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LED Lighting goes Beyond the End of the Rainbow

By Rudy Ramos | Mouser Electronics

LED lighting is increasingly taking over from more traditional forms of illumination, thanks to its many and varied benefits. Longer lifespans help cut maintenance costs. Increased efficiency reduces energy bills. And the smaller physical size, greater robustness and ability to switch LED lights on and off without negative effects, mean whole new applications are now possible.

We all have noticed LEDs springing up in new places: the headlights in our cars, the strip lights in our offices, and even many of the bulbs in our homes. These lamps create visible white light, which incorporates spectrums of red, green and blue light. But what many people aren’t aware of is the impact LEDs are having beyond the rainbow spectrum that is visible to the human eye.

Beyond blue and violet light
Our eyes sense colored light that has a wavelength between around 400 nm and 700 nm. Wavelengths between 495 nm and 430 nm appear to us as blue, while wavelengths below that appear as violet. Once the wavelength drops under about 400 nm, we can no longer sense the light: we are now in the ultraviolet (UV) spectrum. This in itself has three ranges: long-wave (UV-A) goes from 400 nm to 315 nm. Below that is middle-wave (UV-B), down to 280 nm, followed by short-wave (UV-C), between 280 nm and 100 nm.

Ultraviolet light with these shorter wavelengths but higher frequencies is mostly used to destroy harmful biological substances, including in air and water purification systems and on medical equipment.

When you produce low-wavelength, high-frequency light, the photons have more energy than you get from visible-spectrum light, which has longer wavelengths and lower frequencies. This energy is what can be used for sterilization and germicidal purposes.

However, manufacturing LEDs that produce light with such short wavelengths is challenging, because of the bandgap levels of the associated semiconductor materials. Consequently, there are relatively few UV-B and UV-C LEDs in the field.

However, UV-A LEDs are more common, with several semiconductor makers having created materials and products capable of emitting this type of light. And there are three areas in particular where these LEDs are being used: UV-curing, industrial inkjet printing and scientific instrumentation.

UV-A LEDs in industrial inkjet printing
Many of us will be familiar with how inkjet printing works: laying down tiny droplets of ink on some kind of substrate (often paper or plastic). This ink then needs to dry. Oil-based inks do so via oxidation, while their solvent-based counterparts dry through evaporation. Infrared light can be used to speed up the drying process, but risks shrinking or warping the substrate. Printer manufacturers therefore created UV-A-curing inks, which stabilize through photochemical crosslinking. What this means is that polymerization occurs when you bring together the pigment, binding agent and a photoinitiator (the UV-A light). The energy from the UV-A light produces free radicals, which combine with other elements to create strong links between the pigment and binding agent. This process does not expose the substrate to heat, meaning it remains smooth and uniform. Moreover, curing with UV-A light is more eco-friendly, since there is no evaporation or oxidization, meaning no materials are emitted into the environment and 100% of the ink remains on the substrate.

The process of UV-A curing has long used mercury vapor lamps. Thanks to their wide, multi-spectral UV wavelength output, these lights speed up the inkjet process by maximizing the reactive components in the ink. UV-A LEDs, on the other hand, are monochromatic, so are more limited when it comes to UV curing. That said, because you still get all the benefits of LEDs we have discussed, there is still value in using UV-A LEDs in inkjet printing. Ink manufacturers are working to optimize their products to work with latest-generation UV-A LEDs. Their usefulness is currently primarily in low-productivity printers, as opposed to industrial-scale, high-output, high-performance systems. But as UV-A LEDs develop further, their usefulness in this space will increase, meaning we are likely to see them spread further into more industrial applications.

Some UV LED options
Everlight Electronics, Lumileds, Wurth Electronics, Luminus Devices and Led Engin are among the LED manufacturers bolstering their UV-A offerings. The compact Lumileds LUXEON UV Line LEDs, for example, are designed to fit into spaces where earlier UV LEDs could not. With no optics, these surface-mount devices can be combined into arrays with spacing of just 0.2 mm. Because the dies are so small, you benefit from very precise optical control.

Figure 1: Lumileds UV U1 LEDs

Figure 2: LED Engin LZP00UB00 Series LED Emitters

In the high-power space, LED Engin has three lines of very bright UV LED devices in the 385 nm to 410 nm range. These can be used to cure inks, adhesives and dental fillings, as well as to whiten teeth, check documents for counterfeiting, sterilize equipment and for various other medical purposes.

Beyond red
At the upper end of the visible spectrum is red light, beyond which lies infrared (IR). The boundary between visible red and IR is a more blurry one – generally somewhere between 700 nm and 740 nm, depending on conditions. This is because although our eyes can perceive wavelengths as high as 740 nm as red light, anything above 700 nm becomes difficult to see, and is consequently nudging into IR territory.

Here again, LEDs are providing numerous benefits.

IR light is divided into five bands:
1. Near-IR (NIR, 700nm (0.7m) 1500nm (1.5 m))
2. Short-IR (SWIR, 1.5m 3m)
3. Middle-IR (MWIR, 3m 8m)
4. Long-IR (LWIR, 8m 15m)
5. Far-IR (FIR, 15m 1,000m)

Each band has a variety of commercial and industrial uses. We all focus on NIR LEDs. These LEDs are particularly useful when needing to illuminate a dark area in a way that is outside the range of the human eye but not outside the detection range of electronics. NIR LEDs are particularly good for this type of application because of the number of electronic sensors available with optimal responsivity curves in the NIR spectrum, underpinned by the physics of their design.

Security cameras, CCTV and machine vision are some common use cases, with NIR LEDs used to illuminate subjects. These cameras are able to gather data such as car registration plate information, toll tag details, or biometrics for access control or identification. And because they can be used with silicon photodetectors, they’re also ideal for smoke detectors, touchscreens and gesture-recognition systems.

NIR LEDs in spectroscopy
Anyone who has watched CSI: Crime Scene Investigation will know how much use is made of spectroscopy, including NIR spectroscopy, to evaluate clues. And NIR spectroscopy is being used in other ways, too. A relatively recent development has been to use it to assess the quality and properties of the food or medicines we ingest.

This works by shining light from NIR LEDs onto an object. This light is either absorbed or reflected, depending on the molecular makeup of the item being investigated. Using a wavelength-selective detector, you can pick up the reflected light and consequently determine what materials are present.

Where this would once have required big, complex spectroscopy machines, today’s powerful NIR LEDs can be built into hand-held devices. Suppliers such as Osram Opto Semiconductors (with its IR OSLON Black Series LEDs) and Lumileds (with the LUXEON IR LEDs) are coming up with infrared LEDs for this kind of use case.

One of the techniques that makes these high-power NIR LEDs possible is phosphor conversion, which changes the wavelength of the light the LED produces. The original light may be in the visible spectrum, but by passing it through layers of phosphor on top of the diode, it produces luminescence, increasing the light’s wavelength. In this way, you can produce a variety of NIR wavelengths. Using optical domes is another way to control how and where light is emitted. And remember that you can improve NIR spectroscopy performance and bandwidth by using multiple wavelengths in your system design.

Transforming lighting at either end of the rainbow
The benefits of LEDs mean they are having a massively positive impact for applications in both the visible light spectrum and beyond.

At the violet end, UV light is divided into three bands. The first of these “UV-A” is where LEDs are being used for inkjet printing, curing substances and sterilizing equipment. And beyond red, there are the five bands of IR light, including NIR, where LEDs can be used in spectroscopy.

So in the same way LEDs have transformed the world of visible lighting, they are also revolutionizing the light we cannot see, at either end of the rainbow. How could you use UV or IR LEDs in your designs?

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