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Edmund Optics solves LED coupling problems

06 Mar 2006

The days of the quartz halogen lamp in machine vision applications could be over. Jacqueline Hewett finds out how Edmund Optics went back to basics to improve the coupling efficiency of an LED, turning it into a practical alternative.

Collecting light from an array of LEDs and coupling it into an optical fibre is not an easy task. With LEDs emitting over a range of angles, the amount of light at the end of the fibre will typically be only a fraction of that emitted by the devices themselves.

At Photonics West in January, Edmund Optics of the US unveiled its EOS illumination delivery system which it believes solves the LED coupling efficiency problem. What's more, the EOS system can tune the output wavelength on-the-fly; has a uniform near and far field pattern; and can be applied to other sources such as OLEDs.

The company attacked the problem from an optical standpoint. "The primary motivation was to design something to couple energy very efficiently out of a die and into a useful form," Samuel Sadoulet, Edmund Optics' director of product development, told OLE. "The EOS technology is an optical technology that collects, multiplexes, shapes and integrates multiple sources into a single useful output. It is also scalable and applicable to other sources - not just LEDs."

Edmund Optics believes that LEDs transformed by its EOS technology are poised to replace quartz halogen lamps. A typical 150 W quartz halogen lamp has a lifetime of around 1000 h and is the source of choice in many manufacturing, medical, military and machine vision applications.

While an LED has a substantially longer lifetime, and is more energy-efficient than a quartz halogen lamp, the problem has been coupling enough energy out of the LED to make it practical. Driving an LED harder increases its brightness but also introduces thermal management problems and reduces its lifetime - removing a major advantage over quartz halogen.

This situation could be about to change as the EOS technology eliminates the problematic balance between driving the LED and lifetime concerns. Edmund Optics says that initial tests show that EOS-transformed LEDs are at least 2-5 times brighter than quartz halogen and last 20-50 times longer.

As the EOS technology is still patent-pending, Sadoulet was reluctant to reveal too many details of the company's clever optical solution. "The RGB prototype that we had on show at Photonics West contains multiple LED dies of each colour," he admitted. "Multiple optical stages then mix the different wavelengths together into a source whose colour temperature can be controlled."

This ability to tune the colour temperature on-the-fly has led Edmund Optics to coin the phrase "dynamic filtering". Instead of filtering a white light source, the EOS technology allows the user to manually or electronically turn on the green LEDs and turn off the red and blue devices, for example.

"Dynamic filtering is possible because everything has been mixed both near and far field," explained Sadoulet. "You can now add more LEDs and they will be integrated and mixed together very cleanly. The more LEDs you have, the more power you have."

The first EOS product will be commercially available in August this year and Sadoulet expects to see uptake in both the imaging and microscopy markets. "We expect people in imaging to take this on for fibre-optic illumination," he said. "A wider market is illumination in microscopes. The big markets are probably going to be in South-East Asia for imaging and Germany because of its extensive factory floor automation."

Sadoulet adds that Edmund Optics is also hoping to add ultraviolet and infrared wavelengths into the mix. "As we add on more ultraviolet wavelengths you could imagine that the biomedical industry is going to be a huge consumer of this technology," he added. Other potential applications include infrared military illuminators, automotive headlamps, surgical headlamps and even portable projector systems requiring battery-powered illumination.

Although the company is focusing on LEDs and fibre-optic delivery right now, Sadoulet stressed that the EOS technology can be applied to other sources. "Taking LEDs and pumping them into a fibre is just one of many delivery options," he said. "But you can think of many applications that require high-efficiency coupling, OLEDs and fluorescence emissions, for example."

Edmund Optics is now trying to miniaturize the EOS architecture. "The prototype is around 10 × 15 × 3 cm and the optical core fits in your hand," concluded Sadoulet. "We intend to miniaturize the technology."

Unveiling its prototype at Photonics West gave Edmund Optics a valuable opportunity to talk with its customers and hear their ideas of markets that could benefit from the technology. While some offered applications that Edmund Optics already had in mind, others, Sadoulet admits, were ones the company had not thought of. Having eliminated one of the major disadvantages of LEDs, who knows what other markets and applications will adopt the EOS technology.

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