Future Electronics — Advances in UV LED Technology Promise Faster Rate of Adoption

One battle is already over: the LED has defeated conventional visible light-source technologies in every important indoor and outdoor general lighting application.

But what about applications for invisible light in the ultraviolet part of the spectrum? The same characteristics that have made LEDs so attractive for general lighting also apply, in theory at least, to the applications for which UV light can be used, such as curing, disinfection, chemical analysis and synthesis:

• Small light-emitting surface and small overall package size
• Long operating life, and low maintenance and repair costs
• Low-voltage operation and low power consumption
• Tight control of the wavelength of the emitted light
• Freedom from harmful chemical components

These characteristics have led specialist lighting engineers to propose innovative new uses for UV light. They also give good reason for UV equipment manufacturers to evaluate LEDs as a straightforward replacement for legacy UV light-source technologies.

Although the advantages of UV LEDs look attractive on paper, in practice the rate of adoption of UV LED technology has lagged far behind that of visible light LEDs. Now the introduction of new LED products offering a dramatic reduction in unit costs alongside higher optical-power output looks set to launch a wave of new UV product developments, promising valuable cost, environmental and operational benefits for users.

Why UV LED Light Sources Are in Growing Demand

The classification of the UV part of the spectrum of electromagnetic radiation covers the range of wavelengths between 100nm and 400nm, as shown in Figure 1.

Fig. 1: UV light’s position in the spectrum of electromagnetic radiation. (Image credit: Seoul Viosys)

By convention, this UV part of the spectrum is itself split into three: UVA, UVB and UVC. As Table 1 shows, some applications are better served by one class of UV light than by another.

The value of UV light derives from the powerful chemical and biological effects that it has on irradiated objects, effects which are far greater than simple heating. Many practical applications of UV radiation derive from its interactions with organic molecules.

Table 1: The characteristics of and typical applications for the classes of UV light

For instance, the energy of UVA photons may be used to activate chemical reactions through photo-initiators, a useful function in applications such as curing of paint or other coatings, curing of glue or varnish, and photolithography.

For disinfection and sterilization, UVC light is required. A combination of UVC, UVB and UVA light may be used in curing: for instance, UVC hardens the surface, while the more penetrative UVB and UVA cure the bulk of the material. UVB is also preferred for phototherapy such as psoriasis treatment, for which it provides the best combination of effectiveness and safety.

Today, these applications are typically served by conventional UV light sources. These UV lamp types remain in common use because of their relatively low unit cost, or more properly, their low cost:radiant power ratio, and also the high radiant power capability. The effectiveness of a UV light application is essentially a function of irradiance and exposure time. To disinfect the water in a municipal swimming pool, a UV light source must irradiate a large volume of water flowing at a constant fast rate: this calls for high irradiance because the exposure time is short.

By contrast, disinfection of the drinking water stored in a small pleasure boat’s tank can be performed slowly, giving a long exposure time and thus requiring low radiant power which is an ideal application for UV LEDs.

In fact, UV LEDs are superior to conventional UV light sources in almost every respect: the LED’s chief drawback is that its cost:radiant power ratio is markedly higher than that of conventional UV light sources. This means that swimming pool disinfection will remain, as it is now, an application for UV arc lamps for the foreseeable future.

For many applications, however, the characteristics of LEDs are markedly superior.

