Cinema Screen Specification

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(No Transcript)
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Cinema Screen Specification Design

• Andrew Robinson
• Managing Director
• Harkness Screens

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Cinema Screen Specification Design

• this presentation will focus on importance of the
  screen in a cinema
• screen often given inadequate consideration at
  the design stage
• screen can significantly affect the way the film
  is seen and heard and the overall movie-going
  experience
• presentation relates to 35mm projection but will
  also cover digital cinema

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Choice of screen impacts on the overall
experience

• picture quality/viewing experience
• sound quality/acoustic performance
• It can also affect cinema operating costs

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Screen impact on picture quality

• brightness
• colour rendition
• contrast
• distracting imperfections
• overall uniformity of picture
• The screen should show the film the way the
  director intended.

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Screen impact on acoustic performance

• distortion of sound from behind-screen speakers
• particularly loss of high frequencies
• Selecting the right screen can optimise sound
  quality.

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Screen brightness

• International standards exist for brightness
  levels.
• SMPTE recommendations
• 16 ft. lbts (55cd/m2) at centre
• 12 ft. lbts at corners
• measurements made using white light (no film
  running)
• the centre of the screen should not be
  excessively bright (no hot spotting)

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Screen brightness is affected by many factors

• light emitted from lamp/light source in projector
• light lost between lamp and screen
• - mirror
• - lens
• - port glass
• light reflectance back from screen
• The screen is an important factor in the
  brightness of the image.

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Light sources

• modern cinemas have projectors using xenon lamps
• carbon arc technology still used in some older
  cinemas
• xenon lamps between 2.0 and 7.0 kva are used in
  modern cinemas
• more powerful lamps cost more, have shorter lives
  and consume more electricity

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Light reflectance

• screen is major factor in amount of light
  reflected back to audience
• matt white screens scatter light
• 'gain' screens reflect more light back to
  audience

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Light reflectance
Gain Screen
Matt White Screen
Gain screens reflect more light back to the
audience
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Measurement of gain
Projector
Light meter
Magnesium carbonate block
Screen surface
Gain is measured according to British Standard BS
5382
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Measurement of viewing angle
Gain is measured at intervals of 10 by
comparison with reference standard
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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 risk
  of 'hot spotting'

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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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Benefits of 'gain' screens

• can achieve higher brightness level with lower
  lamp power
• with very large screens it may be essential to
  use gain screen to get SMPTE level brightness
• with digital projection, screen gain helps
  achieve brightness level
• Disadvantage gain screens have a narrower
  viewing angle than matt white screens

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Viewing Angle

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

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Screen type/lamp power

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• Parameters that can affect the presentation

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Screen size/screen type

• Harkness recommend the use of different screen
  gain levels according to screen width
• 35mm projection/scope picture
• up to 35' (11m) matt white (1.0)
• 35' to 45' (14m) mid gain (1.4)
• over 45' (14m) high gain (1.8)

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Screen size

• Choosing the appropriate size of the screen is
  important
• absolute size is not the issue
• screen size relative to auditorium size and
  viewing distance is the important consideration
• optimum screen width gives a subtended horizontal
  angle from "reference" seat of about 45- 50

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Screen Size

• Optimum subtended angle 45-50? with cinemascope
  picture
• A screen with a larger angle may reduce picture
  quality
• A lower angle makes the screen seem small,
  particularly in large auditoriums

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Screen shape

• General guideline is that
• matt white screens should be flat
• gain screens should be curved

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Screen shape

• Light scattered by a matt white screen can fall
  on another part of the screen if it is curved,
  potentially reducing contrast.
• Matt white screens should therefore be flat.

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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
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Screen shape

• curving a large screen may also be desirable to
  increase audience involvement
• with a curved screen it is recommended to use a
  gain screen

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Screen rake

• With stadium seating, raking the screen may be
  beneficial
• Each 1? rake gives 2? improvement in reflected
  light angle
• A rake of more than 5? is not recommended

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Acoustic performance of cinema screens

• screen blocks sound from behind screen speakers
• particularly affects high frequencies
• screens are therefore perforated
• perforation pattern is important
• - optimise acoustic performance
• - be invisible at closest viewing point

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Acoustic performance

• Screens can be perforated with different hole
  sizes and perforation patterns
• Typical commercial cinema screen perforation
  pattern
• 1.0 1.2mm diameter holes (0.04" 0.048")
• 5 open area
• For closer viewing (lt5m/16')
• 0.5 0.6mm holes (0.02" 0.024")
• 2 5 open area

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Perforation Size and Density

