Printer Control (Mechanical Controls)
Principles of Printer Control
Printer light sources and sometimes the optics vary over time, although it is primarily the lamp that changes in intensity and colour quality. Monitoring the printer is carried out in order to set the light source back to a common position. Thus a print made today can be reprinted tomorrow without excessive re-testing.
Many laboratories set the light sources of all the printers in the laboratory to a single common intensity and colour balance so that any printer will print any previously graded film without adjustment. This is really only possible with printers that have the same light source construction, such as a Bell and Howell Additive Lamp house, and is only truly useful if the grading system and printer light cueing system are also identical.
Laboratories that specialise in archive work can rarely manage these luxuries as the printers are probably of widely differing lamp houses and cueing systems. Nevertheless a standard is essential as it allows the control operator to know the difference between one printer and another and permits a conversion, even if only approximate, when a film printed on one printer is transferred to another.
Every printer has some form of adjustment for its lamp house and light source. These come in many forms and could include:
Lamp voltage control
Lamp position in lamp house
Filters
Aperture
Additive lamp houses are fitted with "trim" which are gross setting controls that move the entire range of 51 printer light value to different levels and separately control the red, green, and blue light valves. With these adjustments it is possible to set the light source to a predetermined light and colour level and with daily printer control tests to maintain the same exposure level. There are two methods of controlling a printer to a standard value:
Printer Settings For Different Film Stocks
The records needed for a printer are not only to control the fluctuations of the printer lamp but also to define the changes of trim and or filtration needed to allow the various different film stocks and film stock batches to be used interchangeably. Largely because of this complication running plots are not used, but the following procedure adopted
- Daily a printer control test is printed and processed using one primary material. Most laboratories use a single batch of Eastman Colour Print for all colour printers and one of the Black and White Release print films for black and white printers. The density of the test patch can be plotted on a running plot for each printer.
- On a regular routine the other film stocks and alternative batches are tested similarly in order to prepare a table of adjustments from the standard in 1. above. Each printer has a notice or chalkboard beside it, which lists all the possible alternatives and the settings. Additive colour printers usually just require the new trim settings for each alternative stock.
Interpretation And Printer Control
Exposure is the product of time and light intensity and so a correction to the exposure received in a printer can be carried out by altering the time or the intensity. Colour film printing requires the control of the colour of the light as well.
Time Control
With a continuous printer the only way we can alter the time is by altering the speed of the film past the printer gate.
Some printers have a speed control, either continuously variable or in discrete steps. If it does not have a speed control then the only way to change the speed is by changing mechanical components such as the motor or drive pulleys.
With step printers altering the speed changes the time of exposure of each frame or by changing the duration the shutter is open. This last is not a convenient or accurate way of altering exposure. It can be used for gross corrections such as making adjustment for very slow stocks.
Intensity Control
The usual method of control is to vary the light intensity. There are three main methods:
- Changing the light output of
the lamp by altering the voltage or current of the lamp.
Changing the voltage, which is more common than changing the current [other than on sound lamps which are low voltage high current], will also change the colour of the light from the lamp.
The higher the voltage the bluer the light; the lower the voltage the yellower the light. While this is not significant for Black and White films a colour correction will have to be made for colour films. - Changing the light output by
inserting neutral density filters in the light beam.
Neutral Density filters vary the light transmitted. They are available in increments of 0.1 with an additional 0.05.
A 0.5 is equivalent to 2 printer points [Bell and Howell].
Colour filters are also available in red, green, blue, cyan, magenta and yellow and the same values. Any filters introduced into the printer are not normally in image forming light to avoid affecting the image definition. - Changing the trims if light
valves are fitted.
As covered in the printer chapter, the unit of measurement of exposure on a printer is the printer point. A printer point is the smallest increment that we can vary the light on the printer by when printing. In the early days when there were numerous manufacturers of printing machines each manufacturer used a different system to vary the exposure and the amount the exposure varied was also different.
Today the printer point has been standardised as the Bell and Howell printer point or the increment produced originally by the Bell and Howell Model C light valve. The precise exposure change for each printer point is 0.025 Log Exposure. 12 printer points are equal to one camera stop or a doubling of light.
12 printer points = 12 X 0.025 Log Exposure = 0.3 Log Exposure the Log of 2 = .3013 so the exposure change is a factor of 2.
Sometime [rarely] it is necessary to use all three options on the same printer.