This article deals with vesicular film, the duplication process and equipment for exposure, development, and clearing.
Since most Vesicular films are sensitive to violet light (385-410 nm), all duplicators use lamps which radiate in the violet region. Today the most popular exposure source is an additive type, mercury vapor lamp. Fluorescent lamps, Xenon flash, and standard mercury vapor lamps have all been used over the years. Additive type vapor lamps have become the most popular exposure source because of their emission characteristics, high wattage potential and availability. While many lamps produce some violet energy, the additive type mercury lamp has been tuned, through the inclusion (called "doping") of a chemical inside the lamp, to concentrate more energy in the violet range. When using additive type mercury vapor lamps, the following operational procedures should be kept in mind:
1. In the Vesicular process, image forming gas vesicles (bubbles) are moving within a plastic emulsion layer. Exposure sources should be of sufficient strength to expose the film in less than 10 seconds. Some lamps, such as fluorescent, have been used in the past, but long exposure times result. Exposure times greater than 10 seconds reduce the image-forming capability of most Vesicular films, thus reducing resolution and contrast.
2. Because of vagaries in the manufacturing process, lamps will vary in their spectral emission characteristic. This means that two lamps of the same wattage may not radiate exactly the same kind of energy spectrum. The lamp that puts out more of its light at 410 nm (violet light) will allow your duplicator to operate at higher speeds.
3. As additive lamps age, they will lose some of their exposure energy. The exact fall-off will vary from lamp to lamp, but a good guideline would be 5% loss per hundred hours of operation.
4. Do not leave fingerprints on the lamp's surface. The high operating temperatures will cause the fingerprint to discolor and turn brown. This brown stain will block some of the lamp's radiation.
5. All short wave radiation (<350 nm) is potentially harmful for humans, especially the eyes. Mercury vapor lamps do radiate strongly in the ultra violet. All modern duplicators come with interlocks and protective shields to protect the operation. These safety devices should not be interfered with.
The standard development process for vesicular film requires the adequate use of heat for a specific amount of time. Development is time and temperature dependent. In the development process, the plastic emulsion must be raised in temperature to its melting point for a short period of time. The plastic will flow around the image-forming vesicles and then the plastic must cool rapidly to become a hard barrier from which the vesicles cannot escape. Development temperatures range from 240°F to 300°F depending on film type, type of development, thickness of base, and dwell time.
Dwell times may also vary dependent on the type of developer and film transport. Many different methods of development hardware have been used with Vesicular films, including metal platens, moving rubber belts, rotating metal drums, and hot air. The most common method was to place the film in physical contact with the development source.
Radiant heat energy (hot air) is used in some older equipment and also in machine built today; in the older machines it is not as efficient as contact development and may cause non-uniformity. In any form of development, the heat applied should be uniform and the surfaces of the developer should be clean. Dirt spots, paper, oil or adhesive tapes on the film or in the developer will cause a loss of development temperature, and thus undeveloped spots (less density or clear spots) will form on the developed film.
The third step in duplicating vesicular films is clearing or fixing. "Clearing," a word coined for the step following development, may be compared to the fixing of silver films. In this step of the process, the film is given an overall exposure to violet light, breaking down the unused light sensitive salts that remain in the film after the image forming exposure and development. This clearing exposure turns these salts into nitrogen gas so that the gas may escape into the air and the film becomes fixed or stable, A properly cleared Vesicular film will change color as the clearing process breaks down the unused salts. Properly cleared vesicular films have lost the yellow-green appearance of unexposed film.
MAKING THE COPY
All vesicular duplicators require the master and Vesicular film to be brought together, "emulsion to emulsion," and exposed to the printing lamp. Both density and resolution are affected by the exposure step.
Resolution is the ability of the film to render visible fine detail and is expressed as the number of lines per millimeter discernible in an image. Acutance is the objective measure of the "edge sharpness" of the detail. Both exposure and development can affect resolution and acutance.
Since vesicles (bubbles) produce the image in vesicular film, the size and quantity of the vesicles formed determine the acutance.
Master film resolution cannot be improved upon during duplication. Therefore, efforts should be made to create a master film with high resolution. Some loss of resolution is inevitable during duplication; the amount is dependent on the characteristics of the individual duplicator.
Contact between the master film and the copy film plays an important role in duplication. Poor contact will cause the exposure energy to diffuse between the two emulsions, causing a loss in contrast and image sharpness. Proper contact is a function of the duplicator. Fiche duplicators rely on a pressure pad or platen to assure contact. Both sufficient pressure and even pressure are necessary. A test master with resolution targets in all four corners and center can be used as an evaluation tool. Regular checks of the duplicator’s exposure station (using a test target) are a good practice.
16/35mm duplicators use a combination of tension and/or moving belts to maintain contact. Because 16/35 duplicators print while both the master and copy film are moving, they require care in set-up and maintenance. The machines should be kept clean, wind tensions should be checked regularly, and all idler rollers in the film path should turn freely. A reel of master film containing test targets should be run regularly to check imaging performance.
