Compound Curve Multi-format Screen Design: Ideas and Concepts
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The 70mm Newsletter
|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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|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.
New compound curve screen design
|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.
Lens design and depth of focus
|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.
|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.
|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.
|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.|
|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.|
|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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