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Compound Curve Multi-format Screen Design: Ideas and Concepts
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The 70mm Newsletter
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Written
by: Ramon
Lamarca Marques,
Brian Guckian
and Mike Taylor |
Date:
20.09.2007 |
This article puts forward some new ideas and concepts in
relation to screen design for 70mm, current 35mm formats and
D-Cinema. It is speculative in nature, and is not intended
to be a formal paper, but it is felt that the general
intention of the work is useful.
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Background
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Curved
screen design has a long history and has been used for various purposes.
This article is about achieving greater immersion in the cinema experience
rather than increasing luminance, which is often how curved (gain) screens
are used today.
It is also important to stress that the Cinerama process is not covered
here. This is because it is understood that correct use of the format - of
which the 140 degree curved screen was a function of the process, not an end
in itself - required the use of triple cameras, with three taking lenses and
filmstrips. Whilst a new Cinerama production would theoretically be
possible, the intended use for the ideas outlined here is for application in
the conventional commercial cinema environment.
Where screens are curved today, a common technique is to use either a chord
depth of 5% of the screen width, or to curve to the throw, or to curve via
ray tracing according to the requirements of gain screens. Whilst all of
these methods provide an aesthetically pleasing curve, they do not offer the
immersiveness of former curved screen processes such as those employed by
Cinerama, Todd-AO and Dimension 150.
With the increasing need to differentiate the cinema experience from the
home viewing environment and also to encourage the return - at least on a
modest basis - of the "roadshow" large screen presentation, and further,
given the increasing flexibility and attractiveness of
shooting on 65mm, the
question could be asked if former curved screen processes could be revived
and adapted for the modern cinema environment.
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New compound curve screen design
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Fig
1.
Click image to see enlargement
In
order to do this, it is necessary to look at the properties of the two most
common immersive "single hole" large screen processes - Todd-AO and
Dimension 150. Importantly, though effective, both these processes used
constant cylindrical (circular) curves. This is advantageous for the widest
aspect ratio on screen, i.e. 2.21:1 or 2.76:1, but it can be seen from
Fig.1, and often remarked upon, that narrower ratios are disadvantaged by
the same constant curved field.
The answer to the problem is to "flatten" the central portion of the screen
using another, gentler curve. In researching this work, drafting methods
showed that a curve of chord depth of 5% of the screen width, combined with
the most immersive non-Cinerama curve (120 degrees) to create a compound
curve, provided interesting results (Fig. 2).
(For reference, a sketch of a "family" of screen curves used over the years,
derived from a scale drawing, is given in Fig. 3).
Interestingly, use of compound curves was recommended by Philips in their
seminal publication from the 1960s Planning a Cinema. In this case they
recommended a parabolic curve for multi-format 35mm / 70mm screenings.
However it must be noted that the parabola, being an inclined section
through a cone, achieves its greatest curvature at its apex. This again goes
against the desire for a flatter field for narrower aspect ratios in the
central portion of the screen, so that a compound curve would appear to be a
better choice of geometric form.
The transition point between the major curve and the minor one is most
important. Trial and error through drafting showed that the
intersection of the curves could occur at the edges of the Academy Sound
(1:1.38) portion of the screen. There could be a more scientific way of
choosing the transition point, but it could also be argued that there is a
element of aesthetic choice involved. However an important issue, and
relevant below, is that a fixed transition point is necessary if such a
screen design were to be popularised.
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Lens design and depth of focus
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Fig
2.
Click image to see enlargement
Former
processes such as D-150 required bespoke lens designs tailored to each
venue. In today's more cost-conscious era, such practices are likely to be
unattractive. An elliptical screen design overcomes this by limiting the
required correction to only the widest ratios (i.e. 35mm 'Scope, and 70mm),
and furthermore, entailing only a standard correction method if the screen
curvature transition point were fixed to e.g. the boundary of the 1:1.38
aspect ratio. In this way, the projection field would be standardised across
screens since the transition point would be based on an unchanging point
proportional to the screen width (Fig. 4). Also, since the curves are also
based on screen width (a 120 degree curve can be mathematically derived from
the screen width), screen size is not a limiting factor. Thankfully, use of
the compromised and incorrect "common width" screen layout seen in too many
cinemas is not possible with this design. Lenses for the wider ratios could
be provided with greater depth of focus to accommodate the deeper curvature
towards the screen edges.
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Cross-reflection
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Fig
3.
Click image to see enlargement
An compound screen design also greatly minimises cross-reflection as the
central portion is flatter and the deeper curved sections are well
separated. However, since such screens would have to be retro-fitted or
included in new builds, the opportunity arises to use coatings and other
techniques to eliminate potential problems in the field.
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Seating layout
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Fig
4.
Click image to see enlargement
In order to reduce peripheral perceived image distortion on the part of the
viewer, seating layouts used with this screen design should be curved or
angled, which in any case is in line with best practice in cinema design.
This orientates the viewer towards the central area of the screen,
irrespective of where they are seated, and also ensures optimal viewing
comfort. Seating curvature does not necessarily have to be circular, and
could be of a compound form to match the screen curvature geometry.
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Ultra-wide ratios
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Conveniently, a new compound curve screen
design would also make it easier to accommodate wider aspect ratios than
today's, such as
Vistamorph. This because a
curved screen design inherently has a shorter "straight across" width for a
given ratio than a flat screen does. By extension, this means wider ratios
can be accommodated without the need to greatly expand laterally, and thus
makes it easier to incorporate such ratios into current cinema auditorium
designs that favour "wall-to-wall" flat screens.
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Implementation
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A new screen design such as outlined could be
retro-fitted or accommodated in new builds. An important factor in achieving
widespread adoption is the standardisation of screen curvature and lens
correction. This would keep costs down, and importantly, the multi-format
nature of the design and its immersive selling point would be equally
attractive for both digital and conventional film screenings. Furthermore,
new digital 3-D systems such as that from Dolby, which do not require silver
screens, could be accommodated by the system.
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Conclusion
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Fig
5.
Click image to see enlargement
This article proposes a new approach to screen design, employing a
standardised compound curve with matching standardised lens
correction for only the widest ratios, and drawing on curvature forms
developed in previous eras. The compound curvature would be determined by
screen width with a fixed transition point also governed by screen width and
thus would be independent of screen size. Such a design would increase
audience immersion without compromising narrower aspect ratios, would allow
for wider ratios in the future, and would provide a unique selling point for
large-screen 70mm, 35mm and digital presentations.
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