Exposure collimation, assuming good contact, is the second most important property concerning resolution. Collimation refers to the design of the exposure system in terms of how the light is directed to the exposure plane. Ideally, all the exposure energy should hit the film plane perpendicularly. This is the characteristic of a highly collimated system. Energy that strikes the film plane at a less than 90° angle tends to undercut the image and soften edges. Since collimation is a function of the machine design, it is not something you can do much about once you own the duplicator. Most duplicators provide adequate collimation, but you should evaluate your duplicator for imaging performance before purchase. We would recommend you use a high-resolution test target on film for duplicator evaluation.
Slippage and loss of image resolution is a very complicated problem for dynamic printers. There are no simple answers. The following guidelines may be of some assistance if you encounter imaging problems on your duplicator:
Your duplicator should be set to factory specifications for tensions on your unwind, rewind, brakes and clutches. All idlers in both the Vesicular and master film path should turn freely. The drive system should be running smoothly with no pulsing.
Your lamp should be properly cooled. Excessive heat at the exposure station will cause the gas to diffuse out more quickly.
No lubrication should be used in the film path or on the film itself. Silicone in particular can cause slippage problems. Make sure all drive and brake rollers are cleaned frequently with isopropyl alcohol.
Standard Vesicular films are negative working media. The photographic image will be the reverse of the master printed to the Vesicular, i.e., camera negatives will print as positive Vesicular images. Therefore, the maximum density is a function of exposure increasing the number of vesicles available to be captured by development. The minimum density is the function of the least exposure or no exposure at all. Since Vesicular films are made up of bubbles which reflect and refract light rather than absorb light like silver grains and dyes, evaluation must be made in a microfilm reader or densitomer using an optical path for projection. The best technique for assuring proper exposure is to run a step test beginning with a good master and a small amount of exposure. Increase exposure in steps until the screen image is fully readable without loss of detail.
UNDEREXPOSURE - The lack of sufficient exposure results in overall gray images, poor contrasts and poorly defined characters.
OVEREXPOSURE - Conversely, overexposure produces good black screen images, but characters are filled in, hard to read and also low in contrast.
As previously described, development is a function of temperature and time. Temperature of 240°F to 300°F is the common range of development with most films responding properly at 265°F (130°c). Development times are generally a function of the transport speed of the duplicator.
UNDERDEVELOPMENT: The lack of heat or time results in very small vesicles that do not refract light properly. The resulting image is yellow-brown and low in contrast. The screen image appears weak and washed out as if illuminated by a high wattage light source.
OVERDEVELOPMENT: There are two very apparent changes that take place in vesicular films that are overdeveloped. The first is a surface change in the plastic emulsion. The surface appears to be covered with oil or a scum when viewed in reflected light. Microscopic blisters forming on the surface cause this. The second is a loss in maximum density but without change in color of the image on the screen. The resulting image is grainy and characters are not well defined. Some areas may appear blistered or distorted.
PROPER DEVELOPMENT produces clear, sharp, readable images. When temperature and dwell times are unknown, here again, a step test may aid in achieving proper settings. With the dwell time left constant, temperature is increased in steps of 10°F until blistering or surface scum appears. Once this appears, reduce the temperature 10-15°F. Again, normal working ranges are 2400F _ 3000F (116°C - 149°c).
The clearing or fixing process is very simple to accomplish and evaluate. Clearing breaks down light sensitive salts that are yellow-green in nature. If the clearing process is working properly, the vesicular film will change from yellow-green to a transparent media easily judged by examining the minimum density.
The clearing of vesicular films may be measured with a densitometer and proper filters. Many densitometers are supplied with both a Wratten #106 and 18A filter. The #106 is used to measure and compare densities to their visual appearance in a microfilm reader. The #18A filter is used to measure the ultra violet transmission (printing) density of films. By use of the #18A filter and measuring a minimum density area of vesicular film, the amount of clearing can be determined. Unexposed minimum densities measured with the #18A filter yield densities of .80 or greater, depending on film type and thickness.
To test for full clearing, a clearing step test should be made. Measure the #18A density of the unexposed vesicular film. Give the sample film an overall exposure and again measure. Repeat this process until there is no density change. Most Vesicular films reach a color change from yellow-green to clear around a #18A printing density of .70. Fully cleared films have an #18A density of less than .50. Since there is always some density produced by the base and emulsion coating, a density of zero should never be expected.
Once cleared, vesicular films should be protected from high temperature for at least one hour. Vesicular film not cleared may be used in most microfilm readers without problems provided the reader film plane does not exceed 167° F (75°c). However, un-cleared films will change colors in the reader while viewing. For stability and long use, vesicular films should be properly cleared before use.
DO'S AND DONT'S:
Keep the duplicator clean since dirt will print to vesicular film from master and exposure glasses. Dirt will cause underdevelopment spots when allowed to build up on the developer drum or belts. Dirt-clogged air filters will overheat the duplicator, causing electric problems and pre- and post-development in the exposure and cleaning stations. This will appear as increased minimum density or fog.
Do not allow duplicator to overheat. High temperatures will reduce the life of exposure and clearing lamps. Excessive heat in either lamp may result in lamp explosion. Normal operating temperatures should be below 110°F for both exposure and clearing.
Protect vesicular films from high temperatures before and after duplication. High temperature storage of unprocessed Vesicular reduces photographic sensitivity.
After processing, storage of vesicular films should be below 200°F to prevent the plastic emulsion from beginning to flow and cause image loss. Very few office storage areas reach or exceed 200°F.
Follow manufacturer's recommendations for storage of unused film and for correct development temperatures.