Overhead projector

As an overhead projector (also overhead projector, overhead recorder or overhead projector, in the GDR Polylux, in the Bundeswehr Prokischreiber, Switzerland overhead projector or Prokischreiber ) refers to optical projector that mounted on transparent films writing, pictures, and graphics enlarged projected onto a screen.

In the projector Trajanus of Liesegang with the vertical approach from 1927 all the elements of an overhead projector are included. A horizontal work surface for the projection of transparencies and flat test arrangements, such as magnetic lines. The projector is facing the viewer and projected onto a screen behind the speaker. In 1931, after a basic idea of Walther Bauersfeld at the company Carl Zeiss, the write - projection apparatus " Belshazzar ". Widespread application only reached a 1960 matched by the 3M Company on the market overhead projector.

The cars in the GDR for the overhead projector brand name Polylux gave the telecast Polylux her name.

Function

An overhead projector works much like a slide projector. The position of the vertical slide occupies a horizontal illuminated, usually covered by a transparent film writable working surface. The vertical beam path continues with arranged according to the objective mirror image horizontally to a wall. The work surface is on floor units of a glass pane above the Fresnel lens. The illumination below the fresnel lens consists of a lamp, an infrared transmissive concave mirror and a condenser lens in general, which is often coated infrared reflective. As lamps high-voltage halogen lamps, later, low-voltage halogen lamps or metal halide lamps were used. To increase the lamp life, a saving feature is often possible. The saving feature reduces the power light only slightly, but increases the lamp life considerably.

The vertical beam is focused by the Fresnel lens in the lens, and then continued via a deflection mirror horizontally to a screen.

The deflection of the mirror is made ​​so that the working with the projector speakers, the screen in the back and looks at the audience. He has this to Projecting on the slide the right way forward and it can complement handwritten during the lecture.

During an upward projection on a vertical screen, the image is distorted. A square is a trapezoid. To avoid this distortion, the screen can be tilted forward, or the lens is moved. The mirror remains in this lens adjustment for lens adjustment under an angle of 45 ° and the projection is performed on the vertical screen, but higher.

During an upward projection, only with the corresponding tilt of the deflection mirror, in addition to the distortion of a blur due to the different distances of the upper and lower edge of the picture is produced. The image is partially blurred due to the limited depth of field. By tilting the lens blur this can be compensated.

The projection is as bright that it manages generally without black out, hence the term " overhead projector ". For compact devices, the lighting device is at the top next to the mirror and the work surface is designed as a mirror Fresnel lens.

Designs

Use all types of construction for the size of the object field two standard sizes:

• Starting from 10 inches, the size is 250 millimeters by 250 millimeters
• From DIN A4 portrait / landscape size is 285 millimeters by 285 millimeters.

Stands

By selecting different lenses the ratio of projection distance to image size can be changed. At the same distance from the image increases, the shorter the focal length and the smaller, the longer the focal length. For floor models usually have a focal length of 315 mm is used. For compact devices often is a varifocal lens f = 280-320 mm.

When the device is inserted into a table or projection trolley, all controls should be accessible on the surface of the projector. These include the on-off switch, saving switch, an adjustment option for the lamp house and possibly also the lamp changer. After opening the countertop must the power supply be turned off forcibly and a defective lamp can then be replaced. For this, the condenser lens on the lamp must still opened or removed.

The distance between the work surface and the lens is changed depending on the projection distance when focusing. Since the image of the lamp needs to be mapped in the lens, an adjustment is provided for the lighting device. With the use of zoom lenses this adjustment is not necessary, since the distance to the work surface when focusing is not changed.

As lamps for simple claims to the light power halogen lamps are 24 volt / 250 watts, for larger claims halogen lamps, 36 volts / 400 watts used. For large rooms with large video walls, depending on the ambient light 10 square meters and more, are versions with metal halide lamps 400 watts or 575 watts. Projectors with metal halide lamps should have an immediate re-ignition.

Travel equipment

Overhead compact, with folded column for the transport

Overhead with symmetrical mirror Fresnel lens in projection position

Overhead compactly for transport

Compact travel devices are either built as floor models, but collapsible, or they have the lighting system next to the lens in the lens head and the work surface consists of a, on the bottom mirrored Fresnel lens. The lens head can be folded for transport flat on the work surface.

Foldable travel devices can with brighter lamps, and metal halide lamps are fitted, which is not possible for travel units with mirror Fresnel lenses.

For devices that use a mirror Fresnel lens, the projection beam passes twice through the template. In. from the illumination system to the mirror fresnel lens, it is then reflected by the mirror fresnel lens back to the lens. Therefore, no thick originals, such as motion models or LC - bearing panels can be projected. It lead to duplicate edges in the projected image. The Fresnel lens is smooth on the top and protected with a protective layer against damage. On the bottom of the grooves are mirrored. The lens should be as thin as possible to avoid double edges. To stabilize it is adhered to a metal plate.

Symmetrical Mirror Fresnel lenses cause a shift of the lens from the center. This creates a slight upward projection without distortion. However, the lens must represent a larger image circle.

With asymmetrical mirror Fresnel lens, the lens can stay in the center above the lens, the demands on the lens are less high.

Accessories

On the surface of the countertop, there are retractable locking pins for adjustment of pre- projection screens with frame and corresponding holes. On the side surfaces of the housing mounting options for accessories, for example roller device are high or transversely, or storage space provided. At the lens head can an attachment be provided for an anti-glare, which may be necessary for high -performance devices despite reduced glare Fresnel lens.

• Roller device: for film rolls five or ten meters, even more
• Storage areas: as a storage space for films, pens, etc. in addition to the work surface
• Diffuser: in spite of reduced glare Fresnel partly required
• Diavorsatz: to project slides in the size 5 cm x 5 cm (for floor models until 1998)
• Support panels: for data or video projection (until 1998)

Application

Overhead projectors are a suitable also for a larger auditorium, an alternative to blackboard, whiteboard and flipchart. They are used for visualization of facts at lectures (eg, at school, at university or at meetings ). One can use at an edge contiguous multilayer films containing, for example, the caption of a drawing or different details later on fold-down films. Also suitable for laying flat, sometimes even moving models of transparent plastic (for example, mathematics, physics or engineering classes ) are available. There are also instruments with a transparent display that can be set on this projector.

Also transparent screens have been put on the market that can replace a video projector - Further - sometimes overhead manufacturers themselves. They are placed on the working surface of the overhead projector, and this projects the image displayed on the auxiliary device image on the screen.

Alternatives

Today, the overhead projector in many places by a combination of video projector and visualizer ( document camera ) will be replaced. The image on this Visualizier is transmitted directly to the video projector. This has the advantage that transparent projection screens are unnecessary. It can be normal leaves are used and described.

Also, a presentation on a computer is possible on the video projector. Presentation programs allow a design with multimedia content such as animated text, images and videos. A viewing section of such a program is still called film.