For instance, nail bars use UV light to fast-cure nail varnish. Here the LED offers superior value:

• Small size and light weight, making the curing equipment compact and
  easy to handle
• Robust and tolerant of being knocked or dropped from a work surface
• Low operating voltage, making it easy to ensure the electrical safety of
  the operator

LEDs also support the development of new applications for which a conventional light source such as a mercury vapor lamp is unsuitable. For instance, a small UVC LED may be mounted inside the tank of a coffee machine or drinking-water fountain to inhibit the growth of micro-organisms in the water. This application is particularly well suited to UV LEDs because:

• The LED is small, so is easily accommodated in tight spaces
• The low-voltage operation of the LED in a wet environment is
  electrically safe
• The LED’s long operating lifetime eliminates the need for expensive
  lamp replacement

The same features of UV LEDs are also valuable in new, compact air purifiers or deodorizers. In these devices, UVA light irradiates a titanium dioxide-coated catalyser to generate free radicals which break down large organic molecules. These purifiers can be embedded in refrigerators and air-conditioning systems. In cars, they can remove odoriferous volatile organic compounds such as cigarette smoke or plastic outgassing residue. Combined with bactericidal UVC light, a purifier will keep a car’s airconditioning system fresh and free of airborne pathogens while reducing the frequency of cleaning and filter replacement.

These practical advantages of UV LED light sources are now gaining even greater value with the recent introduction of new LED products with superior specifications.

Fig. 2: Nichia’s NVCUQ096A-D4 UVA module has a footprint of 25mm x 45mm

New LED Products Spur Faster Adoption

For curing applications such as varnish-drying in a nail bar, speed is of the essence, and speed correlates with UV irradiance. Compared to traditional technologies, compact LED light sources are easier to focus and to install close to the target, keeping optical losses to a minimum and producing good irradiance at a small target area.

But the introduction of new multi-die modules enables compact LED light sources to reach even higher irradiance. A good example is Nichia’s new NVCUQ096A-D4 UVA module, as shown in Figure 2. At a peak wavelength of 385nm, it produces an optical power output of 150W in a ±30° beam. The module has a small footprint of just 25mm x 45mm.

Seoul Viosys is also producing interesting innovations in the UV LED market. While mainstream UVC LEDs have a vertical chip structure in a ceramic package sealed with a quartz glass window, Seoul Viosys has developed a streamlined chip-scale package under the WICOP UV brand. This ‘package-free’ surface-mount chip technology removes both the cost and the optical losses associated with the conventional ceramic + quartz glass architecture.

When implemented in new UV equipment designs, WICOP UV LEDs promise to provide a higher optical power output in a smaller space at a lower unit cost.

The WICOP UV technology is rivalled on the UVA side by Lumileds’ LUXEON UV FC Line of products, in which FC denotes FlipChip platform technology, as shown in Figure 3. The LUXEON UV FC Line products, which have a 1mm2 die, are supplied as a Chip-Scale Package (CSP) LED which can be reflowed on to a substrate with standard surface-mount assembly equipment and processes.

Examples of applications for these devices include off-the-shelf, fully-packaged UVA emitters. Both the LUXEON UV U1 (1mm2) and the LUXEON UV U2 (2mm2) are powered by Lumileds’ FlipChip platform technology.

Support for Integration of UV LEDs in End Product Designs

UV LED selection and specification are key tasks in system design. LED manufacturers allocate produced units to bins, allowing the buyer to specify products according to flux (optical power output), forward voltage and peak wavelength.

Selection from a broad market provision of binned UV LEDs will be made easier by use of product comparison software such as Future Lighting Solutions’ Usable Light Tool. The user can specify a required flux and peak wavelength, and the tool will provide a detailed list of products meeting the specification. Use of the tool can save many hours of tedious online datasheet searches, while providing valuable application-specific comparison data.

System designers can also use the tool, together with guidance from Future Lighting Solutions’ applications engineering experts, to specify appropriately the supporting components:

• LED drivers are readily available, providing a constant-current output
  and any standard output-voltage rating
• Optics for beam control are beginning to come on to the market.
  Standard LED thermoplastics are not transparent to UV light, but LEDiL
  has developed silicone optics which are compatible with UVA LEDs.
  Development work on optics for UVB and UVC LEDs continues.

With the support of Future Lighting Solutions’ optical technology experts and software tools, designers of UV lighting equipment can now use the latest UV LEDs in confidence, benefiting from the space, cost and power savings promised by this technology.