GB2134279A

GB2134279A - An overhead projector

Info

Publication number: GB2134279A
Application number: GB08401196A
Authority: GB
Inventors: Jesper Munk Laursen
Original assignee: Memofax AS
Current assignee: Memofax AS
Priority date: 1983-01-21
Filing date: 1984-01-17
Publication date: 1984-08-08
Also published as: FR2539886A1; DK24283D0; DE3401709A1; DK24283A; GB8401196D0; GB2134279B; FR2539886B1

Abstract

An overhead projector whose image-forming and beam deflecting optical system (15, 16) is mounted on a holder (21-25) attached to the projector housing is provided with a diapositive assembly (31) which is carried by a carriage (30) slidably mounted on the holder. The assembly comprises a diapositive frame (18) and a supplementary lens system cooperating with the first-mentioned optical system in one extreme position of the carriage to project a diapositive disposed in the frame. In the other extreme position of the carriage, the assembly is disposed completely outside the beam of light emitted by the source of light of the projector, and in this position the projector projects in a normal manner a transparency placed on its glass plate. <IMAGE>

Description

SPECIFICATION An overhead projector The invention relates to an overhead projector with a preferably box-shaped housing whose top is at least partly constituted by a glass plate and in which there is disposed a source of light to illuminate the glass plate, and with an image-forming and beam deflecting optical device mounted over the glass plate by means of a holder attached to the housing, said projector including a diapositive assembly with a diapositive holder and a supplementary optical system.

In a known overhead projector of this type, the diapositive adapter assembly is in the form of an auxiliary device which is optically placed after the optical device--also called the OHP system in the following-which then does not participate in the image formation, but only acts as a source of light. In this case it is moreover necessary to insert a condenser lens for the picture to be evenly exposed. The structure also involves complications in connection with changes in the height of the image on the screen. Such changes are generally effected by rotating the beam deflecting mirror about a horizontal axis, but this causes a proportion of the beam of light to be cut off by the frame enclosing the diapositive, unless the auxiliary device is also vertically adjusted.

The cut-off proportion of the beam of light involves a risk of damage to the frame by heating.

Another option, when the OHP system cannot or is not to be removed, consists in using a diapositive system which deflects the beam of light so that it by-passes the OHP system completely.

It is common to the known applications of overhead projectors as diascopes that they require assembling operations and storage of auxiliary equipment. To this should be added that assembling and disassembling often cause focusing changes which call for adjustments.

The object of the invention is to provide an overhead projector of the present type which can be shifted between the two applications by a single operation without requiring readjustments.

Accordingly, there is provided an overhead projector with a preferably box-shaped housing whose top is at least partly constituted by a glass plate and in which there is disposed a source of light to illuminate the glass plate, and with an image-forming and beam deflecting optical device mounted over the glass plate by means of a holder attached to the housing, said projector including a diapositive assembly with a diapositive holder and a supplementary optical system, characterized in that the diapositive assembly is firmly mounted on the overhead projector above the glass plate in such a manner that is is movable between a position in which it together with the optical device forms a diascope, and a position in which it is disposed outside the beam of light between the glass plate and the optical device.In this structure, shifting is performed merely by moving the diapositive assembly from one position to the other. The problem of storage has been solved in that the assembly remains in place on the projector, no matter whether it is in use or not. The shifting does not result in changes in the distances which are critical to the image formation, so readjustment is not required.

Another advantage is that the lens or system of lenses forming the OHP system is also incorporated as a component of the system of lenses used for the projection of diapositives.

The height of the image on the screen can be changed merely by rotating the mirrer, irrespective of the application of the projector.

The invention will be explained more fully below with reference to the drawing, in which Figure 1 is a schematic side view of a conventional overhead projector and an image screen and with a dotted beam of light, Figure 2 is a view similar to Fig. 1 but shows an overhead projector of the invention adjusted to project d iapositives, Figure 3 is a perspective view showing the optical system holder for an embodiment of the overhead projector of the invention.

