Overhead Projector

This invention relates to overhead projectors and in particular to an overhead projector which is provided with a projection lens that is movable parallel to the plane of a stage on which an original is placed, and by this movement of the projection lens trapezoidal distortion of a projective image and loss of focus can be eliminated when projecting an original at an oblique angle to a screen at a vertical position.

This type of overhead projector is disclosed in Examined Japanese Patent Publication No. 55-18891 and U.S. Pat. No. 3,547,530 (Patented Dec. 15, 1970).

As illustrated in FIG. 1, when projecting an image to a vertical screen a at an oblique angle .theta., trapezoidal distortion and loss of focus occure on the projected image b of the screen. In order not to cause such trapezoidal distortion and loss of focus, a device is made to move a projection lens c and mirror g in the direction parallel to the plane of stage d.

By the way, in this type of overhead projector, it is required, in order not to lower the illumination efficiency, that the illumination light transmitted to illuminate an original through an illuminating system should always be collected at the position of an entrance pupil of the projection lens irrespective of the movement of the projection lens c.

In the said prior art, such requirement is satisfied by tilting a Fresnel lens h or by moving it parallel to the plane of a stage d on which an original is placed. However, this entails the problem that the construction of the device becomes complicated and the device itself becomes larger in size in order to move a larger and heavier Fresnel lens h alone or together with the stage d. Particularly, the type of device wherein the Fresnel lens is tilted becomes more complicated in construction since a light source e is also moved. Further, a simple and quick change of the lamp can not be hoped for because the light source also moves.

The main object of this invention is to provide an overhead projector whose construction is simple and compact, and advantageous in changing a lamp. The illumination light transmitted through the illuminating system can always be collected at an entrance pupil of a projection lens to whatever position it is moved, as a small and movable portion in the illuminating system also moves according to the movement of the projection lens.

The features to achieve the object of this invention consist in an overhead projector which is provided with a movable projection lens that moves parallel to the plane of a stage on which an original is placed, and it possesses a condensing lens in the lighting system to collect light from a light source to the position of an entrance pupil of the projection lens, and a movable mirror to direct light from the light source to the condensing lens with another means to change the position of light collection by the condensing lens by moving the movable mirror according to the movement of the projection lens.

It is a further object of this invention to provide an overhead projector by which the movement of a movable mirror, in accordance with the parallel movement of a projection lens to the plane of a stage on which an original is placed, is surely accomplished by a simple constructional interlocking means with the conventional overhead projectors.

A further feature to accomplish the object of this invention comprises two pulleys that are different in diameter but rotatable together, an end of one wire is fastened to one of the pulleys and another end of the wire is fastened to a projection lens, while an end of another wire is fastened to an end of the other pulley and the other end of the wire is fastened to a movable mirror. Thus, the movable mirror is made movable with the movement of the projection lens by the pulleys and wires.

Other objects and features of the present invention will be better understood from a consideration of the following detailed description taken in conjunction with the accompanying drawings.

FIG. 1 is a side view of a prior art overhead projector to explain the prior art.

FIG. 2 is a perspective view of an overhead projector as an embodiment of this invention.

FIG. 3 illustrates a side view of the overhead projector shown in FIG. 2.

Referring to the accompanying drawings, an embodiment of the present invention will be described.

FIGS.2 and 3 illustrate an overhead projector which is embodied by the present invention. The device comprises, like those already known, a base stand 1 on the upper surface of which a stage 4 is formed in the horizontal plane to place an original and a projector head 17 mounted above the base stand 1.

Projector head 17 is comprised of a projection lens 5 which projects an image of original on a stage 4 and a deflection mirror 11 which directs projected light from the projection lens 5 to a screen (not shown), and they are supported by a pair of reverse L-shape poles 7 secured at one side of the stage 4 on the base stand 1.

Inside the lower part of a base stand 1, a light source 2 and a movable mirror 3 to direct light from the light source 2 to a stage 4 are mounted and the movable mirror 3 is supported by a horizontal pin 18 rotatably. A Fresnel lens 6 as a condenser lens is mounted right below the stage 4 and collects light from the light source 2 through the movable mirror 3 to an entrance pupil of a projection lens 5.

A projection lens 5 is mounted on a lens holder 8 and it is possible to make focus adjustment. The focus adjustment is done by a rear focusing method taking advantage of the variation in focal length. By supporting the lens holder 8 in such a way as to slide on the crooked upper horizontal portion of a pole 7, it is possible for the projection lens 5 to move parallel to the plane of stage 4 in the direction indicated by arrow A. The movement of this projection lens 5 is made by operating a knob 9 mounted on a slide of the lens holder 8. Utilization of frictional force generated between a shaft of the knob 9 and the pole 7 or gearing of rack and pinion is suitable for this kind of machanism. Stopper 10 is provided at an end of pole 7 to protect the lens holder 8 from sliding off.

Deflection mirror 11 is supported on the upper surface of a lens holder 8 tiltably and movable in the direction of arrow A with the movement of a projection lens 5. It can also be rotated up and down independently on the lens holder 8 in the direction of arrow B. By movement of the deflection mirror 11 in the direction of arrow A with an up and down rotation in the direction of arrow B, light from the projection lens can be directed to a screen (not shown) at a desirable height. When projection is made at an angle of elevation to a vertical screen, trapezoidal distortion and loss of focus will occur on the projected image of the screen. However, such distortion and loss of focus can be eliminated by adjusting the position of the projection 5 in the direction of arrow A.

In a guide tube 12 supported along pole 7, wire 13 as shown in FIG. 3 is inserted for pulling. One end of this wire 13 is connected to a lens holder 8 and an another end is wound around a larger pulley 14 mounted in the base stand 1. The larger pulley 14 is biased in a direction to maintain the wire 13 in tension. A smaller pulley 15 is mounted on the same shaft as the larger pulley 14 to move together. This smaller pulley 15 and a movable mirror 3 are connected by a short wire 16. One end of the short wire 16 is wound around the smaller pulley 15 and fixed, and another end is connected to a lower end of the movable mirror 3.

By this means the amount of movement of the projection lens 5 in the direction parallel to surface of the stage is converted, through wire 13, into the rotation of a larger pulley 14, and between the larger pulley 14 and the small pulley 15, the amount of movement of the wire 13 is converted into the amount of movement of wire 16 at a fixed ratio, and an angle of inclination of the movable mirror 3 can be changed according to the amount of movement of the short wire 16.

By this action, displacement of optical axis X for light collection in the illumination system becomes possible, and it enables the Fresnel lens 6 to always collect light from the light source 2 to an entrance pupil of the projection lens 5. The direction correction of optical axis X for light collection can thus be achieved by the movement of a small movable mirror 3 only which is mounted in the illuminating system. The larger size device in a more complicated mechanism will, therefore, not be required. Further, as the light source 2 can be fixed at a certain position, it is quite advantageous to utilize a quick change system of lamp by preparing a spare lamp in a base stand 1. Still further, in order to correct the direction of the optical axis for light collection, the mechanism for interlocking a projection lens 5 and a movable mirror 3 is comprised of the wire mounted along pole 7, the small pulley in the base stand and another wire. Being simple in its mechanism, it can be easily adapted to a conventional overhead projector, and interlocking action is secured.

While the present invention has been described in connection with a specific embodiment, it is to be understood that the invention is not to be limited to the embodiment described. For example, the means for moving the movable mirror with the forward or rearward movement of the projection lens is not limited to the one described. All alterations and modifications falling within the spirit and scope of the inventions as set forth in the appended claims are included in the invention.