GB2049223A - Photographic projection apparatus - Google Patents

Abstract

For continuously maintaining a flexible viewing screen (14) mounted in engagement with and supported by a frame (64) in an undistorted condition over the opening defined by the frame despite temperature variations of the frame and screen, a plurality of coil springs (76), each having an end attached to a preselected marginal edge portion of the screen and an opposite end connected to a point to the rear of the said opening, provide a generally uniform resilient tensioning force on the screen. The force exerted is within a range in which the screen will not, on the one hand, permanently deform due to creep during thermal variations because of its tight engagement with the frame and on the other hand, will continuously maintain the screen in contact with the said frame despite thermal variations causing different dimensional changes between the screen and the frame while allowing for flexing of the screen. <IMAGE>

Description

SPECIFICATION Photographic projection apparatus This invention relates generally to a photographic projecting apparatus and more particularly, concerns an improved method and apparatus for mounting a flexible viewing screen over a frame defining a projection area.

Rear projector motion picture apparatus in which the projecting system and the viewing screen are combined in a single structural unit are known. An example is disclosed in U.S.

Patent No. 3,740,127. These apparatus have gained public acceptance because they are not only compact and portable but permit satisfactory pictorial displays in relatively high ambient light conditions. In projecting apparatus of this type, use can be made of a flexible, sheet-like, translucent screen for providing a uniform and controlled dispersion of the projected image-bearing light over a given zone. Examples of these translucent screens are described in U.S. Patent Nos. 2,180,113, and 2,287,556.

It will be appreciated that for this flexible viewing screen to be successful in operation, it must not be distorted so that the images projected from a film cassette will be projected through the screen in the intended manner. Should permanent surface distortions, such as wrinkles, cause a nonuniform screen surface, the projection of the transparent images will be correspondingly distorted.

One approach for mounting a flexible translucent screen on a frame, usually made of moulded plastic, on the projector housing is to use adhesive tape. Such tape rigidly secures the peripheral margins of the screen to the frame. Use of adhesive tape serves in a satisfactory fashion under normal circumstances. However, such tape presents problems whenever the projector is heated to temperatures above the projector's normal operating temperature. For instance, if the projector is stored in an automobile trunk, temperatures of about 1 60 F and above might be reached. As the temperature increases, both the screen and the player frame, which have different coefficients of thermal expansion and different molded-in stresses, dimensionally expand.Because of the different rates and magnitudes of expansion and due to the subsequent differences in thermal contraction, the screen and frame are dimensionally displaced relative to each other, both during heating and cooling. Another factor leading to this relative dimensional shifting between the screen and frame is creep which is accelerated at these high temperatures. These factors result in shifting of the screen and the adhesive tape relative to the frame so that forces are imparted to the screen by the tape. Because of the foregoing, the screen and the frame move relative to each other and do not return to their intended position relative to each other after such heating so that uneven differential forces are applied to the screen, whereby the screen surface becomes distorted.

Moreover, with the foregoing rigid connection, the screen is taut over the frame and, therefore, can occasionally have the surface permanently creased when depressed.

Consequently, heretofore approaches for securing a flexible, sheet-like viewing screen to -a projector housing give rise to potential problems, whereby the screen becomes physically distorted whenever subjected to high temperatures or is forcibly displaced from its intended position In accordance with the present invention, the noted potential problems associated with a sheet-like, flexible viewing screen becoming distorted are eliminated. Elimination is brought about by the provision of improved means for mounting the flexible viewing screen to a frame.

A photographic projection apparatus according to the present invention includes a viewing screen made of a flexible material and having a given coefficient of thermal expansion, a frame defining a projection area and having a coefficient of thermal expansion different from the given coefficient and means for generally uniformly tensioning the screen over the frame so as to maintain the screen in engagement with and supported by the frame in an undistorted condition over substantially the projection area despite thermal variations of the frame and screen.