In the drawing 10 is a box-shaped housing whose top, as far as most of it is concerned, is constituted by a glass plate 11. At the bottom of the housing 10 there is provided a source of light 12, e.g. a lamp with a filament in the form of a flat spiral, and under the source of light there is fitted a concave mirror 1 3 which directs the light up against a condenser lens 1 4 placed below the glass plate 11. The lens 14 concentrates the beam of light on an image-forming OHP lens system represented by a convex lens 1 5 in Fig. 1. An inclined mirror 1 6 transmits the beam of light from the lens 1 6 to a vertical image screen 17, on which there is formed an enlarged representation of the image or the text of a transparency placed on the glass plate 11.

In Fig. 2 the same projector 10-16 is shown supplemented with a diapositive assembly comprising a frame 1 8 in which a diapositive 1 9 may be placed, and a supplementary lens system represented by a convex lens 20, which together with the lens 1 5 forms an optical diapositive system focusing the diapositive plane on the screen 1 7. The frame 1 8 cuts off the proportion of the beam which is not used for the image formation.

Fig. 3 shows a practical embodiment of an optical system allowing the projection of both transparencies, as shown in Fig. 1, and diapositives, as shown in Fig. 2, as desired. The system is carried by a curved arm 21 which is secured in a sleeve 22 in a rwhanner not shown in detail; the sleeve 22 can be moved up and down on a vertical rod 24, which is secured to the projector housing (not shown), by means of a rotary knob 23. This adjustment serves focusing purposes on the screen when tranparencies are to be projected.

The arm 21 carries a U-shaped frame 25 between the legs of which a lens plate 26 with the OHP lens 1 5 is disposed. A carrier means 27 for the mirror 16 is so placed in the frame 25 that it can be pivoted about its lower edge to change the height of the image on the screen 1 7. A slightly downwardly bent edge part 28 of the lens plate 26 serves to cut off undesirable light. The legs of the frame 25 have inwardly staggered, downwardly projecting extensions 29 with holes which slidably mount two round bars 30 forming a carriage which carries a dispositive assembly generally designated by 31. The diapositive assembly is composed of a casing 32 substantially in the shape of the frustum of a pyramid and having a base plate which forms the diapositive frame 1 8.

The underside of this frame may be coated with a film (not shown), e.g. of aluminium, reflecting infra-red rays, and a filter plate 33 filtering off infra-red rays is disposed below the frame. Inside the casing 32, the supplementary lens system 20, Fig. 2, is so placed in a generally known and not shown manner that it can be moved up and down on operation of a rotary knob 34 to focus the diapositive plane on the image screen.

When the diapositive assembly 31 is in the position shown in Fig. 3, it is disposed completely outside the beam of light from the condenser lens 14, and in this position the apparatus functions in the normal manner as an overhead projector projecting a transparency, placed on the glass plate 11, on the screen 1 7. All that is required to shift the apparatus to project diapositives is moving the assembly 31 on the bars 30 to the left in Fig.

3, which will place the supplementary lens system 20 contained in it in a position in which it is co-axial with the OHP lens 15 and forms together with the lens 1 5 a lens system projecting a diapositive in the frame 1 8 on the image screen 1 7.

The carriage arrangement allowing the diapositive assembly to be moved with respect to the 0HP system might be designed in other ways than the one shown and described in the foregoing. For example, the round bars 30 might be replaced by edge flanges engaging over each other. The assembly might also be pivotally instead of slidably mounted.

Claims

1. An overhead projector with a preferably box-shaped housing whose top is at least partly constituted by a glass plate and in which there is disposed a source of light to illuminate the glass plate, and with an imageforming and beam deflecting optical device mounted over the glass plate by means of a holder attached to the housing, said projector including a diapositive assembly with a diapositive holder and a supplementary optical system, characterized in that the diapositive assembly is firmly mounted on the overhead projector above the glass plate in such a manner that it is movable between a position in which it together with the optical device forms a diascope, and a position in which it is disposed outside the beam of light between the glass plate and the optical device.

2. An overhead projector substantially as hereinbefore described with reference to and as illustrated in Figs. 2 and 3 of the accompanying drawings.