UV LEDs ramp up the quiet side of the LED market

Below the visible spectrum lies a band of wavelengths called ultraviolet (UV). Ranging from 100 to 400 nm, the radiation can effectively be used to sterilize cosmetics, perform forensic analysis, cure materials (Fig. 1) and disinfect water, among many other applications. Today, as is the case with LEDs in the visible spectrum, led street lighting Manufacturer are only beginning to replace the established UV sources in a likewise diverse array of markets.
FIG. 1. Firefly air-cooled UV-LED-based curing system (source: Phoseon Technology).
FIG. 1.
“Compared to the multibillion dollar LED lighting industry, the UV-LED market is a rounding error at around $30 million, but it is growing rapidly,” said Mike Lim, director of global industrial and new business for Luminus Devices, based in Billerica, MA.
By far, the UV curing market is enjoying the greatest surge in UV-LED adoption due to the relatively recent breakthrough in flux density of UV-LED chips beyond the 4W/cm2 mark at the different wavelengths needed to bring UV-LED curing into production facilities. “For the first time, LEDs have reached the power densities that make UV LEDs competitive with existing sources,” said Uwe Thomas, general manager of emitter components with LED Engin, a provider of UV and visible LEDs based in Santa Clara, CA .

Curing involves the cross-polymerization of a photosensitive material, which can be an ink (printing), adhesive or coating, and is primarily performed at 395 nm, 385 nm or 365 nm, wavelengths which are part of the UV-A spectrum (315 -400 nm). Another important UV-A application is inspection systems based on machine vision. UV-LED flashlights in this range are used to detect fraudulent identification and currency, and offer the benefit of use in well-lit environments, which is difficult using a mercury-vapor lamp.
In the UV-B spectrum (280-315 nm), applications for UV LEDs include curing, medical light therapy, forensic analysis and drug discovery. It has been estimated by market research firm Yole Développement (Lyon, France) that 90% of UV LED applications today are based in the UV-A and UV-B regions (Fig. 4). Yole has stated that it expects the market for UV LEDs to grow by 30% annually from $25 million in 2010 to more than $100 million in 2016 . Fig. 5 shows an even more optimistic forecast from the Optoelectronics Industry Development Association, the International Optoelectronics Association and the European Photonics Industry Consortium.
FIG. 2. Multi-die UV emitter in a ceramic package (source: LED Engin).
FIG. 2.
Beyond these applications, UV-B radiation is known to have beneficial health properties including the natural synthesis of vitamin D ​​in humans who are exposed to sunlight. UV-B also accelerates the production of polyphenols in certain leafy vegetables such as red lettuce. Polyphenols are believed to have antioxidant qualities. “Generally these plants are grown in greenhouses today, which intentionally filter out the UV portion of the spectrum in order to maximize plant growth. Interestingly, we have evidence that when those plants are exposed to UV-B LEDs a short time before harvest, their polyphenol content is boosted without compromising plant mass,” explained Cary Eskow, global director of advanced LEDs and illumination for Avnet Electronics Marketing in Phoenix, AZ, which distributes UV LEDs. He continued “This is a novel method for increasing the appeal of some foods without using chemicals. Polyphenols also have received attention due to their purported anticarcinogenic, antiproliferative and antimutagenic properties.”
In the lower UV-C spectral range (100-280 nm), the primary LED applications are air and water sterilization and a range of analytical tools including those that perform spectroscopic and fluorescence measurements. Chemical and biological detectors also work in this spectral range.
UV in the 250-275 nm range sterilizes water, air and surfaces by breaking-up the DNA and RNA of microorganisms and preventing their reproduction. Specifically, 275 nm is believed to be the most effective wavelength for eradicating pathogens such as E-coli in water. In fact, engineers at Sensor Electronic Technology Inc. (SETi), based in Columbia, SC, have determined that 275 nm is the optimum wavelength for water disinfection. SETi has demonstrated disinfection of drinking water in an in-line flow-through system using less than 40 mW of UV power.
Alternatives to mercury arc lamps
In production curing operations, mercury-vapor lamps are hindered by short lifetime (2000-10,000 hr), slow warm-up and cool-down times, and wide spectral power distribution. “In addition, over 60% of the energy that is applied to a typical mercury-vapor lamp is radiated back out as infrared energy, in other words, heat,” said Eskow. He added that the UV output of a mercury vapor led street lighting Manufacturer drops off rapidly over its operational life because some of its electrode material vaporizes , depositing a film on the inside of the quartz tube which the UV cannot penetrate. As a result, the user cannot easily predict the amount of UV generated at a later time; often this is a critical process parameter.
FIG. 3. UV-C LEDs for germicidal applications, from Crystal IS, Inc. The company was recently acquired by Japan-based Asahi Kasel.
FIG. 3.
The mercury lamp has a main peak at 365 nm but several smaller peaks in the visible and infrared regions (Fig. 7). A downside to these extraneous peaks is the generation of heat during printing and other curing operations. “With plastics and other heat -sensitive materials, there have been real challenges in printing because the medium gets distorted from the heat of the mercury lamp,” explained Steve Metcalf, CEO and president of Air Motion Systems (AMS), a maker of curing systems based in River Falls, WI. He gave examples that include plastic gift cards or credit cards, which use sheet-fed lithographic printing.
Metcalf added that many people who would not have considered UV curing because of the complexity of standard curing processes – the high voltage, heat of mercury bulbs and environmental issues – will now consider UV-LED curing because it subverts the traditional hassles of the old process .
UV LEDs bring such benefits as knowing precisely what power level is being delivered to the curing medium, as well as other advantages LEDs are known for, such as the ability to pulse-width-modulate the output, stated Eskow.
The transition to UV LEDs also opened the door to more environmentally friendly curing formulations. With the next generation of products that ink, coating and adhesive companies were developing, they used the opportunity to phase-out volatile organic compounds (VOCs) associated with the solvent -based formulations of past chemistries.