• Standard Perf Mini Perf
  Mini Perf Super
• 1.2mm diameter 0.5mm diameter
  0.5mm diameter
• Density 5.5 Density 1.7
  Density 4.9
• 
  

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Sound attenuation of different Harkness
perforation patterns
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Other important screen characteristics

• As well as brightness and acoustic performance,
  these other screen characteristics are also
  important
• colour rendition
• - accurately portraying colours
• contrast
• - preserving the contrast variations of the film
• no visible seams or other imperfections

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Picture formats/masking

• Most common movie formats are
• Cinemascope 12.35
• Widescreen 11.85
• Screen size can be adjusted by
• keeping screen height constant
• or
• keeping screen width constant

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2.35
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1.85
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Picture formats

• Constant screen height is optically better
• 1.85 film frame is smaller than 2.35
• less light reaches screen (all other things being
  equal)
• with constant height, 1.85 is the smaller screen
• consequently, light levels are about the same for
  both picture formats
• With constant width
• 1.85 screen is bigger than 2.35 screen
• in this case, less light has to cover a bigger
  screen

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Picture formats

• Constant screen width is popular because can use
  available height better
• with stadium seating
• in small auditoria
• However, it is optically worse
• correct light levels are harder to achieve for
  both formats 1.85 and 2.35

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Picture formats

• With constant screen width
• must have sufficient light for 1.85
• use gain screen if large
• 'lose' light for 2.35 (to avoid excessive
  brightness)
• adjust lamp current or defocus lamp
• use variable aperture lens

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Frames and masking

• Moving masking is often used to give a sharp edge
  to each format
• constant height moving side masking
• constant width moving top and bottom masking
• or moving top masking alone
• if moving masking passes in front of speakers
• use acoustically transparent cloth

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Screens for digital cinema 2k projectors

• SMPTE brightness standard 14 ft-lbts (16 ft-lbts
  for 35mm)
• generally follow same guidelines as for 35mm
  projection
• screen lt11m (35ft) use matt white
  screen
• screen lt14m (45ft) use 1.4 gain screen
• screen gt14m (45ft) use 1.8 gain screen
• light distribution is more even with 2k
  projectors less hot spot risk

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2k projectors film formats

• Changing film formats can be achieved by
  different means
• with constant height can do this electronically
  by reducing the area of DMD that is used
• not all available light from projector is used

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

• this maximises the use of available light
• 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 projectors film formats

• 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.

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D-Cinema brightness levels

• depending on the method used, available light
  varies and therefore screen brightness
• lamps in 2k projectors are expensive
  (particularly the special short-arc xenons)
• using gain screens can compensate for light
  losses
• gain screens can be used with standard lamps
  saving on lamp costs

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E-Cinema projection

• there are no standards for E-Cinema
• brightness level is usually the main issue
• gain screens are usually advantageous
• depending on projector lumens/screen size, gain
  screens from 8m may be necessary
• pixilation interference may also be a problem
  (moiré) use a different perforation pattern

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Screens for 3D

• digital technology gives very high quality 3D
  with single projector
• - can also use film with 2 projectors
• most popular technology uses polarised light
• - passive system
• requires silver screen to maintain polarisation
• - 1301 extinction ratio (linear polarisation)
• - also polarised glasses are needed

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Screens for 3D contd.

• alternative technologies
• - active system uses moving shutter glasses
• - passive system uses colour filters with
  special glasses
• all 3D technologies lose at least 75 of
  available light
• - normally need gain screens
• can use 3D screens for 2D movies

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Maintenance of cinema screens

• in clean environments screens should last 7-10
  years
• install screens in clean dust-free conditions
• - all fit-out should be completed beforehand
• periodic soft brushing of screens (vertically)
• do not wet screens or use chemicals

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Replacing Screens in existing cinemas

• consider screen replacement if
• - screen is dirty
• - light levels are low
• - screen has visible seams or other
  imperfections
• new screen likely to give significant improvement
• - particularly when large matt white screens
  changed to gain screens

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Summary

• Screen can significantly affect movie-going
  experience in terms of
• picture quality
• acoustics/sound quality
• Specification and choice of screen is critical
• manufacturing quality
• gain level
• screen shape (flat/curved)
• relative size to auditoria
• special performance requirements (e.g. 3D)

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Cost consideration

• screen surface cost is low part of total fit-out
  costs
• difference in cost between good screen and bad
  screen is 1-2 of fit-out costs
• potential lamp cost and operating savings from
  using brighter (gain) screens

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Final thought

• Movie-goers spend less than 30 minutes in foyer
  areas
• they may spend 2-3 hours looking at the screen
• money spent on optimising screen performance is
  well worthwhile!