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The simple answer is that a screen is correctly exposed when an emulsion coating has become hardened enough by contact with UV light to stand up during printing. To reach this degree of hardness the emulsion needs to receive a certain amount of actinic light.

When we speak of exposure times, we're simply speaking about how long it takes a particular exposure unit to deliver the necessary amount of UV radiation. A powerful and efficient exposure unit may only take a couple of minutes to produce enough actinic. A weak light source may require the better part of an hour. The amount needed still remains the same.

If all that was needed was enough UV to harden the surface of the emulsion, no exposure would run longer than a few seconds. But UV not only has to harden the surface of the emulsion, it must penetrate deeply into, and actually completely through, the emulsion coating. UV can only penetrate further into the coating as the sensitizer in the outer layers of the emulsion is completely neutralized. For this to take place, the sensitizer must come into contact with so much UV that it can't hold any more. As the sensitizer loses its power to absorb UV, the emulsion becomes more and more transparent to the radiation. Exposure is complete only when virtually all of the sensitzer in the emulsion coating has become neutralized through contact with UV.

So, how can we tell when this has taken place? If you have the good fortune to be working with a diazo or a dual-cure emulsion, this is something you can see. As diazo sensitizer reacts to UV light it changes color. So in these emulsions the degree of color change reveals how much exposure has taken place. We can even use color change to make quite accurate measurements, if we employ an inexpensive tool called an exposure calculator.

There are a number of different types of exposure calculators available. Most large emulsion manufacturers supply them. Kiwo, Autotype, Chromaline, Ulano, Sericol, and Murakami offer inexpensive exposure calculators, most of them priced in the $20 to $40 dollar range.

One of the most common types is a small film positive containing a range of different film densities. The film densities each filter out a different percentage of the UV light. These images when reproduced in the stencil allow us to compare the effects of different exposure times. Determining the correct exposure time is simply a matter of identifying a correctly exposed area and making a simple mathematical calculation. Although exposure testing takes time and involves making a special test screen, correctly exposed screens can save you time and money by eliminating stencil breakdown on the production line. A separate exposure test will have to be made for every emulsion and type of mesh used in your shop.

Using an exposure calculator
The good news is that exposure calculators are very easy to use. You simply use the exposure calculator on a screen in place of a normal film positive. Test exposures should be twice the length of your normal exposures for the calculator to work properly. The results depend on some areas of the emulsion being overexposed.

Obviously, the first time you try using an exposure calculator with a new exposure unit or a new emulsion, you'll have no history on which to base your test exposures. You could simply guess, or you could try looking at the exposure information provided by the emulsion manufacturer. Most manufacturers will give exposure data for their products, either on information sheets supplied with the emulsion or online at their websites. Usually this will be presented as a chart giving the ideal exposure times for a variety of exposure units and mesh counts. You simply have to decide how closely your exposure unit and your screens compare to the ones listed on the chart and adjust your exposure time accordingly. Just be prepared to make several tests to get your exposures zeroed in.

When your screen has been exposed, washed out and dried, it's time to read the results of your test. Examine the image of the exposure calculator that appears on your stencil. The instructions that came with your exposure calculator will tell you how to locate an area where exposure has been close to perfect. Using the numerical factor associated with this area and a simple mathematical calculation you can work out your ideal exposure time.

Many exposure calculators also contain lines of text in different point sizes and test images called resolution targets. Resolution is a measurement of the stencil's ability to reproduce fine detail, and the resolution targets can tell you how well your emulsion is holding detail at your exposure time. It is important to remember that a resolution target alone cannot serve as a reliable guide to exposure time. When it comes to resolution an underexposed stencil may appear to be as sharp, or even sharper, than a correctly exposed stencil. Resolution isn't the problem, it's durability. The underexposed stencil has not been hardened sufficiently to stand up during the wear and tear of the printing process.

Exposure control scales
Once you've established correct exposure times for all of your screen and emulsion types, you may think you're done with exposure tests forever. Unfortunately, that is not the case. At least one factor will continue to change as time goes by. The lamp in your exposure unit will grow steadily weaker as it ages. Its capacity to produce UV radiation will decline much more rapidly than its ability to produce visible light. It may appear to be as powerful as ever, but unless you make regular exposure tests, you will eventually be producing underexposed screens.

