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![]() Exposing Direct Emulsions --- Three Key Exposure Variables: Lamps, Time and Distance
In earlier articles in this series we’ve talked about what happens to an emulsion during exposure and a bit about some of the different types of exposure units available. Now we’re going to take a closer look at the core component of any exposure system, the lamp itself. We’ll also cover two key variables of screen exposure: time and distance.
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Light Sources: Exposure lamps
Fluorescent exposure units The lamps used in fluorescent exposure systems produce actinic light at about ten times the rate of standard fluorescent tubes. They also cost more, although this is still far below the cost of lamps used in single point systems. As a rule, fluorescent exposure units cost significantly less than comparable single point systems. Fluorescent exposure units also have a few other arguments in their favor. For one thing, they require minimal floor space because fluorescent systems enclose both light source and vacuum frame within a single metal cabinet. Of course some single point systems also use this set-up, but in many the vacuum frame and the exposure light are separate units. The distance between them has to be adjusted before making exposures. Not only do separate units take up more floor space, but the area between them has to be kept open. Another advantage of fluorescent light systems is that they do not cause hot spots in the emulsion. These are overexposed areas surrounded by emulsion that remains underexposed. Hot spots result when a single point exposure unit has been placed too close to the screen. This is not a problem in most fluorescent exposure systems because you usually don’t have the option of adjusting the distance. The chief reason fluorescent lamps don’t create hot spots is that they produce output that is very even and diffuse. The diffused nature of their output, however, is also one of their biggest drawbacks, a problem so big that it cancels out many of their advantages. Diffused light strikes the screen from many different angles. Some of this light will strike the positive at such an angle that it will begin to undercut the opaque areas, creating an uneven edge. A straight sharp edge in the positive is transformed into a ragged line in the stencil, the phenomenon called sawtoothing. This may not be a huge problem for someone printing real estate signs, but for a printer doing highly detailed work it can be a nightmare.
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Point exposure units Almost all point exposure units use a single, high-output UV lamp. The power of these bulbs can range anywhere from 1,000 to 8,000 watts, or more. In fact, single point light sources are so powerful that they are usually positioned several feet away from the screen. Even at this distance they produce output more than equal to a bank of fluorescent lamps only inches away from the emulsion.
Point light systems also draw quite a lot of electrical power, so one of the things you need to consider if you’re thinking about buying one is whether the wiring in your shop can handle the load. Ask the manufacturer about the unit’s power requirements; you may need to invest in an electrical upgrade.
Matching the lamp to the emulsion This also suggests that the emulsion manufacturer may be the best place to start your search for an exposure unit. After all, you want a unit capable of producing the most effective output. The right exposure unit using the right lamp allows you to produce better exposures in less time. Even a modest gain in efficiency can make a big difference to productivity if you expose a lot of screens. A single point system using a lamp closely matched to the sensitivity of the emulsion can usually expose screens in far less time than a comparable fluorescent system. These shorter exposure times can mean more than just a boost in screen production; shorter exposures can also mean sharper stencils. Even a light source producing highly collimated output will generate at least some stray light. This is light that strikes the screen at an angle, an effect called light scatter. The longer your exposures run, the more opportunity this stray light has to undercut the edges of the positive. Shorter exposures give it less time to do its damage, which is one of the reasons these stencils often appear sharper. This has led some screenprinters to deliberately underexpose screens. But underexposures can lead to a host of other problems, so the ideal solution is to have short exposure times that produce properly exposed screens. This means investing in a powerful exposure unit with an output that zeros in on the sensitivity or your emulsion.
Types of lamps A mercury-vapor lamp is simply a quartz glass tube containing gaseous mercury. Metal-halide lamps are very similar. They too are filled with mercury-vapor, but in metal halide lamps the mercury vapor has been modified by the addition of other metals like gallium, which enhance the lamp’s output in certain parts of the UV spectrum. Mercury-vapor lamps that have these additives are often referred to as doped lamps.
Shuttered and instant-start systems There are two different solutions to this problem. Some units have been designed so the lamp is simply left on all the time. Between exposures a metal shutter closes, but the lamp itself remains on, switched to a resting cycle that keeps it warm. Other units use an instant-start approach. The lamp is switched off between exposures, but they have been specially designed to rapidly reach operating temperature from a cold start. The adherents of instant-start systems point out the advantage of not having to rely on a system containing mechanical components that can break down. Users of the shutter systems praise the elimination of off-and-on cycles that can reduce lamp life. The choice of a system is often determined by the number of screens to be exposed. Shops that make many screens during the course of a day often prefer shuttered systems; instant-start systems tend to be favored by shops that expose only a few dozen. Whichever system you choose, it’s worth remembering that the fewer times your lamp has to endure the stressful trip from a cold start to operating temperature, the longer it’s going to be around.
Time and distance A couple of things will happen as the light source is moved away from the screen. For one thing, as distance increases the power of light drops dramatically. If you double the distance, the intensity of the light drops to a mere one-fourth of its former strength. You can compensate for this weakened output by lengthening the exposure time, but as exposure times increase so does the effects of light scatter. Fortunately, the further away the light source the more collimated it’s output becomes, which helps offset this problem.
Time Emulsions themselves can have vastly different exposure times. In general, photopolymers will offer speedy exposure times and diazos may require lengthy burns, but there are hundreds of different emulsions on the market and general guidelines can often lead to problems. It’s important to check with the manufacturer for the recommended exposure times. Usually you’ll find that they are based on exposures made with an optimum system like a 6K single point exposure system with a metal halide lamp 40” from the screen. If your equipment varies from this ideal, you’ll have to estimate how the variables will affect your exposure times and make adjustments accordingly. Even if all other factors are exactly the same as those specified, the age of the lamp in your exposure unit could make a difference to your exposure time. As lamps age their ability to generate UV declines. The visible light output may appear to be as potent as ever, but if you fail to make your exposure times longer, you’ll end up with underexposed screens. There is almost no way to detect this gradual decline through direct observation. Fortunately, an easy solution is at hand in the form of an inexpensive device called an exposure calculator. Just what an exposure calculator is and what it does we’ll cover in the next article. We’ll look at some of the different types of exposure calculators available and some of the other devices that can help you make sure all your exposures are right on the mark.
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