Henry Corrado, Director of Tejix s.a.r.l, (left) looks at 3D principles and techniques in cinema and theme parks.
[The following article is based on a paper first published on www.tejix.com]
In our recent article "An Introduction to 3D Technology in Cinema and Theme Parks", we explained the basics of 3D technology and how this is applied to both cinemas and theme parks and gave some insight into how the technology works. Today, we’re plunging further into the technology and explaining what principle and techniques can be applied, compatibility issues for the attractions industry and an overview of the systems that are currently available.
Today’s attractions visitors are demanding and new technology alone is no longer sufficient to hold their interest. However, this means that designers can get more creative with the attractions they create and take 3D technology and apply it to new concept rides such as dark rides or people movers.
Whilst more effective and efficient techniques now exist to replace the older standard of running two films simultaneously, the practice is still widely used in the attractions industry. With this mature technology, theme parks synchronise two projectors and fit them with polarizing filters. This allowed the light from one image to be filtered in one direction whilst the other is filtered in the opposite direction; filters on both the projector and viewer’s glasses blocked unwanted images.
Both sets of filters share the same kind of filters, but the glasses have a 90° offset so the opposite eye image is "blocked". This technique has been maintained within the industry as, to begin with, it has been widely used and so is trusted and has become an industry standard.
The use of the older technology also allows for a show to be run on one, traditional 2D, projector in the event that one of them fails and the video players which are generally used can be synchronised with each other and other show elements, such as lighting and special effects.
Another consideration is the media rights management (MRM) system that is used to right piracy is not only compulsory, but also adds unnecessary complication to the technical set up. However, whilst this is true for cinemas, theme parks are much less likely to find rights management to be such an issue.
Despite the fact that this trusted method of two-projector systems is widely used, the picture quality and 3D effect of this is somewhat less than perfect as the tiniest misalignment of filters or projects can result in a degeneration of the 3D effect. Additionally, the linear filters that are used for the most part mean that viewers are obliged to keep their head straight in order to maintain the 3D effect.
To overcome this, a different technique which uses the sequential projection of left and right images from a single player onto a single projector has been made possible by digital projectors and high bit rate video sources.
This means that rather than being projected at 24 frames per second, the rate is 48 images and a double or triple flash is used in order to avoid flicker. This makes the actual rate 96 or 148 fps as the image is flashed onto the screen two or three times.
This updated technique means that it’s possible to block opposite eyes by knowing which eye the projected image will be received on, and then applying different techniques.
- Polarizing filters: A single image being shown at a time reduces the risk of ghosting significantly and it’s possible to use linear filters, or the new circular filters such as those made popular by RealD. These allow for decent 3D quality even when the viewer is not aligned with the screen; however, the use of polarizing filters requires a metalized screen.
- Spectral filters: Panavision and Dolby use special optical filters which have the ability to project onto any surface, not just silver screens. This makes them the ideal choice for rides and people movers as chromatic irregularities are more or less non-existent and 3D is all but perfect. The only disadvantage to spectral filters is the need for more powerful, bigger projects as the light efficiency of systems such as these is quite low.
- Active glasses: otherwise known as shutter glasses, these are close to models designed for home use and contain electronic circuitry which controls an LCD shutter. These also don’t need a silver screen and their implementation is straightforward; the downside is that it can be an issue to properly manage expensive and sensitive equipment such as this in the park environment.
Each technology has various commercial processes which we will take a look at in a little more detail:
- RealD: uses circularly polarized filters and pre-processing is applied in the player when images are being decoded in order to reduce ghosting. RealD produces two projector filter models: Z-Screen and XL. It’s necessary to have a silver screen for both. Z-Screen is an electronic filter that’s easy to install in front of the projector lens and XL is larger and a little more difficult to adjust, but gives better performance. The glasses can be re-used once they have been cleaned and are described as ‘passive models’. In the Wings of Courage theatre in Futuroscope, Tejix used an XL and reports that the brightness is nearly 10 times the SMPTE minimum requirement.
- Master Image (right): this is surprisingly simple and effective and considered by many to be the brightest. A standalone device is rolled in front of the projector and the upper part of the unit is a filter disc rotating at 4320 rpm in sync with the imagery. A metal screen and passive glasses are necessary for use with this system and a problem exists for parks wanting to use it on rides as the Master Image rotating filter is very loud. This means that the noise can be heard from the room if there is insufficient sound proofing between projection booth and cinema, making it unlikely to be suitable for use on rides.
