Digital 2D and 3D Overview

When installing DCI-compliant projectors, screen choice may have more impact than at first thought. Auditorium shape, screen gain, projector set-up and even choice of 3D technology can all make a difference.

Consider screen gain level together with lamp power; these can affect projector choice (model size) as well as lamp selected. It is important to consider this in new-build cinemas; in existing auditoria screen replacement may be beneficial to help reduce lamp power. If showing 3D, other considerations also apply (see below)
Matt white screens usually have a gain of 1; higher-gain levels are available typically 1.4, 1.8 and 2.2 – gain is measured according to international standards such as British Standard BS5550. 14-fl is the recommended brightness level for digital projection.
For a given screen brightness level, lamp power can be reduced as screen gain increases. This can confer financial benefits in initial investment, as well as ongoing operating and maintenance costs. Xenon lamps for digital projectors are more expensive than for 35mm, and lamps from 1.2-7kw are available.

Viewing angles of gain screens and the impact on screen light distribution should be considered.

High-gain screens should be curved to improve the light distribution. 1.4 gain screens can be mounted on flat frames, although a slight curve is preferable. Screens may need to be raked with steep stadium seating.

Auditorium shape can be important, particularly for high-gain screens. Rectangular shapes (long throw) are better than square when using gain screens. The 'half-gain angle' is important; the off axis angle at which the screen gain is half the peak. Seats outside this angle will suffer a noticeable variation in brightness.

Intrinsic light distribution on-screen from digital projectors is more even than from 35mm models, so digital projectors are less likely to 'hot spot' with high-gain screens.
Manufacturers’ quoted brightness levels for digital projectors may be unrealised in practice, depending on how aspect ratios are set up; lenses, lamp choice and age, port glass losses etc. all affect screen brightness.

Interference fringes can occasionally be experienced (between pixels and perforations). Adjusting the projector set-up can usually overcome this; screens with different perforations can also be used. This is not normally a problem in cinemas.
If a screen is damaged or old, changing it when installing new digital projection will ensure the overall quality of the projected image. Certainly it will be important to know the gain of the screen; there are simple ways of doing this in theatre.

Several technologies exist. For single d-projectors, the main technologies are polarised light; spectrum separation or 'active' glasses.
All single projector 3D systems 'lose' light – up to 88% of that available in 2D mode.

3D systems need gain screens and more powerful lamps to compensate for light losses. Lower screen brightness levels are accepted for 3D (typically 5fl compared with 14fl for 2D).
Polarised light systems use 'silver' screens, which are intrinsically high gain (around 2.4). The maximum screen size which can achieve 5fl with a single projector, is about 14m (20m+ with Real D XL).
Other 3D technologies use 'white' screens but also require high gain. Recommended gain levels are 1.4 from 8-10m; 1.8 from 10-12m; 2.2 above 12m.
All 3D screens require special eyewear, which varies according to technology used. 'Silver' screen systems use cheap glasses; 'white' screen systems require more expensive glasses that are recycled.
Polarised light systems will always require a screen change (to silver); other 3D systems may require a screen change to higher gain.
When 3D auditoria are showing 2D content the screen will generally be too bright, so lamp power should be reduced.

Harkness Screens' Digital Screen Selector is available to guide screen selection when installing digital projectors for both 2D and 3D.

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