Rear projection
With rear projection (engl. Rear projection ) to describe the projection of static or moving images "from behind", that is, behind the screen in the direction of the viewer or of the recording medium (in contrast to the shadow , the only silhouettes shows). It is used in particular in film production and playback.
The inventor of rear projection is the engineer Josef Behrens , who patented the first rear projection process in 1918 and a second process optimized for color film in 1935.
production
In production, rear projection is used to combine actors in the film studio or on the stage of a theater with separately recorded background shots. To do this, the actor stands in front of a screen onto which the background images are projected from behind.
The front projection represented a further development . Today, such scenes are usually realized using blue screen technology .
The rear projection process shaped the appearance of films for an entire epoch. Heavy cameras, the studio principle and limited budgets made it necessary in certain cases to combine the action of the actors in the studio with a background action. The whole team didn't have to drive to a remote location, instead the second camera team shot the background templates, which were combined with the foreground action in the studio using rear projection. The rear projection was often used for car trips, ship shots or close-ups of the actors.
After the use of this technology has declined more and more in recent decades, there has now been a trend towards rear projection again in recent years: With new materials and technologies - including new generations of projectors - rear projection is once again taking on a stronger role and replaces front projection in some areas (for example, in television studios, in window advertising, point of sale , at concerts and cultural events, in control rooms and so on). Often there are settings that involve artistic alienation or dream sequences that can be produced in this way without digital compositing .
Procedure
The basic principle of rear projection is that actors act in front of a large, partially transparent surface (screen) onto which a stationary or moving image is projected from behind. In addition to the film camera , a powerful projector and a screen are also required.
Slide projectors with a large-format slide, so-called rear projectors, were used as projectors. These projectors were able to illuminate a large area evenly. For this purpose, very powerful projection lamps with up to 5000 watt incandescent light or up to 10,000 watt carbon arc lamps were used. These are able to deliver the required luminous flux of 40,000 to 60,000 lumens . Special ventilation systems are needed to remove the heat and fumes from the arc lamps from the studio. Often the projectors had their own cooling system to cool the lamp house and the film level.
The rear projectors had to be soundproofed to enable full tone recordings in the studio. There should be a remote control for sharpness, which is located in front of the screen near the camera, because this is where the sharpness of the image can best be assessed.
In addition to still images, moving backgrounds were also projected. However, normal cinema projectors are not suitable for moving rear projections because they cannot deliver the required amount of light without burning the film. Film projectors have a three-wing shutter to avoid flickering (the image changes during one of these three shutter speeds). Rear projectors, on the other hand, only have a single-wing lock with 240 ° light sector and 120 ° cover. This increases the usable amount of light. The shutter sectors of the camera and the rear projector are synchronized via synchronous motors .
The highest demands are placed on the image position of rear projectors for moving subjects. The companies Mitchell and Bell & Howell, who have also earned a very good reputation with film cameras, built projectors with special gripper mechanisms. These gripper mechanisms work with two additional locking grippers, which ensure that the image remains in good shape.
The DEFA studios' backpro system was equipped in 1971 with a variable 35/70 mm projector from the Dresden camera manufacturer Pentacon and delivered very good results.
In the 1930s and 1940s, when the film materials were not yet that sensitive, three projectors were sometimes used, which projected the image simultaneously onto the rear projection screen with the identical template. Luminous fluxes of up to 125,000 lumens were achieved.
Special screen walls are required for rear projection. They used to be made from acetyl cellulose . For this purpose, the liquid was sprayed onto large glass plates and drawn off after drying. Later on, screens were made of nylon or perlon fabric. The former rear projection system of the DEFA studio was equipped with an 8 × 16 m screen made by Harkness in England.
Rear projection screens must meet certain parameters. On the one hand, they must deliver a sharp, high-resolution image that is evenly illuminated right up to the edges, but must not form a “hotspot”. In this context, the "hotspot" is the direct shining through of the projector light through the screen. Modern rear projection screens represent a compromise between diffuse scattering and permeability ( transparency ).
The more transparent the screen, that is, the greater the brightness, the greater the fall-off of light towards the edges of the screen. This fall-off in light is a critical problem for the credibility of the combined settings. The light rays must be deflected more strongly by the projection optics at the edge of the screen towards the camera than in the middle, and the distance is greater in each case. This leads to the aforementioned decrease in light. This is why long focal lengths are used on the projector and the camera because the difference in brightness between the central rays and the edge rays is smaller.
