Screen Selection for Digital 2D

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Screen Selection for Digital 2D 3D Cinema

• Andrew Robinson
• Managing Director
• Harkness Screens
• ICTA January 2009

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This presentation will cover the following topics

• Brightness standards for digital cinema.
• Differences between digital and 35mm projection.
• Screen selection for digital projection.
• Use of gain screens.
• Digital 3D screen implications.
• Mixed digital 2D/3D applications.

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Standards for screen luminance

• SMPTE luminance standard for digital cinema is
• 14 fl in screen centre /- 3fl.
• 75 of centre brightness at the sides, minimum 9
  fl. SMPTE 431-1-2006.
• Luminance standard for 35mm projection is 16 fl.

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Differences between digital and 35mm projection

• Digital projectors use short arc, high-pressure
  Xenon lamps which are more costly than 35mm
  Xenons.
• Light distribution from digital projector is more
  even than 35mm.
• Setting up and changing film formats on digital
  projectors can result in significant reduction of
  available light.

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Digital Cinema Projectors

• Light outputs of up to 30,000 lumens are quoted
  by projector manufacturers.
• Achieving maximum lumens requires optimum set-up
  and operating conditions
• - this may be costly in terms of operating costs
• - in reality, incident light on screen may be
  significantly less
• Screen selection is important to achieve
  recommended light levels and to moderate
  operating costs.

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Screen light efficiency

• Screens cannot create light but they can
  optimise the reflection of it.
• Reflectivity of a screen is measurable and is
  usually called gain
• Gain is measured by comparing reflectivity of the
  screen with a reference standard and is expressed
  as a factor (e.g. 0.8 or 1.2).
• There is a British Standard for gain measurement
  (BS 5550), which uses a reference standard to
  compare screen materials.
• There are other measurement standards, but the BS
  is widely recognised and is used by Harkness to
  classify screen gain.
• This enables screen materials to be compared in
  the laboratory.

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Measurement of gain
Projector
Light meter
Magnesium carbonate block
Screen surface
Gain is measured according to British Standard BS
5550
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Types of cinema screen

• Screens are available with various gain levels
• matt white 'gain' typically 0.8 1.0
• 'gain' screens up to 2.0 or more
• typically mid gain
  (1.4)
• or high
  gain (1.8)
• above 1.8 gain there
  is a risk
• of 'hot spotting'

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Light reflectance
Gain Screen
Matt White Screen
Gain screens reflect more light back to the
audience than matt white screens
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Measurement of viewing angle
Gain is measured at intervals of 10 by
comparison with reference standard
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Gain vs. viewing angle

• with all screens, perceived brightness reduces as
  viewing angle increases
• brightness of gain screens reduces more than matt
  white as viewing angle increases

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Gain curves of typical screens

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Picture formats / masking

• 2 most common movie formats are
• Cinemascope 2.351
• Flat 1.851
• Screen size can be adjusted by
• keeping screen height constant
• largest picture is scope
• or
• keeping screen width constant
• largest picture is flat

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2.35
1
1.85
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2k digital projectors film formats

• Native scope screen
• The full resolution of the DMD is 1080 x 2048.
• Changing film formats with constant height can
  be done electronically by reducing the area of
  DMD that is used.
• Not all available light from projector is used.
• Up to 37 loss.

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2k digital projectors film formats
alternatively using an anamorphic lens for
scope picture

• this maximises the use of available light some
  light loss from lens
• requires activation of anamorphic lens
• these anamorphic lenses are expensive
• this is the only practical approach on large
  cinemascope screens

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2k digital projectors film formats

• Native flat screen
• Changing film formats on constant width
  screens.
• This is easily achieved electronically.
• With digital projection, light is reduced
  changing from 1.85 screen to 2.35 screen, but so
  is the screen size. Brightness levels are
  maintained. (This is an advantage over 35mm when
  the opposite happens).
• Up to 24 loss.

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Screen light levels

• Depending on set-up, unused light can be as much
  as 37 with a digital projector.
• In practice there are other light losses also.
• Screen gain level can make a big difference to
  screen brightness or lumens required.
• Medium to high gain screens (1.4 to 1.8) will
  reduce lamp power needs and can significantly
  reduce operating costs.

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Screen light levelsLumens required to achieve 14
ft lamberts
The above lumens are for incident light on the
screen (after set-up losses).
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Screen light levels contdLumens required to
achieve 14 ft lamberts
The above lumens are for incident light on the
screen (after set-up losses).
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2k digital projectors film formats

• Optimum aspect ratio for digital projection is
  1.901 (20481080).
• If movies were made in this format and cinema
  screens were this format, there would be
  significant benefits with digital projection
• use all the DMD (no set-up losses)
• use all the available screen
• no blockbusters on a smaller screen
• no need for moving masking
• Many modern US theatres have constant width
  screens 1.851 which is very close to 1.901.
• Could 1.901 become a new standard?