One form of apparatus according to the invention, for projecting images from a photographic film, includes a screen of flexible material subject to thermal contraction and expansion, a housing having an end portion for supporting the screen, the end portion being subject to thermal contraction and expansion to a different degree from that of the screen and defining a projection opening for permitting the projection of images on to the screen, and means for mounting the screen on the end portion substantially to cover the projection opening, the said mounting means including means for tensioning the screen across the projection opening with a preselected, substantially uniform force for continuosuly maintaining portions of the screen in contact with the end portion and the portion of the screen located over the projection opening in a generally undistorted condition despite differential thermal expansion and contraction of the screen and the end portion, wherein the tensioning means provides a force within a range in which the screen will not, on the one hand, permanently deform due to creep during thermal variations because of its tight engagement with the said end portion and, on the other hand, will continuously maintain the screen in contact with the said end portion despite thermal variations causing different dimensional changes between the screen and end portion while allowing for flexing of the screen.

Such an apparatus permits controlled repositioning of the screen to a preselected relationship relative to the housing end portion after being forcibly displaced from such preselected relationship.

In order that the invention may be better understood, an example of apparatus embodying the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a diagrammatic perspective view of a rear projection motion picture player embodying the principles of this invention; Figure 2 is a diagrammatic view, in perspective, more clearly shows the novel mounting arrangement of this invention; Figure 3 is a diagrammatic view, partially in cross section, illustrating the player and mounting arrangement; and Figure 4 is a plan view of the viewing screen for use in an apparatus embodying the present invention.

As shown in Fig. 1, the photographic apparatus of the present invention preferably takes a form of a rear projector motion picture player apparatus 10. A similar type player is disclosed in the aforenoted U.S. Patent No.

3,740,127. Since the player apparatus 10 itself is not an aspect of the invention, only details of its construction and operation necessary for an understanding of this invention will be set forth. Essentially, the player apparatus 10 comprises a generally hollow box-like housing 1 2 defined in part by a viewing screen 14, details of the latter will be subsequently set forth.Positioned rearwardly of the viewing screen 14 and in adjoining relation to a top surface 1 6 of the housing 1 2 is a cassette-receiving chamber or well 1 8. In use, the cassette-receiving well 1 8 is constructed to removably receive a motion picture photographic film cassette 20 as well as extends to and is in communication with an opening 22 in the top surface 1 6 of the housing 1 2.

Pivotally mounted at the forward edge of the opening 22 is a door member 24 mounted for pivotal movement between a closed position, blocking the opening 22, and an open position, permitting insertion and removal of the film cassette 20.

The film cassette 20 is of the multipurpose motion picture photographic film type such as described in U.S. Patent No. 3,895,862.

Typically, the film cassette 20 includes a parallelepiped housing 26 which encloses a pair of motion picture film spools 28 and 30.

Upon such film spools 28 and 30, there is a motion picture film strip 32 disposed for transport between one spool and the other, in an arrangement wherein portions of the film strip are advanced in a plane parallel to the lower wall of the film cassette. Within such lower wall, there is formed an exposure/projection opening (not shown) for facilitating exposure and subsequent projection of the film strip 32 after processing of the latter.

Means are also provided in the film cassette to permit entrance of suitable illumination and for redirecting of such illumination in a generally downward direction through the film 32 and out of the exposure/projection opening.

An opening 34 is formed in one side wall of the film cassette 20, and a reflector element such as a mirror or a prism (not shown) is included within the cassette to redirect light entering opening 34 through the incremental, transparent frames of the film 32 and out through the exposure/projection opening. A conventional projection lamp (not shown) is mounted in the player apparatus 10 for suitably directing illumination into the cassette 20 for the above-noted purposes. Also included in the film cassette 20 is a processing station (not shown), such as described in the aformentioned patent, which is selectively operable for distributing processing fluid in a uniform layer over the film strip 32 during selective advancement of the latter beneath the fluid processor.Further included within the housing 1 2 and in adjoining relation to the cassette-receiving well 18, are means (not shown) for driving the film spools 28 and 30 and for progressively advancing the film strip 32 therebetween.

Mounted beneath the cassette-receiving well 18, in alignment with the exposure/projection opening is lens assembly 36. In the illustrated embodiment, the lens assembly 36 is constructed and arranged to project positive transparent images of the film strip 32 to the viewing screen 14 as the film strip selectively passes the exposure/projection opening so as to focus it at a given distance from the film strip. In this embodiment, the lens assembly 36 projects the images vertically downwardly along a path following optical axis 38. A reflective mirror 40 is mounted in the path of the optical axis 38 at an acute angle thereto so as to redirect the projected light-bearing image along a second optical axis 42, forwardly of the mirror and inclined upwardly from a horizontal axis 44, to the viewing screen 14. Preferably, the viewing screen 14 is disposed in substantially normal relation to the second axis 42 for purposes of reducing pictorial distortion.

