The resurgence in 3D have largely fueled the digital rollout, helping cinemas to justify the massive cost of converting from existing 35mm projectors to digital ones. There are several technologies involved in 3D, and many questions arise. The FAQ below is designed to try to help newcomers to cinema to understand the different technologies involved.
There are about half-dozen different key vendors, with a variety of different technologies. Real-D and Master Image both use circularly Polarised light, Imax uses linear polarisation, Dolby uses colour filters and Xpand uses Active glasses
Active glasses have LCD shutters that can block off either eye in the glasses. In this way the projection system projects both left eye and right eye images onto the same screen but the frames alternate between left eye and right eye. The glasses keep up with these images by each lens blocking out the image that is meant for the other eye. When the left shutter is open the right shutter is closed, and vice versa. These glasses are battery powered and are reusable - they need to be collected and washed at the end of each performance.
In most cases light is not polarised; it arrives in random orientations from all around us. However, it is possible to filter out some light depending on its orientation. With polarising glasses the 2 images are projected simultaneously on top of each other, but at 2 different polarisations, one for the left eye and the other for the right eye. The polariser for each eye (which can be pictured like a grate that only lets light pass through the gaps in the grate, and so blocks those that are not arriving parallel to a slot in the grate) blocks the light that is not intended for that eye so that we see the 2 different images that creates the 3D effect. There are 2 kinds of polarising light methods for 3D, that which is affected by the orientation of the glasses (tilting head sideways destroys the polarising effect) such as Imax and those that are not (such as Real-D and Master Image). There are benefits and trade offs to both systems. These glasses are typically inexpensive and recycled after each use.
Dolby's system can most easily be pictured by considering the red/green lenses of previous 3D incarnations. However, whereas the red/green solution used cheap filters that allowed only one colour to pass to each eye the Dolby system splits the visible spectrum into many more segments, alternating one thin sliver of colour to the left eye and the next this sliver of the spectrum to the right eye, repeating this for the entire visible spectrum. In this way full colour is achieved in both eyes. These glasses are reusable and so need to be collected and cleaned after each performance.
For a 2D projection system all light is visible to both eyes. However, in the case of 3D it is necessary to half the light going to each eye (by blocking half of the light to each eye) and so even in an absolute best case we might hope for 50% light efficiency compared to 2D projection. However, no 3D system is anywhere close to this efficient and there are other factors involved that further reduce the light onscreen. For this reason whereas the standard projection target for 2D is 16 foot-lamberts, the standard for 3D has been 4.5. There have been some efforts to increase this to 6 foot-lamberts as it is generally considered that brighter 3D images are better. However, it is necessary to master each image to the target brightness that is being projected at for best results.
Real D's 'XL' introduced some new technology to the cinema's 3D industry. Instead of continuing to follow the tranditional method of simply blocking the light that is not wanted, this technology 'recycles' the unwanted light by re-polarising it into the correct orientation that is needed, helping to increase brightness levels without impacting any other aspect of the projection system.
This is cross-talk between the images. In an ideal system only each eye sees the image that is intended for it. However, if some of the light bleeds through the wrong lens than it is possible to slightly see the image also that is not intended for this eye and this effect is called 'ghosting'.