Fortunately, once there is one type of exposure measuring tool that you can use without making a special test screen. This is the exposure control scale or grayscale exposure calculator. Once you've established your ideal exposure time using a regular exposure calculator, the exposure control scale can help you keep your exposures on track.

The exposure control scale is basically a small film positive strip of containing a range of graduated neutral film densities. Often this is referred to as a hardness scale because the range of densities can tell you how your exposure has affected the emulsion's ability to adhere to the mesh.

The Stouffer 21 Step Sensitivity Guide and the Kiwo Quick Check are two popular exposure control scales. Both are small enough to be tucked away in an out-of-the way corner of almost any production screen. Using them regularly is a great way of keeping tabs on the lamp in your exposure unit.

Make your own exposure calculator
If you don't have time to run to a supplier for an exposure calculator, you can make your own. The homemade exposure calculator works a little differently than the film positive type we talked about earlier. It takes a little more work, because you have to make several exposures. instead of just one. But the results they produce can be just as accurate.

Cut 4 equal-sized strips of photo-opaque material. Rubylith or some other type of masking film is ideal. Tape one of your normal positives to a coated and dried screen, and lay these photo-opaque strips side by side until they cover almost the entire positive. (Use clear tape to hold the strips in place.) Leave an area equal in size to one of the strips open.

Expose the screen for one minute, but don’t wash it out. Remove one filmstrip and make another one-minute exposure. Continue to remove strips alternating with minute-long exposures until all the strips have been removed, and you're left with a screen that contains exposures that range from severe underexposures to overexposure. Now remove your positive and wash out the screen.

The extremely underexposed areas should be easy to find. The gentle spray of water used to develop the stencil will probably be powerful enough to knock them right off the mesh. Other signs of underexposure may be a little harder to spot. As you begin to wash out the screen, check the wet stencil for signs of milkiness.

Spotting underexposures can be helpful, but what you really need to look for is an overexposed area. Chances are, the area right next to it will be the area that has been correctly exposed. Signs of overexposures are harder to spot, but one dead giveaway is lines that look significantly thinner than they do in the positive. Take a close look at fine details. Have some of them filled in?

The almost complete lack of pinholes is another indicator of overexposure. Most of us curse pinholes, but we also know that one or two can be expected to turn up in almost every screen thanks to tiny airborne dust specks that settle on a screen when the emulsion is still wet. During exposure they act like tiny positives and block the UV light. When you wash out the screen you dissolve the dust or wash it away leaving the tiny opening behind. If the screen has been overexposed, however, enough light has struck the emulsion to work its way around these specks. The pinholes get filled in just like the fine details in your positive. If you can't find any test area that reveals signs of overexposure, double your exposure time and try the test again.

You can buy a commercial version of the homemade calculator called a step wedge calculator. Though usually quite a bit cheaper than the film positive type, step wedge exposure calculators can produce test results that are every bit as accurate. And the step wedge calculator also allows you to decide how long you want to make your test exposures. With the film positive type, the exposure times tested depend on the relative film densities that appear on the calculator. With a step wedge calculator, exposures can be as long or as short as you'd like so long as they are identical in length. You could make the individual tests only a few seconds long if you wanted to narrow the range of exposures tested.

Using an exposure calculator with non-diazo emulsions
SBQ photopolymer or one-pot emulsions do not change color according to the length of exposure to UV light as diazo-based emulsions do. The grayscale exposure calculators can still convey some information about the stencil's relative hardness. Rather than revealing different colors, the scale will appear as a range of shades in a single color. Underexposed areas will appear lighter because they will be thinner than correctly exposed emulsion.

With photopolymer emulsions, underexposed stencils can often be spotted at the wash tank, where the underexposed emulsion will look milky when the water first hits it. Pay special attention to the squeegee side of the screen, where signs of underexposure are more likely to turn up.

Another useful tool for evaluating exposures is a magnifying device like a jeweler’s loupe, a linen tester, or a pocket microscope. There's nothing like taking a close-up look at the threads of the mesh and the stencil attached to them to evaluate the quality of your exposures. While invaluable for evaluating stencil resolution and definition, a magnifying device can also reveal the filled in details and thinning lines that signal overexposure.

For the final article in our exposure series, we’re going to take a look at several high tech methods of controlling screen exposure, specifically electronic exposure controls and measuring tools like the integrator and the digital radiometer. We'll also explore some of the more revolutionary stencil-making methods including projection exposures and CTS or Computer-To-Screen systems. Maybe your shop belongs at the cutting edge.