CA1075941A

CA1075941A - Pressure plates for cinematograph projectors

Info

Publication number: CA1075941A
Application number: CA273,127A
Authority: CA
Inventors: Michael J. Edwards
Original assignee: RFD Systems Engineering Ltd
Current assignee: RFD Systems Engineering Ltd
Priority date: 1976-03-04
Filing date: 1977-03-03
Publication date: 1980-04-22
Also published as: GB1577736A; FR2343270A1

Abstract

ABSTRACT OF THE DISCLOSURE

In a cinematographic film projector, there is provided at the film gate an apertured pressure plate which presses the film against a film guide. When the film is arrested in the film gate to project a stationary image formed by one film frame, heat emitted from the light source tends to buckle the film causing distortion of the image. To overcome or reduce buckling in these circumstances, the pressure plate is provided with a transparent member extending across the aperture in the pressure plate to present to the film within the area of the aperture a film contacting surface which opposes buckling of the film within the area of the aperture.

Description

This invention relates to pressure plates for supporting films in film gates of cinematograph projectors which have the ability to arrest movement of the film and to project a stationary image formed by one frame of the stationary film.
In such projectors, heat emitted from the projection lamp tends to buckle the film when the latter is stationary, causing distortion of the image. This can generally be corrected by refocusing, but the need to refocus each time the film is stopped or started represents a considerable disadvantage which the present invention is intended to overcome.
According to the invention a cinematograph film projector comprises a film gate including film guide means and an apertured pressure plate to press the film against the film guide means, the pressure plate including a transparent member extending across the aperture in the pressure plate to present to the film within the area of the aperture in the plate a film contacting surface so disposed as to oppose buckling of the film within the area of the aperture in the plate.
The transparent material is preferably glass. Said one surface of the window is preferably slightly recessed with respect to spaced contact rails of the plates so as to minimise abrasion of the film as it passes over the window.
An exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which:-Figure 1 is a side view, partially in section, of the film gate and surrounding structure of the projector, 3~
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Figure 2 is an enlarged view of the pressure plate shown in Figure 1, Figure 3 is a fragmentary view as seen on the line III-III of Figure 2, and Figure 4 is a fragmentary sectional view taken on the line IV-IV of Figure 3.
The projector has guide grooves 1 and 2 through which the film 3 passes. Between the guide grooves 1 and 2 the film passes through a film gate 4 where the image is projected by light from a tungsten halogen projector lamp 5. The pressure plate 6 is mounted at the film gate and has a rectangular aperture 7 through which the image is projected onto a screen (not shown). A heat filter moves into the position shown in Figure 1 when movement of the film is arrested and a stationary image from one frame is projected.
A rotatable shutter 9 has fingers 10 which impede the passage of light each time a bar between adjacent frames of thè film 3 is passing the aperture 7.
The film passes between a film guide 12 and the pressure plate 6 which is urged by two helical compression springs 13 towards the film guide 12 so as to maintain the film in a flat condition when in the- film gate. A semi-circular plate 14 is secured in the projector and serves as a fixed abutment for the springs 13 which surround respective studs lS.
Referring to Figures 2 to 4, the pressure plate 6 is in the form of an elongated strip of metal having a long-itudinally extending shallow groove defined by a base 16 disposed between slightly raised longitudinally extending contact rails 17 and 18. The contact rail 17 has a shallow iO759~1 recess 19 to reduce friction on the passing film 3, and the other contact rail 18 has a deeper recess 20 to accommodate the teeth of a rotatable sprocket which engages the sprocket holes of the film 3 to drive the latter. The rectangular aperture 7 is formed in the base 16 of the pressure plate 6.
In accordance with the invention, the aperture 7 is covered with a glass window 22 one surface 23 of which engages and supports the film 3 to prevent buckling of the film and distortion of the image when the film is stationary. The interposition of the heat filter 3 between the projector lamp 5 and the film gate when the film is stationary reduces the amount of heat reaching the stationary film in the gate.
By using a pressure plate according to the invention it has been found that the film 3 can be st!opped and a stationary image projected without loss of focus for a period of 25 seconds or more.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A cinematograph film projector comprising a film gate including film guide means and an apertured pressure plate to press the film against the film guide means, the pressure plate including a transparent member extending across the aperture in the pressure plate to present to the film within the area of the aperture in the plate a film contacting surface so disposed as to oppose buckling of the film within the area of the aperture in the plate.

2. A projector according to claim 1, wherein the pressure plate includes spaced parallel contact rails extending along opposite edges of the aperture in the plate and having film contacting surfaces which contact the film along the longitudinal edges thereof, and wherein the transparent member is located between the rails with the film contacting surface of the transparent member set back slightly in relation to the film contacting surfaces of the rails so as to minimize contact with the film as the latter passes through the film gate.

3. A projector according to claim 2, wherein the contact rails of the plate are upstanding from a flat base portion provided with the aperture of the plate and wherein the transparent member is mounted on the base portion on the same side thereof as the rails.

4. A projector according to claim 3, wherein the transparent member is in the form of a transparent plate bridging the aperture in the base portion and wherein the film contacting surface of the transparent member is flat over the area of the aperture in the plate and is curved in a direction away from the film at the entry and exit ends of the gate.

5. A projector according to claim 4, wherein each of the contact rails on the pressure plate is formed with a longitudinally extending recess to reduce the area of contact thereof with the film.

6. A projector according to claim 1, 2 or 3, comprising light source means to produce an image of the film in the film gate, the light source means being mounted on the same side of the film as the film guide means.

7. A projector according to claim 1, 2 or 3, wherein the transparent member comprises a glass plate.