Smart sensor distinguishes between film and leader material in film-processing machines
Deluxe Laboratories processes original negative film that studio cameramen shoot and produces finished film prints that are distributed to theaters. The company has a global capacity to process more than 5 billion feet of negative, release, and trailer film annually. In addition to Hollywood, CA, and Toronto, Canada, it maintains film laboratories in London, Rome, Madrid, Barcelona, Bangkok, and Sydney.
Part of the Deluxe operation involves processing original camera negatives. These negatives are used to produce intermediate positive copies from which internegatives are made. These internegatives are used to produce theatrical release prints, keeping the original negatives safe. This part of the production of a motion picture is a high-speed, high-volume process that requires the same productivity, quality, and reliability as an industrial manufacturing or assembly process. The seven developing systems that produce the prints run at more than 800 ft/min and develop more than 1.1 million ft of finished film per day, depending on the individual machine.
Each of the Deluxe film-processing machines, manufactured by Technical Film Systems, holds approximately 12,000 ft of film, including both film and film leader. At the beginning of the process, the leader runs through the machinery until the system is stabilized. Then the film is attached to the leader and processing can begin. The film is developed in a continuous process. Accumulators at both ends of the machine enable operators to remove finished film or add film to be processed without interrupting the development process.
Leader and film
When the leader is running through the system, there is little depletion of the chemicals used for film development. As soon as film is introduced, however, these chemicals need to be continuously replenished. To properly control this process and begin replenishing chemicals, sensors that examine the product as it passes through the system need to be able to distinguish between leader and film. If the sensors cannot discriminate between leader and film, the system cannot control the chemical-replenishing system correctly. This interferes with the chemical balance in the system, and film may be over- or underdeveloped. For this reason, sensors are calibrated—using the Omron ZFV-C Teach function—to recognize new leader and used leader that may have a different color (see photo).
In the past, Deluxe installed infrared (IR) through-beam sensors in the film path to distinguish leader from film, since film had a carbon-backing layer that would stop the IR beam. However, in recent years, film manufacturers have removed the carbon backing from the film, so that IR sensors could no longer distinguish leader from film. If the chemical balance in the system is incorrect, film can be over- or underdeveloped.
Smart sensors
Many vision systems that Deluxe previously considered to detect film from leader were prohibitively expensive, and laser sensors tended to burn through the thin film. Because of this, the company chose the Omron ZFV-C smart sensor, an integrated machine-vision unit that consists of a camera head, integrated light source, and image-processing unit. Because the system can detect variations in hue from an input reference color, it can easily distinguishing the white leader from the clear, dark blue of the film. While the leader is opaque, the processed film is translucent. The ZFV-C detects and distinguishes between those different colors, while the thickness, transparency, or other physical qualities of the material have no bearing on the result.
ZFV-C camera heads are mounted at the exit of the drying portion of the development system, before it enters the drying process. Mounting the sensor at the beginning of processing would risk fogging the film with its light source. The camera, which includes an integrated LED light source, is mounted at an angle to the film and faces a black background beneath the film to eliminate reflection or glare. When it ceases to detect leader, film is present.
By interfacing to an Allen-Bradley SLC 5/04 PLC, the smart sensor can enable the system to automatically replenish chemicals used in the system. At present, Deluxe has installed a ZFV-C sensor and an Omron S8VM-03024CD power supply on each of its Hollywood print-developing systems and has ordered more for other systems worldwide.
James Robertson is supervisor of engineering, Deluxe Laboratories, Hollywood CA, USA; www.bydeluxe.com
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