New UV Lamps but Low Output?
By Tim Reece
We all understand that conventional ultraviolet lamps diminish energy over time. Therefore, we use UV light meters to continually monitor the performance of these lamps so that we may adjust main and back exposure times. Certainly, some could argue that they don't need a UV light meter to indicate when they should adjust exposure times because they perform exposure step tests religiously, and that is indicator enough. Honestly, it's pretty hard to disagree with that logic.
However, have you ever purchased a new set of UV lamps, expecting your main or back exposure times to decrease significantly with a surprisingly lackluster result? What we know is that a UV light meter gives you the ability to put a numerical value on the power output of the lamps. What some do not know is that along with milliwatts per square centimeter of UV energy, temperature also plays a factor in the polymerization of photopolymer.
As the lamps increase in temperature, the speed at which the material polymerizes also increases. So, one could come to the conclusion that putting in a set of new UV lamps most likely would yield a higher amount of energy along with a higher temperature. This is often the case. But there is one more factor that isn't always common knowledge.
Exposure units for photopolymer platemaking have fans that aid in keeping temperatures consistent during the exposure process. Some more sophisticated exposure units monitor temperature, and the fans come on and off automatically to maintain a consistent temperature. With less sophisticated units, the fan runs continually. In addition, some units have an exhaust to evacuate the heat from the machine and away from the operating area, whereas others do not.
So, here's the catch. As lamp temperatures exceed 140°F, the gases within the UV lamps are affected by excess heat. The result is that energy output actually begins to drop. So you could have placed a new set of lamps into your exposure unit with disappointing results because the lamps, in essence, are overheating, and gases within the lamps are being consumed, resulting in sub-optimal power output.
The good news is that the fix is pretty easy. Fans, fan guards, and screens need to be cleaned and debris removed to allow proper airflow into the exposure area. Be sure, after cleaning, to thoroughly remove any debris that could settle on the plate face. If your exposure unit does have an exhaust, there is one more area that you need to address. It's difficult to measure airflow from the exposure exhaust without the right equipment; however, this area, too, can become congested and require cleaning. In many cases, there is a baffle in which you can adjust airflow manually.
So, if you are confused or upset the next time you don't see your power jump through the roof when you replace your UV lamps - a little bit of basic housekeeping is often all that is required.