A suitable rectangular shaped rim or bezel member 46 is mounted to the open front area defined by the player housing 1 2. The bezel 46 has a pair of retaining tabs 48 (one of which is shown in Fig. 3) which have a retaining recess 50 formed therein for purposes subsequently mentioned. In the assembled condition, the tabs 48 rest on the bottom wall 52 of the player housing 1 2. Included in the player housing 1 2 is molded plastic light shield or cone member 54; such as described in U.S. Patent No. 3,865,475, issued February 11, 1975. This cone member 54 is defined by cooperating baffle plates 56, 58 and 60 together to provide three sides of a light excluding tunnel extending from the mirror 40 to an end portion behind the viewing screen 14.As constructed and arranged, the baffle plates 56, 58 and 60 are disposed to extend from the reflective mirror 40 to the viewing screen 14, whereby the image-bearing light reflected along the optical axis 42 is easily accommodated. The fourth side of the cone member 54 is defined by the bottom wall 52 of the player housing 1 2. Formed in the top baffle plate 58 is an opening for permitting transfer of the image-bearing light from the optical axis 38.

Mounting of the screen 1 4 is further enhanced by the presence of an elongated rod 62 having its opposite ends retained in and by the retaining recesses 50. Due to this arrangement, the rod 62 allows the screen 14 to be pulled uniformly over its longitudinal extent. Accordingly, the rod 62 and the end portions of the baffle plates 56, 58 and 60 constitute a frame 64 defining the projecting area 65 over which the screen 14 substantially covers. Further, the recess 50 could be formed in the bottom of the tab 48. Alternatively, the light cone 54 could have the recesses formed thereof.

The rod 62 enables the screen 14 to be housed with the confines of the player housing 1 2 in an economical manner.

Referring back to viewing screen 14, such substantially provides the full face of the player housing 1 2. It is, as noted, arranged relative to the mirror 40 so as to have the image extend over substantially the full area thereof.

In this particular embodiment, the screen 1 4 is comprised of a translucent, flexible, sheet-like material which provides for even illumination without hot spots. Such a translucent viewing screen 14 may be of the type more completely described in the copending U.S. Application Serial No. 899,792, filed April 25, 1 978. Details regarding the particular composition of the screen need not be given insofar as it does not by itself form an aspect of the present invention. However, certain characteristics of the screen are important from the standpoint of better understand ing how the present invention is addressed to overcome the problems associated with such types of screens. Generally, the screen 14 is self-supporting and essentially made of plastic materials.In particular, the screen 14 is made out of a polymeric material having particles of a discontinuous phase and a polymeric contin uous phase and therefore is subject to stress relieving at high temperatures, such as 160"F, and creep. Further, the screen 14 has a given coefficient of thermal expansion which is different from those of the plastic baffle plates 56, 58 and 60 and as well rod 62.

Reference is made to Fig. 4 for best showing the viewing screen 14 as being comprised of a generally rectangular main section 66 which has along the margins thereof marginal flaps 68 with a generally trapezoidal planar shape and a folded over portion 70. An eyelet member 72 connects each of the flaps 68 with a folded over portion 70 for strength.

Referring in particular to Figs. 2 and 3, there is best illustrated the improved screen mounting arrangement 74 of this invention which compensates for thermal variations of the player 10 causing the screen 14, and the cone 54 and the player housing 1 2 to expand and contract. Essentially, the mounting arrangement 74 is comprised of a plurality of resilient coil springs 76 having preselected spring constants. Each of the springs 76 has one end attached to a corresponding one of the eyelet members 72. The opposite end of certain ones of the springs 76 is connected to suitable openings 78 formed in the baffle plates 56, 60 and the wall 52 as well as about a retaining lug 80, extending inwardly from a side wall of the player housing 1 2.

It will be best observed in Fig. 2 that the springs 76 are generally equidistantly spaced along each marginal edge of the viewing screen 14. In this manner, the springs 76 tension the screen uniformly. If unbalanced forces were applied, screen 14 would end up being distorted.