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Operating Savings
 
 
 
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Practical Implications

• New installations of digital projectors
• - consider projector / lamp options together
  with screen gain level
• potential operating cost savings
• capital cost savings if can use smaller projector
• Existing installations of digital projectors
• by changing to high gain screens, may be able to
  reduce lamp size and operating costs

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Measuring screen gain in existing theatres

• Relatively easy to do this using light meter and
  reference.
• Should also measure absolute light levels in foot
  lamberts.

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Gain screens and viewing angles

• Perlux 140 (1.4 gain)
• half-gain angle 50 (0.7 gain)

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Gain screens and viewing angles contd

• Perlux 180 (1.8 gain)
• half-gain angle 34 (0.9 gain)

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Viewing Angle

• In this typical theatre almost all seats have
  better than half-gain level even with 1.8 gain
  screens.

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Screen shape
A curved gain screen reflects more light back to
the audience
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Screen shapeGain screen
curvature
Harkness guideline is 5 curve R.O.C. 5 of
chord 50ft screen width
2.5ft screen depth
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3D Cinema

• Stereoscopic 3D is growing fast, particularly in
  US.
• Over 2,500 3D screens operational worldwide end
  2008.
• Forecast of 5,000 3D screens by end 2009 and
  8,000 by end 2010.
• Around 12 3D titles releasing in 2009 more in
  2010.
• Single digital projector gives economic 3D
  performance.
• High quality of digital 3D images.

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3D Cinema Technologies

• There are 3 main technologies used in digital 3D
  cinema
• Polarised light systems
• polarising filter/silver screen/passive eyewear
• e.g. Real D, Master Image, Imax
• Colour filters
• colour filter wheel/white screen/passive eyewear
• e.g. Dolby
• Active eyewear
• white screen/electronic shutters in eyewear
• e.g. Xpand

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3D Cinema

• All systems use eyewear.
• Real D, Master Image
• - 1
• Dolby
• - 30 need recycling
• Xpand
• - 60 batteries need recycling
• All 3D systems absorb a large amount of light
  compared with 2D mode because of filters and
  eyewear
• - typically 85 light lost
• - acceptable screen brightness levels 4.5 fl.
  (14 fl. 2D)

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Screen implications for 3D Cinema

• Polarised light systems (e.g. Real D)
• Screen must maintain polarisation on reflection.
• Silver screens (aluminium flake coatings).
• gt1301 signalnoise ratio to maintain quality 3D
  images.
• Silver screens are intrinsically high gain
  (typically 2.4).
• Compensates light losses from 3D.
• Screens up to 45 ft. with a single projector (70
  ft. with Real D XL system is possible).

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Screen implications for 3D Cinema

• Colour filter wheels (e.g. Dolby 3D)
• Use white screens (do not need to maintain light
  polarisation).
• Gain screens required to overcome light loss
• - up to 2.2 (maximum gain available)
• Up to 45 ft. wide screen possible with single
  projector / 2.2 gain screen.

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Screen implications for 3D Cinema

• Active eyewear (e.g. Xpand)
• White screen.
• Gain screen needed for larger screens.
• Screen up to 50 ft. with single projector / 2.2
  gain screen.

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Mixed 2D / 3D applications

• Acceptable brightness level with 3D is 4.5 fl
• efficiency level 15
• Recommended brightness level for 2D is 14 fl
• Switching 2D / 3D modes
• 2D image too bright
• change lamp / reduce power in 2D mode
• Real D XL system 2D / 3D modes in balance
• Increased risk of visible defects due to high
  gain
• All Harkness 3D screens will support 2D content
• useful for mixed programming

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Theatre shape

• 3D systems use high gain screens.
• High gain screens have reduced viewing angles
• Spectral 240 half-gain angle 24º
• Perlux 220 half-gain angle 25º
• Best to use theatres for 3D which are long
  relative to width
• throw minimum 1.5 x screen width
• With 3D, curving the screen is strongly
  recommended.

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Viewing Angle

• This theatre has most seats with an acceptable
  viewing experience.

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Summary

• For digital projection, it is recommended to use
  gain screens.
• Benefits in power use/lamp life can give big
  operating cost savings.
• Screen must be consistent with 3D technology
  chosen.
• For 3D digital screens, it is usually necessary
  to change the screen for the highest possible
  gain.