D displays, please consult the following section.