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Magic lantern
The first light source ever used in the magic lantern was a candle.
Candles however were comparatively weak illuminants, also, when they burnt
they became shorter and the position of the wick got lower, so the flame
continual became out of alignment. Therefore soon the candle was replaced by
the oil lamp.
Later the standard 'Candle' became a measurement of light source intensity.
It was originally defined as a one sixth pound candle of sperm wax, burning
at the rate of 120 grains per hour.
Oil Lamps
The first little oil lamps burned a variety of oils including vegetable
oil, sperm oil, paraffin, whale oil and others. The oil lamp was one of the
simplest to use. However some care was needed to position the flame at the
focal point of the lens, and it was also necessarily to trim the wick
properly because an overlong wick produced a wobbly and unfocused light.
Lime Light
The limelight is a very bright gas lamp, invented in 1825 and widely used
for magic lantern illumination and theatrical lighting until about 1900. It
consists of a block of lime (calcium oxide) heated in an ox hydrogen flame.
The two gasses oxygen and hydrogen are fed in by a pipeline system with
valves. The flame from the nozzle heats the lime block, which then glows
white hot. The jet is fitted with a mechanism with lever taps and cogwheel
adjustment for turning and raising the lime block.
This limelight produces up to 1,000 candles.
Now you know where that saying came from? "In the lime light" !!
Carbon arc
Some of you might have a welding device using this technology, I have one
:). The biggest projection devices often used carbon arc lamps, which were
used for a considerable time. A couple of touching carbon rods were
connected to an electric power source. When the rods were pulled apart
slightly, this produced a bright light, caused by the incandescent carbon
particles burning between the points.
Since the carbon rods are gradually consumed, one had to adjust the rods
continually to maintain the correct distance between them, and the correct
position relative to the optical centre of the lantern, during the show.
This difficult job was later often achieved by an automatic regulator.
Acetylene illuminant
This simple and cheap illuminant was invented in 1836 by the English chemist
Humphrey Davy. Acetylene gas is generated from calcium carbide, a hard
greenish substance with a pungent odour. When burned with the correct amount
of air, acetylene gives a pure, white light.
The carbide was placed in a generator and allowed to come into contact with
a controlled supply of water. Then the acetylene gas was formed that was
passed to the burner through rubber tubing.
Acetylene was not only popular with lanternists, but also used for
illumination in locations where electric power was not available, and now
also used for metal welding. However the burners were never of sufficient
brilliance to replace limelight or carbon arcs and were generally used for
small scale projection.
Animated pictures
The first machine to project animated pictures onto a screen was the
praxiscope invented in 1877 followed two years later by the British
inventor Eadweard Muybridges zoopraxiscope. Then followed by Frenchman
Lumiere, inventor of the Cinematographie, camera, film processor and
projector in one unit.
Modern projector
A projector is the key piece of technology in showing movies in cinemas
around the world. To show a modern film based movie, there are five things
you absolutely need:
- A way to advance each frame of the film.
- A way to project the image from the film.
- A way to read the audio.
- A surface to project the image on.
- A system to play the audio.
A projector provides the first three items on the list. While movies are
usually projected onto a screen, or a large white wall. A movie projector is
a device that continuously moves film along a path so that each frame of the
film is stopped for a fraction of a second in front of a light source.
The light source provides bright illumination that casts the image on the
film through a lens onto a screen. It takes an large amount of film to
make a movie. Most movies are shot on 35mm film. You can get 16 frames on 1
foot (30.5 cm) of film. Movie projectors move the film at a speed of 24
frames per second, so it takes 1.5 feet (45.7 cm) of film to create every
second of a movie.
Calculations
- One second
1.5 feet (24 frames per second divided by 16 frames per foot)
- One minute
90 feet (1.5 feet per second multiplied by 60 seconds)
- One hour
5,400 feet (90 feet per minute multiplied by 60 minutes)
A two hour movie plus five minutes of previews = 2.13 miles (11,250 feet
divided by 5,280)
You can use this formula to figure out just how much film it took to show
the next movie you go see. Just multiply the number of minutes in the movie
by 90 to get the number of feet of film.
