OVER HEAD PROJECTOR

Abstract: Over head projector for projection of opaque matters comprising an image-forming unit consisting of a downward facing, relatively large, ring-shaped, concave mirror (5) and a smaller, upward facing, convex mirror (6) which embraces the image formed by the concave mirror (5) of the matters to be projected. The curvature of the convex mirror (6) is such that the light rays reflected from it pass through the hole in the concave mirror (5), reflect for the plane mirror (9) and form an image at a suitable distance from the projector. A glass pane (11) seals the opening on the upper side of the concave mirror (5) and another glass pane (12) closes the entire opening under the concave mirror (5) and supports the convex mirror (6). The glass panes (11, 12) stop dust from entering the reflecting surfaces of the curved mirrors. The space under the convex mirror (6) provides room for an illumination device (3) and a reflector (4).

Description: - -- - The invention is an over head projector which can project opaque materials, such as texts and pictures from books. The over head projector is an extremely useful educational tool. The educationist has the instrument in front of him while facing the pupils, and he can work with the original as it is projected on a screen behind him. Unfortunately, only translucent originals can be shown with the known over head projector. This is a serious limitation. Opaque materials must be projected with an episcope. This is heavy and expensive device. Turth-er. ore it is impossible to work on the original during projection. The li ht output when pr ecting opaque materials is low. Ξ-piscope producers compensate for this by giving the original a very intense illumination. It is the problem of low light output which makes the open construction of the over head projector unsuitable for opaque materials. In addition the need for correction of dif ractional aberrations demands a more complicated lens than the single element lens of the over head projector. A well corrected lens with a great aperture is expensive. The over head projector as described permits pictures, texts or other objects on non-translucent background to be pro¬ jected with reasonable brilliance even if the illumination of matter to be projected is modero.te. The projection is not subject to optical aberrations caused by diffraction. This is attained by replacing the lens in the known over head projector by an optical system with great aperture con¬ sisting of two curved mirrors. One of the mirrors is a ringsha'.ed, spherical or paraboloid, concave mirror with, a short focus. It faces downward towards the material to be projected. The other mirror is smaller, convex and faces upwards. It is big enough to cover the area of the image reflected by the concave, downward facing mirror above it. It is placed so that all light rays reflected by the BAD concave mirror hit it before reaching their focal point. The shape of the convex mirror is such that the light rays are reflected from this mirror on slightly converging paths. They pass through the hole in the concave mirror and are reflected from the inclined, plane mirror above it. At a suitable distance from the projector - they form a sharply focused picture, right side up, on a vertical sur¬ face. A construction of the over head projector as described is shown in thedrawing. The materials to be projected 1 are placed on the table 2 within the circular area lighted by the illumination device 3 with reflector - . The light rays from the projected material which enter the ring-shaped opening between the concave mirror 5 and the convex mirror 6, contribute to the projected image. These light rays reflect from the concave mirror 5 - so that all rays from a given point ~ on the projected material, tend to converge against a common focal point 8. However, before they reach this point, the light rays hit the convex mirror 6 and are reflected. They maintain a converging path and pass through the hole in the concave mirror 5- After re¬ flection from the plane mirror 9 5 t ey converge at point 10, at a suitable distance from the projector. The pro¬ jection is free from errors caused oγ diffractional aberrations, and it will be right side up, as seen from the position of the pupils. A glass pane 11 covers the hole on the upper side of the ring-shaped, concave mirror 5 > and stops dust from reaching the reflecting surfaces of the curved mirrors. Another glass pane 12, covers the entire underside of the ring-shaped, concave mirror 5. It serves to stop dust from entering the reflecting surfaces, as well as holding the convex mirror 6, the reflector and the illumination device 3 i place. Current for the illumination device 3 is led through a wire in the glass pane 12. The focusing knob 13 can adjust the distance between the curved m i rr o rs 5,6 and the projected material.

Claims: Patent claim: 1. An over head projector, comprising an illumination device (3) eventually with reflector 0+) , to light the original (1) which is to be projected, and image-forming elements (5>6) which via preferrably a plane mirror (9); form an image (10) in a suitable distance from the projector c h a r a c t e r i z e d b.y : that the image-f rming elements constitute two curved mirrors (5.6); one of which (5) is relatively large, ring-shaped, concave and faces the original (1) to be projected, while the other (6) is smaller, convex and faces the hole in the ring-shaped, con¬ cave mirror (5) > that the convex mirror (6) is placed in such a way, and is big enough, to embrace the area con¬ taining the image reflected by the concave mirror (5)• 2. Over head projector as described in claim 1 , c h r a c t e r i s e d b y : that the two curved mirrors (5> ) are sealed by, and held in place, relative to each other by two antireflex coated glass panes (11,12); one of which (11) covers the hole on the upper side of the concave mirror (5)> the other (12) closing the ntire underside of the ring-shaped, concave mirror (5) and sunoorting the con¬ vex mirror (6). 3. Over head projector as described in claim 1 and 2, c h a r a c t e r i z e d b y : that in the cavity below theconvex mirror (6) is room for the illumination device (3) with reflector (*+). OMPI BAD RlGW/^ ^N A TlOJ^

Inventors: BAKKE INGE, BAKKE, INGE
Applicants: BAKKE INGE, BAKKE, INGE
Application year: 1983
Main classification: G03B21
IPC codes: G03B21