The larger the angle of view of the camera and projector, the greater the fall-off of light at the edge of the projected image. The so-called "bend" angle describes the angle that results from the extension of the edge rays from the projector and camera. As a rule, this angle should not exceed 17 °. To achieve this, lenses with a long focal length are used on the projector and lenses with medium focal lengths on the camera. The combination of a 150 mm projection lens with a 75 mm lens on the camera, which results in a "bend" angle of 14 °, is common for rear projection.
The following example demonstrates the effort required to show an actor in a medium long shot in front of a rear-projected winter landscape. In order to project the background image onto a screen 2.40 m high and 3.40 m wide and record it with the camera, a studio at least 31 m long is required. The projector with a 150 mm lens is 20 m away from the screen and the camera is 11 m away on the other side. The depth of field of the camera optics ranges from 8 m to 11 m with an aperture of 2.8. The actor can act at a distance of 9 m. This example demonstrates the space requirements of a rear projection system.
At DEFA in Berlin there was a special rear-pro studio with a screen of 8 × 16 meters. Behind the screen was the so-called "rear tube". The projection room was 53 m long, with the projector on a rail in the middle.
templates
In the preparation of backpro scenes, the background motifs must be determined and rotated. It is beneficial to have a detailed storyboard . From this, the viewing direction and setting sizes can be recognized.
The camera height, the focal length and the angle of inclination of the background camera and the studio camera should roughly match. If the background action of a street scene is later to be combined with a dialogue scene between two actors in the studio, it is advantageous to work with doubles when setting up the background . You can use them to set up the picture. The background action, such as passers-by or road traffic, should not come too far into the foreground, as this would have a disillusioning effect. The horizon line of the sky is defined by the focal length, the angle of inclination of the camera and the camera height.
The following data should be noted and noted on the flap:
- Time of day
- Position of the sun
- Focal length and camera type
- Sharpness settings and aperture
- Scene number and variant
- in the case of templates for car journeys, the speed of the camera vehicle
Very wide-angled and very long focal length lenses should be avoided with the originals, as they can only be combined with unsatisfactory results in the rear projection. There should not be any blurred objects in the foreground in the template, otherwise the impression could later arise that there are two levels of focus, one in the background template and one in the foreground action. For static recordings, the camera should be absolutely fixed and not move or vibrate.
Car trips
Rear projection was often used for realizing car journeys. This means that these recordings can be made relatively cheaply and quickly, and easily integrated into the shooting schedule. There were a number of arguments why this technology was used for over 40 years, although it was never perfect and is mostly noticeable to the trained eye today.
There are several things to consider when including the templates. Before the templates are rotated, the season, time of day, light and weather mood must be determined according to the script or follow-up sequences. It is important to know which vehicle it is, how high the cab or the seats are. People in a bus sit higher than in a car.
Then, depending on the resolution, the viewing directions must be determined. A default setting is to record the driver and passenger from the front or the side. However, it is also possible to adjust it at an angle. If you want to cut between two perspectives, the background and the lighting mood must be connected. Either you rotate the respective perspectives one after the other or with two cameras at the same time. It is important to note the different speed effects when looking at different directions. If the camera films in the direction of movement forwards or backwards from the vehicle, the impression corresponds to the real speed, but if you look sideways at a 90 ° angle to the direction of movement from the vehicle, the speed appears faster than the real speed. A phenomenon that also applies to trailer trips. At the same speed, setting at 90 ° will produce a faster impression. In order to achieve the same speed impression during assembly, the lateral settings should only be rotated at 60% of the original speed. Normally focal lengths between 35 mm and 40 mm are used for background shots with 35 mm film, for side shots it is advisable to use shorter focal lengths in order to minimize the speed effect and motion blur.
Nowadays, when car journeys are filmed in real life and the trailer drives through a forest area, a multitude of interactions arise. Light falls through the trees and hits the actors, reflections on the windshield reflect the treetops and the sky. One possibility would be to use a second rear-view system and a screen above the car to reflect reflections on the windscreen.
