Machines used to duplicate motion picture film.

Types of film printers

All motion picture printing machines have either step or continuous motion. Both of these two kinds can be either be exposed in contact or using optical methods, so there can be four kinds of machine:

  1. Step contact
  2. Continuous contact
  3. Step optical
  4. Continuous optical

Movement of the film

A step printer copies the film, one complete frame at a time, in a manner comparable with a camera, which photographs the original scene, one complete frame at a time, or a projector which shows the film frame by frame. The movement of the film through the gate of the machine is therefore intermittent as in a camera or projector.

A continuous printer treats the film as an unbroken ribbon and draws the film through it at a uniform speed. There is no intermittent movement.

Exposing the image

A contact printer has the original film and the raw stock, onto which the copy is to be made, in intimate contact with each other, emulsion surface to emulsion surface, at the point in the machine at which the exposure is made. The two films will reach and depart from this point by separate paths, but will be in contact at the point of exposure.

In an optical printer, the original film is placed in a projector head, the equivalent of an ordinary projector, which projects the picture which is on the film, via a lens, onto the raw stock in the camera head. The two films are separate and follow completely different paths. An optical printer may also be thought of as a camera photographing a picture which is illuminated in the projector.

Differences for an archive

An important difference between the contact and optical printer is that a contact printer, perforce, makes a copy of the same size as the original, whereas an optical printer can make a copy of the same size, or larger or smaller. Thus if an Archive has any films on any of the many different gauges which have been introduced, but which did not find wide use or have since become obsolete, and the need is for a copy on a currently used gauge, then the copy will have to be made on a form of optical printer. Even so, a projector will have to be found or made which will accommodate the unusual gauge. Some of the film gauges to which this paragraph refers are 60mm, 65mm, 28mm and 17.5mm.

Another property of an optical printer is that, in the case of damaged film, it may be possible to pass it through the projector head where only the original film is, when it might not be possible in a contact printer where there have to be both original and copy films in the same gate.

Light sources

We need to consider also two forms of the light which exposes the copy film. These are specular and diffuse.

In specular illuminations, the exposing light reaches the film virtually all from a single direction. There is a beam of light like a projector. This is produced from a light source which is small in size, and is then concentrated into a beam in virtually a single direction by a concave mirror or by condenser lenses or by both.

Diffuse illumination is like daylight on a day when cloud covers the sky. Light reaches all places from all directions and there are no hard shadows to be seen. The light from a fluorescent lamp is diffuse – it emits light not from one small area, but from the whole length of the tube, and it cannot cast a hard shadow. Diffusion is usually achieved in a printing machine by inserting an opal glass in the light path.

The form of illumination used has a profound effect upon the results of motion picture printing.

Consider contact printing. Although we speak of the films being in contact, this contact is not perfect, particularly when the original is buckled. With specular illumination, at any point of the picture where the two films are not quite in contact but are separated by a minute amount, that point of the picture will still be sharply rendered on the copy. In other words, the specular beam will still cast a hard shadow of that picture point onto the copy film. In the case of diffuse illumination, in which light comes from many directions, the shadow of that picture point on the copy will be diffused. Therefore when using diffuse illumination in contact printing, it is vital to achieve very good contact.

So, why not use specular illumination exclusively? If the original film has any dirt or abrasion on its base surface, this dirt and abrasion will also be sharply imaged onto the copy – an effect which is not desired. With diffuse illumination, if very good contact between the emulsion surfaces of the original and copy film is achieved, then the wanted picture contained in the emulsion will be sharply copied, but dirt and abrasion on the base surface, which is perforce separated from the copy emulsion by the thickness of the base of the original, is not sharply copied; its image is diffused, and may be sufficiently so that no image of the dirt is apparent at all.

In the case of optical printing, the same is true, but in the case of specular illumination, the unwanted effect is perhaps more severe, and in the case of diffuse illumination, the suppression of dirt and abrasion is probably less effective and there is a great loss of light which has to be provided for by slower printing or providing more powerful illumination.

Suppressing scratches

There is another way of suppressing scratches in optical printing. This is what is known as 'wet’ or 'liquid’ printing.

Debrie TAI wet gate printer

In this case, the original film at the time of exposure is coated with a liquid which has a refractive index near to that of the film base. This liquid fills up the abrasions and presents a smooth surface for copying and so the abrasions do not appear in the copy. The liquid most commonly used are perchloroethylene and trichlorethylene. These are volatile, which minimizes the problem of drying the film after printing; and they are toxic, ventilation must be provided.

It should be noted that wet printing will suppress abrasions, but will not suppress any opaque particles of dirt which may be on the base surface.

Diffuse illumination helps to suppress both dirt and abrasion.

Wet gate printing is also effective in contact printing, but there is a difficulty with shrunken original film. In printing, the same number of perforations of original and of copy film must go through the machine, but the original is shrunken; therefore, the actual length over any given number of perforations is less. So, as the two films go through the machine they must slide against each other slightly at the areas where they are in contact. The presence of the wetting liquid inhibits this sliding. Wet contact printing is widely practiced in the cinema industry, but with normal dimension, good condition films.

