One of the most beneficial uses for a digital radiometer is to check on the health of your exposure lamp. A lamp's UV output drops as the lamp ages. Unless exposure times are regularly adjusted to compensate for this reduced output, you will eventually be producing underexposed screens. Regular readings taken with a digital radiometer can let you know when and by how much to adjust your exposures.

You can also use a digital radiometer to check on a screen exposure as it is taking place. Place the radiometer's sensor behind the screen that is on the side opposite your exposure lamp. When your exposure begins, your radiometer will give a reading of zero. All of the UV radiation is being absorbed by the energy-hungry sensitizer in the unexposed emulsion. However, as the exposure continues the sensitzer will gradually take on energy until it can hold no more. At that point it can no longer block the UV and as more and more UV penetrates into the emulsion, your radiometer readings will slowly rise. When all the sensitizer has been used up and UV can now pass through the emulsion coating virtually unhindered, your readings will level off.

Despite its obvious usefulness, many screenprinters will never lay eyes on a digital radiometer. That is, unless the shop is printing with UV inks. In UV printing a digital radiometer is often regarded as a necessity for checking the output of the UV curing unit. Beware, though, in many cases the radiometers used to monitor curing units are not the same as those used to measure exposures. If you're thinking about buying one, prices for most digital radiometers start close to the $500 mark, although Chromaline offers a digital radiometer called the UV Minder, currently selling for $175.

Why go to so much trouble over screen exposures?
Screenprinters who do long print runs or who use mainly water-based inks might consider a digital radiometer a very practical investment. Underexposures are one of the principal causes of stencil failure, so wherever stencils will be subject to more than the normal amount of stress accurate exposures become vital. To understand why underexposures play havoc with the durability of stencils, we need to think about what happens during the exposure process.

Exposure begins with the layer of emulsion closest to the light source, that is, on the print side of the screen. From there, it progresses slowly on through the emulsion coating as the sensitizer in the emulsion gradually loses its power to block UV. The last part of the emulsion coating to be exposed is the emulsion on the squeegee side of the screen.

This layer may be the most important part of the entire emulsion coating when it comes to durability of the stencil. Not only does it play an essential part in locking the stencil to the threads of the mesh, it must withstand the abrading passes of the squeegee blade. If the emulsion on this side of the screen remains underexposed, a critical part of the stencil will forever be soft and weak.

Proper exposure times can eliminate this potential vulnerability. Regular tests with an exposure calculator can help, as will regular measurements with a digital radiometer of the output in your exposure lamp. But when we get busy, exposure tests tend to be the last thing we think about. Long intervals may pass between tests during which the UV output of the exposure lamp may drop significantly. Without regular exposure tests, your exposure times are really based on nothing more than guesses. To put an end to the guesswork once and for all you really need to take control of the amount of UV striking your screen.

Integrators
This can be a lot easier than you think. In fact, thanks to a device called an integrator, it can be almost automatic. Like a digital radiometer, an integrator measures the intensity of UV radiation, and like the radiometer, it uses a filtered photocell as a sensor. But an integrator not only measures the intensity of UV output, it also acts like a dosimeter and keeps track of the total dose of radiation aimed at the screen. When the correct amount of UV has been produced, the integrator switches the exposure lamp off. The big difference between radiometers and integrators is that a digital radiometer is merely a measuring device, an integrator is a control.

Integrators are not used as independent instruments. They are components of an exposure system. But they can be purchased separately. Although, according to Cornel Jurca, Customer Service Manager of Olec Corporation, most customers shopping for integrators simply want to replace or upgrade existing equipment. Integrators from Olec range in price from $390.00 to $895.00 USD. Jurca says integrators are most in demand for industrial processes where "repeatable exposures" are essential, such as in the manufacture of circuit boards.

Overcoming Exposure Variables
Integrators bring consistency to screen exposures. This can be especially important when screens have to match one another as closely as possible, as in four-color printing. Even if you expose your screens months apart, screens should be exactly the same. The exposures should be identical even if you made the first using a brand new exposure lamp and the second with a lamp approaching the end of its life. The integrator-controlled exposures are based only on one single factor: the amount of UV generated. Exposure times are adjusted automatically to keep the exposures consistent.

The placement of the photocell, however, can make a pretty big difference. If the photocell is mounted on the exposure unit itself, the only thing it can measure is the output of the lamp. But if it's placed on the vacuum table close to the screen, the photocell can register the actual amount of UV light striking the screen.

If the photocell is mounted closed to the screen, it can also compensate for variations in UV intensity due to distance. Distance can have a dramatic effect on output. For example, if your exposure unit somehow got positioned at twice its usual distance from the screen your exposure would have to be four times longer to compensate for the drop in intensity. If you were making a time-based exposure and failed to adjust your exposure time, you would end up with a seriously underexposed screen. But an integrator-equipped exposure unit would automatically compensate for the lower intensity by making the exposure longer. The actual amount of light striking the screen would never vary.

Electronic UV measuring and control devices give us the ability to produce accurate and consistent screen exposures. Digital radiometers and integrators can be wonderful tools for maintaining the pace and consistency of screen making in busy shops, but are they for you? That really depends on the type of jobs you take on now or hope to take on in the future. If you plan to go looking for more complex work the consistency these devices bring to your work may give you the confidence you need to tackle bigger jobs. On the other hand, if your print runs are frequently interrupted by stencil breakdowns, stop and take a minute to add up the costs of screen failures. These electronic exposure helpers may begin to look like very wise investments.