Should higher than normal operating temperatures be reached, for example in the order of 160"F, such as when the player 10 is in the trunk of an automobile then the cone 54, the player housing 12, and the rod 62 will expand at different rates and magnitudes than the viewing screen due to the disparity of their coefficients of thermal expansion. Owing to these differences, the noted components will have a tendency to move relative to each other.

Moreover, at such high temperatures the molded in stresses of the plastic materials are relieved and the effects of creepage are increased. These factors will also effect relative movement between the components. Upon cooling, the viewing screen 1 4 and the player housing 1 2 including the frame 62 will contract, but, however, at different rates and magnitudes. Ordinarily, the differences of these thermal coefficients of expansion together with creep and stress relieving cause relatively small relative movements of the components in the order of a few thousands of an inch. Without the mounting arrangement 74, even such small differences would distort the viewing screen 1 4 over the projection area 65.The differential rates and magnitudes of relative movement occurring because of thermal heating and cooling of the noted materials are compensated for by the springs 76 maintaining the screen 14 in constant contact with the cone 54 and the rod 62 despite the fact that such cone and rod are expanding and contracting relative to the screen 1 4 due to the noted thermal variations.

Moreover, the mounting arrangement 74 enables the screen. 14 to reposition itself from the preselected planar arrangement covering the area 65 after being forcibly displaced therefrom. The foregoing advantageously allows the screen 14 to be accidentally depressed when in use without becoming creased and without need for a back-up support. Advantageously, the spring constant of each of the springs 76 is selected to ensure that the screen 14 remains constantly engaged against the cone 54 and the rod 62, which, as noted, defines the frame 64 for supporting the screen 14. For purposes of illustration and not limitation, it has been determined that with using a screen 14 and player 10 of the type noted, the spring force of each of the springs 76 should be approximately 1/2 pound with a range of- + 1 /4 pound. With such a spring force available, the spring arrangement 74 allows the screen. 14 to reposition itself after being depressed, but simultaneously ensures constant contact between the screen and the frame 64 without causing the screen and the cone member 54 to permanently deform especially at high temperatures of the type mentioned.

Claims (7)

1. A photographic projection apparatus including a viewing screen made of a flexible material and having a given coefficient of thermal expansion, a frame defining a projection area and having a coefficient of thermal expansion different from the given coefficient and means for generally uniformly tensioning the screen over the frame eo as to maintain the screen in engagement with and supported by the frame in an undistorted condition-over substantially the projection area despite thermal variations of the frame and screen.

2. A photographic projection apparatus for projecting images from a photographic film, the apparatus including a screen of flexible materialsubject to thermal contraction and expansion, a housing having an end portion for supporting the screen, the end portion being subject to thermal contraction and expansion to a different degree from that of the screen and defining a projection opening for permitting the projection of images on to the screen, and means for mounting the screen on the end portion substantially to cover the projection opening, the said mounting means including means for tensioning the screen across the projection opening with a preselected, substantially uniform force for continuously maintaining portions of the screen in contact with the end portion and the portion of the screen located over the projection opening in a generally undistorted condition despite differential thermal expansion and contraction of the screen and the end portion, wherein the tensioning means provides a force within a range in which the screen will not, on the one hand, permanently deform due to creep during thermal variations because of its tight engagement with the said end portion and, on the other hand, will continuously maintain the screen in contact with the end portion despite thermal vairations causing different dimensional changes between the screen and end portion while allowing for flexing of the screen.

3. A photographic projection apparatus according to claim 2, wherein the mounting means further permits repositioning of the screen to a preselected relationship relative to the opening after displacement thereof from the preselected relationship.

4. Apparatus in accordance with claim 2 or 3, wherein the tensioning means include a plurality of coil springs, each one having one end portion attached to a preselected marginal edge portion of the screen and the opposite end portion connected to a preselected portion of the housing to provide the substantially uniform force.

5. Apparatus in accordance with claim 2, 3 or 4, further including an elongate rod connected at each end to the housing, the rod cooperating with one marginal edge portion of the screen so as to facilitate the folding of the screen over the rod.

6. Apparatus in accordance with claim 6 wherein each coil spring has a spring force in the range of 1.1 to 3.3N.

7. A photographic projection apparatus substantially as herein described with reference to the accompanying drawings.