The present invention relates to an overhead projector, and more specifically relates to an overhead projector of a so called reflecting type in which the light source lamp is on the same side of the original to be projected as is the projection lens.
Overhead projectors are currently in wide use nowadays in various business, scientific, and educational establishments as simple, inexpensive, and convenient enlarging projection devices. Typically, an overhead projector has more flexibility during use than does a slide projector, and is usually used for simultaneously projecting an enlarged image of an original image which is being drawn by the hand of the operator on an original image plate or film which is resting upon a mounting table. Thus a lecturer can illustrate a lecture by making a drawing on said original plate or film in an ongoing fashion, elaborating various parts of the drawing as his lecture proceeds, for example.
The known types of overhead projectors are through type overhead projectors and reflecting type overhead projectors. The general form of overhead projector in use nowadays is a through type overhead projector, in which the light source lamp which illuminates the original is on the opposite side of the original from the projection lens which forms the projected image; in other words, the light rays for forming the image pass from the light source lamp on one side of the original, through the original, to the projection lens on the other side of the original. Typically, just below the original there is placed a Fresnel lens, on the horizontally disposed flat upper surface of which the original rests, and this Fresnel lens concentrates the light from the light source lamp so as to illuminate the original more strongly. Typically the projection lens is mounted on a frame above the original as it rests on this Fresnel lens, and further, typically, near the projection lens there is mounted a planar deflecting mirror for deflecting the path of the light rays which will form the image on the screen through an angle of approximately 90.degree., so that these rays shine substantially in the horizontal direction to impinge upon the substantially vertically mounted screen, which is apart from and in front of the projector.
On the other hand, a so called reflecting type overhead projector has been heretofore known for a long time, but has until recently not been in wide use. In such a reflecting type overhead projector, the light source lamp which illuminates the original is on the same side of the original as the projection lens which forms the projected image; in other words, the light rays for forming the image pass from the light source lamp on one (usually the upper) side of the original, through the original, are reflected off a mirror just below the original, pass through the original again, and then pass to the projection lens on the same side of the original as the light source lamp. Typically, just below the original, between it and the reflecting mirror, there is again placed a Fresnel lens on the horizontally disposed flat upper surface of which the original rests, and this Fresnel lens again concentrates the light from the light source lamp so as to illuminate the original more strongly. Typically the projection lens is mounted on a frame above the original as it rests on this Fresnel lens, and again near the projection lens there is mounted a planar deflecting mirror for deflecting the path of the light rays which will form the image on the screen through an angle of approximately 90.degree., so that these rays shine substantially in the horizontal direction to impinge upon the substantially vertically mounted screen, which is again provided as apart from and in front of the projector.
In this connection, as a modification of the reflecting type overhead projector itself, it is considered that the lower flat surface of the Fresnel lens may be covered with a mirror coating so that the Fresnel lens and the reflecting mirror are formed as a unitary element, or alternatively the combination of the Fresnel convex lens and the reflecting mirror is replaced by a Fresnel concave mirror which has a reflecting surface on the grooved upper surface thereof. It is to be understood that these modifications should be included in the concept of the reflecting type overhead projector of which the improvement by the present invention is contemplated, as will be seen hereinafter. In any case, the function of the reflecting table in which a Fresnel lens and a reflecting mirror, or alternatively a Fresnel mirror, are incorporated is analogous to that of a concave mirror.
This configuration of reflecting type overhead projector is reasonably serviceable, and has the advantage that very little space is needed between the original and the lower part of the projector, and is thus simpler, lighter, and more compact than the above described type of through overhead projector; and further, since the light source lamp is located in the projector head, i.e. above the original, near the projection lens, out in the open, cooling of the light source lamp can be accomplished much more easily, without any danger that the rising heat from the lamp should damage the original; but such reflecting type overhead projectors have in the past suffered from the disabling problem that no good multilayer composite formed of a Fresnel lens and a reflecting mirror, of sufficiently high reflectivity, has been commercially available. Accordingly, up until recently, the reflecting type overhead projector configuration has not been in wide use.
However, recently a good multilayer composite formed of a Fresnel lens and a reflecting mirror, of very adequately high reflectivity, has become commercially available, and accordingly the reflecting type overhead projector has become a practicable and useful type, especially in virtue of its inherent advantages as previously explained. However, a new problem has become recognized and troublesome with respect to such a reflecting type overhead projector configuration, as follows.
Since in the reflecting type overhead projector configuration the light source lamp, which is necessarily quite powerful, is located above the original and shines directly downwards onto said original, a danger exists of the hand of the projector operator being burned, if the operator should put his hand into the path of the light rays. Since, as mentioned above, one of the principal operational modes of such an overhead projector is for simultaneously projecting an enlarged image of an original image which is being drawn by the hand of the operator on the original, for example when a lecturer is explaining a lecture by making a drawing on the original in an ongoing fashion for elaborating various parts of the drawing as his lecture proceeds, such a putting of the operator's hand into the path of the light rays shining from the light source lamp onto the original is quite likely and common. Accordingly, the very real danger of burning of the operator's hand should be protected against, in some way, if the reflecting type overhead projector configuration is to be viable. However, merely reducing the power of the light source lamp in order to avoid such burning of the operator's hand is not a proper way to handle the problem, since brightness of the projected image on the screen is very important, from the point of view of performance of the overhead projector system as a whole.