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Light Sources: Exposure lamps
Actinic light is light that has the ability to cause chemical reactions. It can be found in the upper part of the ultraviolet range and in the lower part of the visible light spectrum. The actinic light output of an exposure unit depends on its lamp. Both the lamp’s power and its ability to generate UV radiation determine the unit’s effectiveness.

Fluorescent exposure units
Some exposure units are able to generate sufficient actinic light using only a single lamp. These are called single point systems. Others require a number of lamps to do the same job. Most multiple lamp exposure units are fluorescent-based systems. The output of individual fluorescent lamps is weak, so a number of them are usually needed to produce sufficient UV to expose screens. They also have to be mounted very close to the screen, usually within six to eight inches, and fairly close to one another. Fortunately, fluorescents generate so little heat there is no danger of heat damage to the emulsion. Some exposure units contain more than a dozen lamps. The number used is limited only by the size of the screens that the unit has been designed to accept.

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.

Point exposure units
The best exposure units produce highly directional light that travels in a straight line from source to target. This is known as collimated light. Collimated light strikes the positive head-on, which helps transfer the image to the stencil with great accuracy and little undercutting. The best sources of collimated light are point exposure systems, which explains why they have become the first choice for printers doing demanding halftone and four-color printing.

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.

Exposure Unit A single point exposure unit using an unshuttered system. Operators have the option of controlling exposures either by a timer or an integrator. This unit features a lamp with high UV output. (Not visible in this photograph.) Note the relatively small size of the highly efficient reflector. (The clear area in the middle of the top part of the unit.) Just visible in the lower right-hand corner of this photo are measurements printed on the concrete floor in bright blue, which allows the operator to quickly position the exposure light the correct distance from the vacuum frame.

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
It may seem odd to start your search for a lamp with the manufacturer of your emulsion, but that’s where you can find a vital piece of information. You need to know the part of the spectrum your emulsion is most sensitive to. If your exposure unit allows you a choice of different lamps, buy the one that produces output concentrated in that area.

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 number of different types of lamps have been successfully used in point light systems: mercury-vapor, metal-halide, incandescent quartz, halogen, and pulsed-xenon. Mercury-vapor and metal-halide lamps produce the most intense UV output, and tend to be used in many of the more expensive exposure units, where a typical lamp may cost $200.00 or more.

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
Whatever the type of lamp used in your system it will undergo the greatest amount of wear and tear when you first switch it on, as it warms to operating temperature. This means that switching a lamp off and on between exposures can dramatically shorten its life. The other problem is that some lamps take several minutes to warm-up and this period represents dead time that can slow screen production to a crawl.

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
In most point light systems before you make your exposure, you have to determine the correct distance between the exposure lamp and the screen in the vacuum frame. The minimum distance should be no less than the diagonal measurement of the image to be exposed. In other words, measure the image from corner to corner and that's about how far away to place your light source. If the image itself is relatively small, you may want to use the dimensions of the screen for your guideline, or screens, if you’re exposing more than one at a time.

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
We already know that the further away our exposure unit is from the screen, the longer we have to make our exposure time. But many other factors also affect exposures. Mesh can make a considerable difference in exposure time. A screen stretched with colored mesh will require an exposure 50% to 100% longer than a screen stretched with white mesh. The thickness of the emulsion coating can have an effect as will the amount of humidity present in the emulsion at the time of exposure.

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.