Choosing An Exposure Unit
Question
There are mercury-vapor, metal halide, fluorescent, incandescent quartz, and halogen lamps for exposure units each with its own wattage rating. What type is better? How do I compare lamp types and wattage in order to make an intelligent choice?
Solution
You have to remember that stencil materials only respond to a narrow range of wavelengths. Namely those corresponding to UV, violet and blue light, together known as actinic light. This is the reason that we can work safely under yellow lights when coating and drying screens.
The best types of bulbs for exposing screens should therefore have a high actinic output. Incandescent quartz, white fluorescent tubes and halogen lamps spread their output evenly over a wide spectrum to give the appearance of white light. Most of the electrical power going in is not producing the type of light you need to shoot screens. Therefore any unit designed with these types of bulbs will have a low intensity and long exposure times.
High power mercury-vapor bulbs come in different versions, each of which emits a great deal of actinic light. The most effective bulbs for exposing screens contain doped mercury vapor, and there are two main types. The diazo or metal halide lamp, which contains gallium iodide, and the multi-spectrum or tri-metal halide, which, in addition to the gallium, also contains some iron salts.
These additional ingredients, along with mercury vapor, boost the amount of useful actinic light (mainly violet/blue) which these bulbs emit.
The result is that you get about twice the output in comparison to standard mercury-vapor lamps. This means a higher intensity of light delivered to the stencil surface, and shorter exposure times.
Another measure of the efficiency of an exposure unit is the evenness of coverage. When exposing screens, depth of cure is very important. In other words, you have to harden the stencil evenly all the way through in order to avoid pinholes and premature breakdown.
To shorten exposure times, the temptation is to push the exposure unit closer to the screen. With larger vacuum frames, this leads to the formation of a "hot spot" in the center that gets overexposed and loses detail. However, with a rapid fall-off in intensity towards the edges of the frame, screens here can be underexposed and full of pinholes. A good reflector design can minimize or compensate for this effect. Once the lamp gets close enough to the screen, however, direct illumination from the bulb will always produce a hot spot.
The moral here is to make sure your exposure unit has enough kilowatts to enable you to pull the lamp back far enough to get even coverage, while keeping exposure times at a reasonable length.
On the subject of evenness of intensity, this is really the main claim to fame of exposure systems based on fluorescent tubes. Fluorescent tubes have been developed with a special phosphor coating on the inside of the glass that produces an actinic output around ten times greater than that of high output white fluorescents. The best type of tube only emits a very intense blue light of the optimum wavelength for exposing photostencils.
Typical intensity of this type of exposure unit is equivalent to a 6kW multi-spectrum at about 60" from the screen. Since 60" is the minimum recommended distance for exposing an image of around 40" x 40", this gives these fluorescent-based exposure units the edge in speed when it comes to exposing very large formats.