UV Light, LEDs, and Mercury Vapor Lamps|
Article- March 2012 By Larson Electronics.com
Larson Electronics Rechargeable UV LED Light
Ultraviolet light is below the visible light band on the light spectrum, yet provides almost as much practical use potential as visible light. While visible light has obvious benefits tied to providing basic illumination, ultraviolet light’s possibilities lie more in the direction of using light to perform work as well as specialized tasks which incorporate artificial forms of visioning. Among the many applications wherein ultraviolet light is applied, the most common are, machine visioning, sterilization, adhesive curing, forensics, and water treatment. Ultraviolet light is just like any other radiation found within the light spectrum, only it resonates at wavelengths of 100 to 400Nm, making it invisible to the naked eye. Like all forms of light, UV can be focused, reflected, intensified and diffused. Applications requiring the use of UV radiation have typically relied on mercury vapor lamps fitted with filters to produce the needed ultraviolet output. LEDs however are fast replacing the mercury vapor lamp due to their greater efficiency, precise spectral output, longer life, and cooler operation.
Applications where UV is used to facilitate curing usually involve compounds such as adhesives, ink, and polymers that are photosensitive. UV radiation is applied in these instances to speed up or activate on demand the curing process in the materials as well as produce a more even and thorough curing process, resulting in stronger adhesion and durability as well as enhancement of beneficial material properties. UV light of 365-395NM is typically used in these applications and is produced using high intensity mercury vapor lamps equipped with filters which block out light radiation above and below 365-395NM. The problem here is that up to 70% of the mercury lamps’ output is wasted as heat and undesirable visible light output that is blocked by the filter. In essence, while the mercury vapor may be producing 1,000 lumens, only 30-40% of that output is resulting in useable light. The need to maintain the mercury vapors’ UV output at required levels also creates additional problems with maintenance, and these lamps must be periodically checked to ensure they are maintaining output within the desired wavelengths and at the proper intensity. These lamps experience a steady decrease in UV output over the course of their short life time, thus increasing the operational costs associated with mercury vapor UV lamps as only replacing the lamp can restore UV output to the required levels.
UV light within the 315-395Nm band, which is known as UV-A, is also effective for use in machine visioning systems. In these applications, UV light is used in conjunction with sophisticated sensors and software to detect minute differences in materials and surfaces. Machine visioning applications include aircraft and aerospace inspection systems wherein the surfaces and materials used in the construction of aircraft are inspected for normally invisible fractures and cracks that could lead to potentially serious failures. Manufacturing and technological applications also employ machine visioning to inspect machined products and complex circuits at an almost microscopic level to determine if any abnormalities or imperfections exist. In these applications, again, mercury vapors lamps are the typical choice. As noted above, problems with efficiency, heat, and maintenance costs are present here as well. In addition to the aforementioned issues, machine visioning and inspection applications relying on mercury vapor lamps do not fare well where high levels of visible ambient light are present as the intensity of the UV produced is rarely strong enough to prevent dilution.
UV light within the 280-315NM range, also known as UV-B, is typically used in applications such as forensic testing and analysis, medicine, and law enforcement. In medical applications, UV-B has proven effective in the treatment of skin disorders such as psoriasis and vitiligo. UV-B radiation also plays an important role in how the human body processes vitamin D, the metabolism of calcium, and even the production and release of insulin. As with other applications, the mercury vapor light sources used to produce UV radiation carry inherent limitations and shortcomings that significantly affect the performance and effectiveness of UV in these applications as well as impacts their costs.
In just about every application that relies on UV radiation, practicality of light sources, costs, and reliability are all major factors. Current mercury vapor setups often require constant monitoring of a mercury vapor lamps ‘UV output to avoid variances and errors in readings, and these lamps generate prodigious amounts of heat which make implementing them problematic in applications where materials are susceptible to heat damage. Although the fluorescent UV lamp offers some solution in some applications, they too present additional issues in the form of impractical design and limited output. Recently, developers of UV reliant equipment have begun implementing LEDs into their designs in an effort to combat these issues with promising success.
LEDs hold the potential to completely replace mercury vapor lamps in UV reliant applications. In categories such as longevity, spectral output, cost effectiveness, and material compatibility, LEDs offer greater versatility and suitability. Much of this is due to the unique light producing characteristics of the LED and its ability to produce “clean” light. Clean light as used here simply means that the LED can produce light over a specific band of the light spectrum. For instance, in applications where a filter is normally needed to filter out the undesirable visible and infrared spectral output of a mercury lamp, LEDs can be designed to produce ONLY light in the UV band. This is a major advancement for technologies such as machine visioning and medicine as equipment can be made smaller and accuracy and reliability greatly increased by substituting bulky and hot running mercury vapor/filter lens assemblies with ultraviolet LEDs.
LEDs offer to reduce expenses associated with UV reliant technologies as well through reduced maintenance and servicing costs. Whereas a typical mercury vapor lamp may have a rated lamp life of 24,000 hours, this must be assumed to be far less in many applications due to the need to maintain UV output at high levels and the mercury vapors’ tendency to drop in quickly UV output as it ages. LEDs on the other hand have common life span averages of 50,000 hours or more, with many being able to maintain up to 70% of their output at the end of their operational life. In applications such as machine visioning or medicine where UV levels must be maintained at specific levels for maximum effectiveness and accuracy, this means greatly reduced maintenance intervals and increased reliability.
LEDs offer further benefits in the form of cool operation and high efficiency. Although mercury vapor lamps are indeed efficient, they run at high temperatures and can be difficult to implement in applications where heat sensitivity is an issue. LEDs can maintain efficiency, and in many cases improve it, while greatly reducing the heat associated with UV light creation. This too offers the potential to reduce the size of UV reliant equipment and devices as lamp assemblies will require less hardware and can be moved closer to the working surfaces.
For most developers of UV lighting technologies, LEDs have pretty clearly spelled the fast approaching end of the mercury vapor lamp in their applications. Current limitations on LED use are shrinking at a rapid pace, and as LEDs are adapted to producing light over a larger range of specific wavelengths, the range of UV applications open to LED use grows.