Active, Passive and Active/Passive 3D — Selecting the Proper 3D Technology for your Application

As 3D visualization becomes more mainstream, it is important to understand the basic differences between Passive and Active Stereo Visualization.
-- As 3D visualization becomes more mainstream, it is important to understand the basic differences between Passive and Active Stereo Visualization.

Passive Stereo is one of the two methods commonly used to create high quality 3-dimensional projected imagery, and the system architecture is actually pretty straightforward. The imagery from two projectors is overlaid onto a single screen, with one projector dedicated to content intended to be viewed by the left eye and the other dedicated to content for the right eye. Opposing polarizing filters are then placed in front of the lens of each projector. Next, the projectors display the distinct left and right eye images onto a specially coated screen that maintains the left eye / right eye polarization of the reflected light. The viewer wears “passive” polarized glasses that match the polarizers installed on the projectors. Thus, the viewer’s left eye only sees the content rendered by the left eye projector, and the viewer’s right eye only sees the content rendered by the right eye projector. Those distinct left and right eye images are interpreted by the human brain as 3D.

Passive stereo projection may be a benefit for applications in which a large audience will be viewing the 3D content. Passive polarizing glasses are typically much less expensive than the active LCD shutter glasses used in Active 3D visualization. In addition, there can be an economic benefit to distributing the actual 3D signal for Passive Stereo visualization. Since each projector is being tasked with displaying a traditional 60 Hz signal, bandwidth requirements are not extraordinary and standard cables and signal distribution can be employed. However, as mentioned above, Passive 3D demands a special screen to maintain polarization. Furthermore, viewing angles can be limited with passive stereo visualization. If the highest quality 3D imagery is required, most experts agree that Active 3D projection provides a superior 3D experience.

Active stereo systems rely solely on one high frame rate-capable projector, and that projector can be employed on just about any front or rear projection screen surface, including matte white surfaces. Electronic LCD shutter glasses are used to create a 3-D experience for the viewer. Active stereo systems work by presenting the left and right eye images interleaved in rapid succession. As an example, in a 120 Hz active 3D system, the 60 odd frames (from 1-119) would be intended for the left eye, and the 60 even frames (from 2 – 120) would be intended for the right eye. The glasses are actually LCD shutters that allow the user to see only one image at a time, through either their left eye or their right eye. The individual lenses in the glasses actually alternate between being opaque and transparent, operating in sync with the images being displayed from the projector via either an infrared emitter, or through an imperceptible sync “flash” that the projector displays during the blanking between frames. In either case, when a left eye image is being presented by the active 3D projector, the left lens of the glasses is transparent while the right eye is opaque, and when a right eye image is being presented by the active 3D projector, the glasses’ right lens is transparent while the left is opaque.

While the single, high bandwidth, Active 3D capable projector is generally a bit more expensive than a single standard projector, the total cost of the system and the long-term cost of ownership is much less expensive than Passive 3D systems that require two units. The reasons are straightforward – in an Active 3D installation, the user only needs to purchase and maintain a single projector, and no maintenance is required to assure the critical alignment is maintained between the two projectors employed in a passive system. Over time, the simplicity of the single projector Active 3D solution delivers very significant savings in terms of lamp cost and routine maintenance.

Additionally, there is a third popular approach to creating high quality 3D that is a combination of both the Active and Passive technologies. Referred to by many as Active/Passive 3D, this design is typically applied when the highest quality 3D is desired for larger audiences, where the cost of Active 3D glasses would be prohibitive. In short, a single, wide bandwidth Active 3D projector is employed along with a rapidly switching LCD polarizing shutter that is installed in front of the lens. Much like a full Active 3D system, an Active/Passive system operates at 120 Hz, alternating left and right eye content frame by frame. At the same time, the LCD polarizing shutter switches polarization, frame by frame, in sync with the projectors’ display of each frame. Since the active polarizing shutter is installed on the projector, the viewer wears passive polarized glasses. This Active/Passive system design delivers all of the simplicity and cost of ownership benefits of a single projector in a full Active 3D system, while also delivering the reduced cost benefits of passive polarized glasses. Since it is based on polarization, the Active/Passive design does require specialized screen that preserves the polarization of the projected light, just as is required by a full Passive 3D system.

Digital Projection offers a comprehensive line of 21 Active Stereo 3D capable projectors. Our 3D product line includes SXGA+, WUXGA and 1080p displays, with light output performance from 2000 – 30000 lumens!. All of DP’s 3D projectors can be applied in either full Active or Active/Passive 3D applications. In addition, all of our precision displays are compatible for use in Passive 3D system designs. Talk to your DP regional manager today to get more information on our extraordinary lineup of industry-leading 3D displays.