There are two dominant technologies for generating UV light. The most common is the arc lamp. This technology involves passing an electrical current between two electrodes which are enclosed in a quartz tube with a mercury fill and an inert gas (usually argon). When a voltage is applied across the electrodes, current begins to flow. This current rapidly heats up the fill material generating a plasma arc between the electrodes. The mercury and any other fill materials in the arc emit visible and UV light.

The second main technology is the microwave - powered or “electrodeless” lamp. These systems utilize lamps which are basically sealed quartz tubes containing the fill material and argon gas. Microwave energy is used to heat the fill material and generate the plasma. This operates in principle much like a microwave oven. Since the microwaves can transmit energy through the walls of the quartz tube, no electrodes are needed inside the sealed tube. The microwave energy to power these lamps is generated by magnetrons. These are high voltage vacuum tubes specially designed to generate microwaves.

Each technology has its advantages and disadvantages. Accordingly, each technology has gained dominance in applications where its advantages have clear value to the end user. The main advantages of the arc lamp technology are its relative simplicity and ability to be packaged into small profiles at whatever length is needed. For this reason, most UV lamps in printing applications are the arc lamp technology.

The major disadvantage of the arc lamp technology is the presence of the electrodes inside the quartz tube. Fill materials and electrodes interact, causing a loss of output over time. These problems have been solved by the lighting industry, however, and most quality UV lamp producers know how to prevent this degradation. It is not uncommon for a high quality arc lamp with a mercury spectrum to obtain a useful service life of 3000-5000 hours.

Electrodeless lamps have the main advantages of a smaller diameter bulb and a more stable output over time, especially with metal halide lamps. This combination of tight focus and stable “D” bulb output has made the microwave lamp the dominant technology in the optical fiber industry.

The magnetron is the is the major disadvantage of the microwave powered lamp. It must be mounted inside the lamp housing, making for a bulky and heavy assembly. Magnetrons, like all vacuum tubes, experience output decay over time and must be replaced every 4000-6000 hours of operation. This significantly increases the operational spares cost compared to the arc technology. Due to the physics of the microwave discharge, these electrodeless lamps are limited to a maximum length of approximately 10 inches. Longer lengths are obtained by placing 10 inch modules end-to-end. Installed costs of such longer length arrays are higher than comparable arc technology. Additionally, magnetrons are prone to failure when rapidly cycled on and off. Microwave power lamps are not well suited to applications which require rapid on/off cycling.

There are two other alternative technologies which are sometimes used in UV curing applications. The first is the xenon flash lamp. This lamp uses a lamp with a xenon gas fill and no mercury. The lamp discharges in a “flash” pulse which emits a burst of UV, visible, and infrared. Because the pulse is short, the lamp does not heat up. Compared to arc and microwave-powered lamps which operate at a quartz tube surface temperature of 600-800 degrees Celsius, the Xenon flash lamp exhibits much lower heating of the part being cured. By supplying a train of pulses, sufficient curing energy can be delivered to the UV curable material.

While the pulse nature of the xenon lamp makes it impractical for use with moving webs or sheets, it can be effectively used on stationary parts. The main disadvantage of the xenon flash lamp is the life of the lamp, which is rated in numbers of pulses. Depending on the number of pulses needed to cure a given part, the flash lamp life, stated in number of parts cured, could be much less than a comparable arc or microwave powered lamp.

The second alternative technology is the excimer discharge technology. This type of lamp uses a combination of two gasses to create a discharge of energy at a specific wavelength. An example of the excimer technology is the XeCl, or xenon chlorine lamp which has a peak at 308nm. There is relatively little energy outside of this peak.

While experimental high pressure microwave-powered eximers have been offered in the past, the only commercially viable product is the barrier, or flat plate discharge. Since the UV curing process is relatively complex, requiring various wavelengths to obtain complete cure, the excimer discharge technology has found limited use in the UV curing field.

Heat generated by the UV lamp must be removed through some active method, as the power densities normally employed in UV curing are too high for ambient cooling. The two methods used for heat removal are air cooling and water cooling.

Air cooling is the simplest and cheapest approach. In this method, ambient air is drawn into the lamp and across the quartz tube and other surfaces which require cooling. The resulting heated air is then discharged outside the plant. A temperature sensor located in the exhaust duct and tied to a damper with temperature control allows the cooling air flow to be regulated to maintain a desired temperature. While relatively simple and inexpensive, this type of cooling draws large amounts of air from the work area. If the work area is heated or air-conditioned, this can substantially increase the HVAC operating cost. A solution to this problem is the “push-pull” system. This approach uses a second blower to provide make-up air to replace the air drawn out by the exhaust blower. Inlet air filtering, balancing, and interlocking are required to make this system work reliably, but once accomplished, it is an effective method of air cooling.

Water cooled UV lamp housings consist of components with water cooling passages. A flow of cool water removes the heat from the components without the need for large amounts of cooling air. Consequently, water cooled lamps tend to be much more compact than air cooled lamps. For this reason, almost all UV sheetfed lamps are water cooled. The quartz tube must operate at an elevated temperature and cannot be water cooled. Lower wattage UV water cooled lamps often operate in ambient air, while higher wattage lamps employ a small level of air cooling to maintain proper quartz tube temperature.

So what is the best technology? The simple answer is that it depends on your application. If you are looking to convert to UV, start by making a list of the process parameters that are most important to you. Then ask plenty of questions to the chemistry companies and UV equipment suppliers. They should be able to explain to you how their products support your process.