An episcope is an optical device used to project mostly opaque media. The original is illuminated and the diffusely reflected light is thrown via a mirror through an objective onto a projection screen . Episcopes belong to the family of projectors and work according to the principle of front light projection , which is why they are also referred to as front light projectors in today's parlance .
The expression comes from the ancient Greek επί epi “on it” and σκοπεῖν skopein “to look”. Synonyms are antiscope , paxiscope and mural projector .
Episcope by Eduard Liesegang (around 1926)
Classic episcope from Leitz
Classic episcope by Heinrich Malinski
Modern episcope of the Enna company (template is placed on top of the device)
An episcope is used to project opaque flat objects, pictures or illustrations from books onto a screen. The episcope consists of a support surface for the template, a lighting system to illuminate this template and a lens. As a rule, a deflecting mirror is attached in front of or behind the lens, which directs the beam onto the vertical screen.
The lighting system is uniform and high submission illuminance illuminate. A steep incidence of light increases the effect. However, smooth, shiny surfaces, e.g. B. smooth art paper, reflections that get directly into the lens and then appear as light spots on the projected image and reduce the contrast (legibility). These reflections can be reduced by varying the angle of incidence of the lighting.
The illuminance on the original can be increased by using a concave mirror behind the lamp and a condenser lens ( convex lens ) between the lamp and the original. Lightening mirrors on the sides of the support surface also support the bundling. As a result, the light is better bundled on the template and the luminous flux of the lamp is better used.
For the projection beam path, only the small portion of the diffusely reflected light from the original can be used that radiates in the direction of the lens. The rest is lost and can not be used for image generation . However, the quality of the lens, its diameter or its aperture ratio has a decisive influence on the light intensity of the projected image on the screen. The deflecting mirror directs the light from the horizontal template onto the vertical screen. Without a mirror, the image would be reversed or mirrored.
In episcopes with larger template areas, lighting systems with lamps that have a long light field, e.g. B. halogen floodlights or metal halide lamps , Liesegang Episkop E10, or several reflector lamps, Sofortpresenter plus, VEGA international doo. With this arrangement, the original can be illuminated more intensely and more evenly.
The different designs of episcopes depend on the template area and areas of application.
Small episcopes with a template area about the size of a postcard are more used for home use. With these devices, the template is usually placed on the template field from above.
Episcopes for school use have a template surface of approx. 19 cm across and 16 cm high. This means that A4 templates can be projected upright in different sections, for example with the Leitz LE 19 or the Liesegang E11-522 / 260 episcope. The projected image is easy to read on a screen of approx. 1.8 m due to the almost ten times magnification. The illuminance on the screen shows a contrast that (depending on the room light) still enables good visibility.
Large devices with a template area of up to 30 cm × 30 cm provide a good overview of large templates, such as maps, prints, drafts, etc. The overview is at the expense of legibility, as only five times the magnification is achieved on the same screen.
For many devices there are versions in which the template is pressed against the template surface from below using a template holder. If the original holder is removable, the projector can also be placed directly on a large original. With some large devices, the template can also be placed on the template surface from above. Modern episcopes have a break light so as not to dazzle the user when changing templates. As soon as the document cover is opened, the powerful projection lamp switches off and a weaker lamp switches on.
The antiscope as a sign episcope is a special design. The projector is adjustable on a vertical column and projects onto a drawing table. The magnification can be adjusted from 0.5 to ten times and more depending on the distance.
Combinations of episcope and slide projector are called epidiascopes .
Due to the front projection, non-transparent as well as transparent (with restrictions) templates of episcopes can be projected. The templates do not have to be processed beforehand for projection, i.e. H. copied onto transparent media and, if necessary, brought to a specific format, such as daylight or slide projectors . Possible originals for direct projection are, for example: photos, images, documents, fabrics, inlays and more. The best results are achieved with the most matte templates possible. A plane mirror on the template surface only results in a dark field on the screen. When using transparent media (not too dense) it is sufficient as a compromise to provide this with a white, matt base and to insert it into the episcope. With modern episcopes, a template can be projected onto a screen to a size of 1.6 × 1.6 m and larger, depending on the room light.
Because of the small proportion of diffuse reflection that is reflected in the direction of the lens, lamps with a high output must be used in contrast to back-light projection. In the past, the power required for this could only be provided by arc lamps, which made the devices large and unwieldy. With the development of halogen projection lamps and metal halide lamps with a higher luminous flux per watt, the devices became a bit more manageable, but the enormous heat development was still a problem. So that the housing could withstand the heat, episcopes were mainly made with a metal housing. A sufficiently active cooling system, such as a fan, was also necessary. With the development and use of the low-voltage halogen projection lamps, the heat problem could be reduced considerably, as was the use of two projection lamps which illuminate the original from two sides (mostly from above and below). The housings were so more compact and could also be made of lighter plastic, which made the devices much more manageable. The performance of the cooling system could also be throttled and the noise level lowered during operation. In addition, through infrared-permeable mirrors behind the lamp and blocking infrared filters between the template and the lamp, parts of the thermal radiation on the template could be reduced. The exposure of the template due to the extremely high illuminance in connection with the temperatures that occur should be taken into account in order to avoid damage to sensitive templates. Further disadvantages result from the nature of the template. Due to the front-light projection, this should be as matt as possible, rich in color and contrast in order to achieve a good projection image. Furthermore, no changes can be made to the template during the projection, such as markings in text passages, inserting graphics, etc. To do this, the original must be removed and then reinserted. In contrast to overhead projectors, the process of changes cannot be followed on the screen.
The so-called visualizers represent a further development of the episcopes . In terms of their functionality, they differ considerably from the classic episcopes projection type. As with an episcope, the Visualizer can be used to record any type of template (books, photos, three-dimensional objects, etc.) using a camera or scanner and then deliver a high-resolution image to a video projector, for example .
The modern version of the episcope is the instant presenter, which also weighs around 20 kg.
Episcopes are rarely or not at all used in school lessons or presentations. They have been replaced by the easier-to-use overhead projectors , where changes to the template can be better tracked and which are therefore much better suited for teaching and presentation purposes. Because non-transparent templates can be used, episcopes are popular among painters , artists and copyists . They are used for preliminary or tracing of sketches, drawings, photos, etc. For some time now, episcopes have also been used more often by airbrush artists. Some also convert them to projectors.
- leica-wiki: pictures
- F. Paul Liesegang: The projection system . in: Scientific applications of photography first part . Published by Julius Springer, Vienna 1931, page 251.
- Wolfgang Grau, Hugo Heine: Technique of projection . Beuth Verlag GmbH, Berlin 1980, page 393, ISBN 3-410-11227-8 .
- Gottfried Schröder, Hanskarl driver: Technical optics . 9th expanded edition, Vogel Buchverlag, 2002, page 121, ISBN 978-3-8023-1923-5 .
- F. Paul Liesegang: The projection system in: Scientific applications of photography, first part . Published by Julius Springer, Vienna 1931, page 253
- FWU: Equipment sheet, Liesegang E11 - 522/260. FWU Institute for Film and Image in Science and Education, Grünwald, 1982
- Wolfgang Grau, Hugo Heine: Technique of projection. Beuth Verlag GmbH, Berlin, 1980, page 43, ISBN 3-410-11227-8 .
- Wolfgang Grau, Hugo Heine: Technique of projection. Beuth Verlag GmbH, Berlin, 1980, page 339, ISBN 3-410-11227-8 .
- Gottfried Schröder, Hanskarl driver: Technical optics. 9th expanded edition, Vogel Buchverlag, 2002, page 221, ISBN 3-8023-1923-0 .