- Expand: This offers an active glasses solution in which an infrared transmitter that has been installed in the room synchronises the occultation of shutters on the electronic glasses. The system is simple to implement and can be used on all surfaces, making it suitable for a wide variety of scenarios. However, the glasses are expensive and the electronics inside them make them fragile and difficult to clean – it’s also not ideal to have this kind of equipment at a park as they will be difficult to manage and so will need replacing often.
- Dolby: these are licensed to employ a clever technique in theatres which was developed by German company Infitec. Theme parks are not obliged to use the Dolby distribution channel and so can source glasses and filters elsewhere. This method relies on the spectral division of light, a little like anaglyphic colour filters. Images in the video server must be treated in order to compensate for chromatic aberration during the process of offsetting the wavelength of each primary colour around a median value.
The projector will need to be disassembled in order for a small spinning wheel to be installed, which holds the spectral filter in the light path inside the projector. These projectors can be obtained pre-fitted with this wheel from the manufacturer Barco. Guests wear passive glasses, which are relatively expensive, fitted with the high tech optical filters but overall, the Dolby system offers a number of advantages.
A silver screen is unnecessary and switching from 2D to 3D is fully automatic, doing away with the need to move the filter manually, and 3D effects and colorimetry are perfect. The main concern with Dolby is with efficiency, as 2-3 times less light from the projector is passed by the system. This means that good illumination is necessary which requires a larger projector, which may impact investment and operating budget. It’s also worth taking note that the Dolby 3D system also works with fixed filters in two-projector configurations.
- Panavision: these have teamed up with Omega Optical, a manufacturer of optical telescopes, in order to offer a 3D system inspired by the same principles as Dolby and Infitec. This means that the same constraints that can be seen with the Dolby system apply here too, despite the fact that Panavision claim the system to be more efficient. With Panavision systems, the light spectrum is divided by a rotary filter in 10 bands, 5 of which are visible to each eye, thanks to the passive glasses that are equipped with advanced filters; the lenses in Panavision glasses can also be used with conventional projectors.
Theme park attractions that have 3-4-5D attractions use exactly the same screens as those found in cinemas; a PVC canvas with a white coating on a metal frame. If speakers are used behind the screen, then the canvas is perforated with holes of 0.5 to 1mm at a 5% density, which can’t be seen from a distance of a few metres away.
If active glasses or light spectral division is used, images can be displayed on any surface, not just a white screen. This makes them ideal for rides which are moving as they are not sensitive to the positioning of the viewer. However, the most widely used 3D processes use polarized light rays which pass the omnidirectional light rays at the projector through a filter, which only allows rays to go in a given direction.
This means that it is not possible to coat the PVC fabric used for the screen with a white surface as it must retain light rays' polarization properties. The screen instead is coated with aluminium oxide, which is what gives the screen its silver appearance. Metallic screens are ‘high-gain’, which means that they reflect much more light than they receive and a gain of 2-2.5 is typical. This gain is obtained at the expense of the viewing angle and viewers installed on the sides will get a much dimmer image than those at the centre.
This results in a ‘hot spot’ in the middle of the screen and renders it impossible to have uniform light on a metallic screen. To overcome this, seating should be installed in a rectangle with a width which is less than the base of the screen.
White screens tend to be more robust than their metallic cousins and it is possible to clean them as long as no detergent is used and no excessive pressure applied to the surface. However, metallic screens are considerably more fragile and can only be dusted gently every now and then – the material for both types of screens should be replaced every few years.
Glasses present the greatest logistical problem for parks; these have to be distributed, cleaned and then re-distributed and in a high-capacity venue, this is more constraining than in a cinema, where the glasses are often sold. Glasses should be cleaned by using disinfectant wipes or in a dishwasher and care must be taken that this is carried out correctly so that bacteria doesn’t pass from one guest to another.
However, cleaning affects the effectiveness of the filters and the 3D effect is impaired after a few washes. This makes for a difficult choice for managers who have to decide whether to purchase good-quality, longer lasting glasses or those that are less expensive and will need to be replaced in the space of a week or two.
This problem has been somewhat addressed by Miele, who have manufactured a special version of one of its models to handle a 40 ° dishwasher cycle, as it’s high temperatures that quickly degenerate the glass’ optical filters. They also offer baskets for 3D glasses but the 5 to 8000 Euro price tags may cause some operators an issue.
Advances in 3D reproduction, both in homes and cinemas, has provided a number of benefits for theme park operators. Sharp 3D images can now be obtained by employing advanced techniques such as spectral division and this can be applied to all kinds of surface and viewing scenarios. Theme park operators can of course ensure the best performance from the most reliable systems by partnering with industry experts such as Tejix.