Because a feature length film is so long, distributors divide it into
segments that are rolled onto reels. A typical two-hour movie will probably
be divided into six reels. In the early days, films were shown with two
projectors. One projector was threaded with the first reel and the other
projector with the second
The projectionist would start the film on the first projector, and when
it was 11 seconds from the end of the reel, a small circle flashed briefly
in the corner of the screen. While the second reel was rolling, the
projectionist removed the first reel on the other projector and threaded the
third reel.
Platter
 In
the 1960s, a device called a platter began to show up in cinemas. The
platter consists of two to four large discs, about 4 feet in diameter,
stacked vertically about 2 feet apart.
A payout assembly on one side of the platter feeds film from one disc to
the projector and takes the film back from the projector to spool onto a
second disc. The discs are large enough to hold one large spool of the
entire film, which the projectionist assembles by splicing together all of
the lengths of film from the different reels.
Once a projectionist splices the film and loads it on the feed platter,
he threads the film through the platter's payout assembly and into the top
of the projector. A strip of film has small square holes along each side
called sprocket holes. These holes fit over the teeth of special gear-like
wheels called sprockets. The sprockets, driven by an electric motor, pull
the film through the projector.
The film needs to advance one frame, pause for a fraction of a second and
then advance to the next frame. This is accomplished using one of two
mechanisms. The first one uses a small lever known as the claw, which is
mounted on a bar next to the film's path. The claw is connected to the outer
edge of a wheel that acts as the crank.
The circular motion of the crank makes the claw lift up and out to come
out of a sprocket hole and then down and in to catch onto another sprocket
hole. This causes the film to advance one frame. The speed of the sprockets
is closely synchronized with the lever action of the claw to make sure that
the claw is consistently advancing the film at a rate of 24 frames per
second.
The key element in a projector is the light source. Carbon arc lamps have
been used since the early 1900s but have a very short life. Xenon bulbs are
the most commonly used lamps today. Xenon is a rare gas with certain
properties that make it especially suited for use in projectors
- In dense enough quantities, it will conduct electricity.
- As a conductor, it glows very brightly.
- It will provide bright illumination for a substantial amount of time
(4,000 hours +).
The xenon bulb is mounted in the centre of a parabolic mirror located in
the lamp house. The mirror reflects light from the bulb and focuses it on
the condenser. The condenser is a pair of lenses used together to further
intensify the light and focus it on the main lens assembly. The heat
generated by this focused light is incredible. That's why film melts so
quickly when the projector stops spooling it.
As the focused light leaves the lamp house and enters the projector, it
is intercepted by the shutter. The shutter is a small, propeller-like device
that rotates 24 times per second. Each blade of the shutter blocks the path
of the light as it comes to a certain point in its revolution. This blacking
out is synchronized with the advancement of the film so that the light
doesn't project the fraction of a second when the film is moving from one
frame to the next.
Without it, the film would seem to flicker or have faint impressions of
the images out of sync. Many projectors use double shutters that rotate in
opposite directions. This causes the light to be cut off from both the top
and bottom of each frame, further reducing the possibility of flicker.
Before the light gets to the film, it also passes through an aperture gate.
The aperture gate is a small, removable metal frame that blocks the light
from illuminating anything but the part of the film that you want to see on
the screen.
From the aperture gate, the light passes through the film and into the
main lens. The lens is removable and can be changed depending on the format
of the film. The two most common lenses are flat and Cinemascope. Many
projectors have a turret that allows both types of lenses to be mounted, and
the projector will rotate the required lens into place.
From the projector, the light goes through a view port at the front of the
projection booth and travels to the front of the auditorium until it reaches
the screen. Finally, the images from the film appear on the screen.
Projectionist
A projectionist may perform the following tasks
- Make sure the film is in good condition
- Thread film into two projectors
- Start the projector, adjust the sound level, focus the image and
keep an eye on the projectors
- Change over and close down projectors
- Perform minor repairs and basic maintenance to equipment and make
safety checks
- Operate slide projectors, spotlights or similar equipment
- Operate computerised equipment (called 'bio-box') used in large
cinema complexes
- Supervise other bio staff and trainees
- Ensure compliance with safety regulations for cinemas
- Operate air conditioning equipment, emergency lighting, curtains,
music and control heating levels
- Work in the ticket box, candy bar or office
- Assist with general theatre maintenance.
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