Guide to UV Measurement
Spot Cure Basics
| UV “spot” curing systems are in wide use for a variety of industrial manufacturing applications. These devices direct discrete ultraviolet and/or near visible light energy to small target “spots” to instantly polymerize or cure special adhesives, coatings, maskants or potting materials. The generated radiant energy is channeled to the target site by lightguides, waveguides or focusing arrays. |  |
Construction
The vast majority of ultraviolet spotcures employ similar functional components: power supply, bulb, reflector, shutter control and some means of directing the light, which is generated. Spotcure bulb construction and product application requirements dictate power supply design. Modern power supplies today are very robust & reliable, and are normally microprocessor based for powerful programming routines as well as status/alarm indications. Bulb
s are mercury-based, usually in an inert Argon or Xenon background, with varying pressure and arc qualities. The best designs not only produce light strong in desired UV content, but also very stable and long lived.
Reflectors can either be integrated into bulb manufacture or designed as a separate, discrete component. While separate reflector designs cost a bit more initially, the ROI is higher because reflectors only need be replaced a small fraction as often as the bulb itself. For spotcures, all reflectors are elliptical rather than parabolic so the focal point is a highly concentrated spot. Some reflectors are constructed of dicroic materials to separate unwanted IR (infrared) spectral components, while other designs use precision polished metallic constructions in combination with IR filters to accomplish the desired heat reduction. The advantage of separate filtration is that in many R&D as well as process applications, IR is actually desirable for thermal add in two part curing. Additionally, flexibility in filter changes allow different spectral pass-through for varying chemistries.
Spectral Output
Most UV lightcure chemistries today require energy in the UVA to effectively trigger free radical adhesive & coating formulations. UVC energy is also necessary for both surface cure of free radicals, as well as many cationic formulations. Additionally, light in the near visible range is also employed by a growing variety of chemistries. Therefore, a spot cure with a wide range of spectral output and/or one matched with interchangeable filter flexibility is ideal.
The generated UV energy is then projected through a shutter and/or intensity control assembly before exiting the spotcure. The simplest systems employ basic shutter on/off operation. More expensive systems employ iris or other shutter modulation, and a few even employ microprocessor controlled “optical feedback” loops to regulate power to the bulb to compensate for intensity drop-off over time.
Light Guides
UV energy can be directed to final application site through lightguides, waveguides or focal arrays. By far the most common means is lightguides. These devices can be constructed of either specialized optical fibers, or specialized liquid-filled reflective tubing with quartz lens caps. Liquid lightguides have the advantage of better short length optical efficiency and usually lower cost. Fiberoptic guides are more useful for very long deliver lengths and multiple site applications using a single light source. Fiberoptic guides are also longer lived and more useful in the extreme intensity applications popular today for sub-second curing in high speed process lines.
Waveguides are highly polished aluminum or stainless steel devices used on wider area spot cures. They generally last indefinitely and suffer no loss of UVC energy. Lens arrays can occasionally be useful for directing light energy greater distances through ambient air than with lightguides alone. However, they are rather expensive and suffer from additional energy loss through lens conversions.