Realization of rear projections
Once the original has been rotated, the negative is developed and copied onto a soft positive material on the optical printer . For the rear projection template, each setting should be copied three times and it is practical to insert a sharpness test board at the beginning of the roll in order to be able to check the image sharpness before each run. Before each setting there should be a counter leader so that the foreground action can be coordinated. There should be no glue points in the rear projection, as this could damage the projector's locking catch and lead to fluctuations in the image position.
Decisive for the success of a rear-projection recording is the reduction of the hot spot and the drop in light to the edge. There are different possibilities for this:
- Gradient canvases: These canvases are a little less transparent in the middle than at the edge and compensate for the fall in light.
- Reduction of the "bend" angle due to the large focal lengths of the camera lenses
- If you want to pan during the rear-projection recording, pans over 40 ° should be avoided, as they lead to distortion and loss of light
- If the camera is pivoted slightly out of the optical axis (3–4 °), the hotspot can be placed outside the image.
- negative aperture in the beam path of the projector (metal star).
Another compromise has to be found in the choice of the camera lens, the distance between the actors and the rear projection screen and its lighting. The longer focal length reduces the hot spot, but at the same time reduces the range of depth of field. The actor and the screen must be in the depth of field of the camera optics without the actor being too close to the screen, as the light that illuminates the actor would otherwise hit the screen and reduce the contrast of the projected background image.
So that the light that illuminates the actor does not hit the screen, a “scissor light” is often used from two sides or a very steep light from above - incident light is to be avoided. The light sources should be carefully hidden from the screen. It is advisable to paint decorative objects black from behind, otherwise the light from the screen will be reflected by them and reduce the contrasts. Another problem is the alignment of contrast, gradation, color temperature and color contrast of the background action and foreground. The decisive factor here is how the original was copied, how high the color temperature of the projection lamp is and how the color temperature is changed by the screen. It is advisable to do a test to try different filterings when copying the rear projection.
Optimizing this process requires many attempts and test runs. The cameramen who have worked with rear projection in the past have built up their own wealth of experience over the years, which can rarely be expressed in formulas or fixed rules. Nowadays, with the increased quality demands, rear projection is only used when the "rear projection" only takes up part of the image, such as when driving a car or looking through a window behind a curtain.
The film materials with higher sensitivity, which have been improved in recent years, would require less high luminous flux from the rear projectors and less foreground lighting - unfortunately, rear projectors are rarely used any more.
In the future, bright digital projectors in HDTV format with a resolution of 1100 lines and 100% image stability will be a possible alternative.
In the theater and show area there are large format slide projectors (15 × 15 cm) that work with 4,000 / 6,000 or 12,000 watt HMI projection lamps. This enables good results to be achieved in conjunction with so-called opera foil .
reproduction
There are various areas of application for rear projection in playback:
TV
In principle, every (tube) television works on the principle of rear projection. The image generation takes place here behind the image surface.
movie theater
In cinemas , the rear projection is often found in movie theaters with limited space. If there is no more space behind the auditorium for a conventional projector room , rear projection is a good alternative if there is enough space behind the screen . This requires specially constructed screen materials with a special translucency . In the case of analog film , the film strip is to be inserted into the film projector in the opposite direction compared to conventional projection . Correspondingly, sound pick-up devices are to be installed reversed. Examples are Bellevue 4 in Zurich ; one of the 4 former Karlstor cinemas , Munich .
Concerts
In cooperation with Crypton Future Media and Sega , music concerts with virtual actors in life size are projected onto a light-absorbing glass wall by means of rear projection, which is particularly popular with fans in Asia. To this day, mainly fictional Japanese anime characters have been used. This made the vocaloid singer Hatsune Miku particularly popular .
Other uses
Rear projections are often used where there is enough space behind the screen and the projection technology should be hidden for aesthetic, design or other reasons (noise and heat development, sensitivity of the devices). These are, for example, exhibition stands, displays, advertisements or stage backgrounds in the theater .
literature
- Claus Grosskopf: Josef Behren's inventions 1918-1947 . Berlin 2007, ISBN 978-3-7861-2560-0 .
- Laura Mulvey: A Clumsy Sublime . In: Film Quarterly Vol. 60, No. 3 (2007), p. 3.
- Johannes Binotto : Looking back on representability. On the aesthetics and expressiveness of the rear projection. In: Filmbulletin 2.13 (2013), pp. 37–43 ( online ).