Shrinkage and printing

Shrinkage of the original film has a profound effect on copying in another respect.

In the case of step printing, the machine moves a complete frame into a printing aperture which is the size of one frame, and there each frame is exposed while the two films are held stationary. The sliding which has to take place, as described above, occurs while the films are being moved, during which time the light is cut off and no exposure takes place.

In the case of continuous printing, the films move smoothly past an illuminated aperture and exposure of some part of the film takes place the whole time. Therefore, this sliding occurs during exposure, and consequently the image is, to some extent, smeared on the copy. It is possible for this slippage to take place continuously and smoothly all the time, in which case the effect is to smear the images evenly, i.e. make not quite sharp all the horizontal lines. In this case the effect may be hardly noticeable – it is just a little diminution of the sharpness of the picture.

But more commonly, and almost invariably, the slippage does not take place perfectly evenly. There is a tendency for the two film surfaces to stay together for a while and not move, so at that point the image is quite sharp. Then it slips and you have quite a noticeable smear over a short period, and so on. This slippage may take place, not over the whole height of a frame but a certain part, e.g. one part could be perfectly sharp whereas another part of the frame could be smeared. It does not take place at the same point in every frame, so there is a very disturbing effect in that different parts of the picture are alternately sharp and smeared.

Sometimes a horizontal line in the picture will be printed as it enters the exposure area; the films will then slide quickly and the same feature will be printed again on another part of the copy film so that there are two distinct images of the horizontal line. This is sometimes called “double imaging”. If the original film is grossly shrunken, it may not sit on the sprocket teeth properly at all and may rise up over the tips of the teeth and jump forward. The result in the copy is that every time that happens, the picture jumps out of frame on the screen. This is known as “roll-up” and renders the copy virtually useless.

A similar effect can occur on a step printer if the pins moving the film in the gate fail to pull a complete frame into position, perhaps because of some damage to the pins.

Unsteady images

There is another potential cause of unsteadiness in a step printer.

Some step printers, like good cameras, use registration pins. One pair of pins, the transport pins, pulls the film down (they do not fit the perforations exactly) transporting it to an approximately correct position, and then the register pins enter into another pair of perforations and hold the film fixed in position because they fit the perforations. But if the original film is shrunk you cannot do that because shrinkage is variable, and if you try to put a standard size pin into a shrunken hole you tear the film to pieces. Moreover, the register pins cannot be the correct size for both the shrunken original and the unshrunk copy film. Therefore, register pins are impracticable with shrunken film.

In that case there is a problem with inertia in the gate of a step printer, because when the films are moving they have the tendency to carry on and do not want to stop straight away. The transport pins which pull the film down do not fit the perforations and the films tend to carry on a bit further after the pins have stopped moving. If the “carry on” is not consistent you will have unsteadiness in the form of a rapid vertical jump. You can resist this “carry on” from inertia by having a tightly sprung gate. This will stop the film but in doing so it could readily aggravate any damage from which the film suffers. Speed of printing is very relevant in as much as the faster you are going the more serious the effects from inertia are going to be, and the resulting unsteadiness is going to be difficult to avoid.

It must be noted that Sound Track can only be printed on a Continuous film printer.

Printing sound tracks

A sound track is a continuous record and if printed on a step printer, a discontinuity is introduced which would be reproduced as an unwanted sound. It is also worthy of note that if the length of the exposing aperture in the direction in which the film passes through the machine is very short, the smearing effect is diminished.

Shrinkage occurs not only in the length of the film, but also in the width.

This leads to the possibility that the original shrunken film can move from side to side while passing the point of exposure in the printer. This usually does not happen rapidly, so that the effect in the copy is that the picture floats gently from side to side. The movement is usually not only slow but slight, and in this case is not intrusive. This 'lateral weave’ could be prevented, in some kinds of printer, by providing separate guided paths, to and from the exposure point, for the two films, the guide for the shrunken film being narrower than the standard width one for the copy film. This provision is not normally made on printing machines.

Commercial labs and shrinkage

Concerning the degree of shrinkage which different machines and laboratories can deal with, it is not possible to make any very hard and fast statement because several factors are involved.

The following must be regarded as an approximate guide:-

  • Commercial laboratories, usually using normal continuous contact printers, will not usually be able to copy anything shrunken more than 1% and are liable not to achieve satisfactory results at more than 0.5%.
  • An archive using its own modified step contact printer can make it cope quite satisfactorily with 2% – and maybe more. Very little film is more shrunken than this.

A commercial laboratory could, of course, also modify a machine, but very few do.
The projector head of most Step Optical printers can have the travel of the transport pin(s) adjusted to accommodate as much shrinkage as is commonly met (certainly 2%).

Printing and buckle

We must consider the effect of 'buckle’ on the copy. When film is buckled it does not lie flat. Therefore when we attempt to put it in contact with a new film for copying, the tendency is that some parts are in contact and some parts are not quite in contact. Then the copy, at the parts which are not quite in contact, will not be sharply rendered. The resultant effect of the picture appearing intermittently sharp and diffuse is known as 'breathing’.


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