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Remote Phosphor Technology – the potential and pitfalls.

LED technology that is starting to penetrate the current LED marketplace known as Remote Phosphor Technology (RPT). RPT was first pioneered in the LED sector some nine years ago but it was researchers at the Lighting Research Centre, Rensselaer Polytechnic Institute in the US that provided a practical way forward. They developed and patented a new packaging method for white LEDs called Scattered Photon Extraction (SPE) which enables higher luminous efficiency by placing the phosphor at a remote location from the die by shaping the lens surrounding the die to extract a significant amount of the back-scattered light from phosphor before it is absorbed by other components within the LED emitter. Even back in 2005, the SPE technique showed a 20-25% improvement in efficiency compared to standard white LEDs which was an incredible feat considering how bright LEDs have become in four years!

The most popular way to create a white LED is to use a Blue LED chip which is coated with a phosphor and there are several common methods used in production as shown in figure 2. Referring to figure 2, the majority of LEDs are created using either (a) or (b) where the phosphor is either conformally coated around the LED chip or dispersed throughout the coupling medium respectively. Conformal coating is considered the better technique as the quality of light output is improved and there is less of a CRI or CCT variation with beam angle.

Philips Lumileds have recently developed a new method, Lumiramic, mentioned in previous articles where the phosphor is placed within a small disk above the die to improve quality and consistency.

The first iterations of remote phosphors placed the phosphors away from the LED chip but still within the emitter package. Although this showed an improvement there would still be significant issues with LED temperature especially as phosphor efficiency and lifetime degrade as the ambient temperature increases.

A further issue is that when blue light which is absorbed by the phosphor it converts the light into a different wavelength commonly know as phosphor conversion. Unfortunately, phosphor conversions scatters light in all directions (not just forwards) and hence studies by the Lighting Research Center (LRC) have shown that up to 60% of the lighting created by the phosphor is scattered back toward the LED die and lost due to absorption.

Even when there is a highly polished reflector used the arrangement did not show much of an improvement in terms of extracted light output over traditional packages where the phosphor was placed close to the die. The LRC uses the SPE technique to extract the back-scatter light from the remote phosphor before it is absorbed by using a secondary optic that efficiently transferred the blue light to the remote phosphor whilst recycling the back-scattered white light from the system so it is not lost.

The SPE technique shows a 60% improvement in light output and efficacy compared to similar commercial white LEDs, where the phosphor is placed close to the LED die. Although this research was undertaken in 2005, when traditional white LEDs had an efficacy of 30 lumens per watt at 350mA, the SPE system showed an efficacy of approximately 50 lumens per Watt. If we applied the SPE or similar light recycling techniques to the latest blue LEDs we would be looking at efficiencies of 150-160 lumens per watt!

More recent studies by the LRC indicate how the CCT of an SPE fixture varies with different phosphor densities.

The variation in phosphor density and thickness is one of the key issues for RPT based white LED fixtures hence why manufacturers have taken a long time to begin to introduce such products to the market.

There are several research groups around the world that have been working on RPT technologies where they have mixed two or more different phosphors together enabling a variable colour temperature or CCT to be achieved by using the relative intensities of two different wavelength LEDs. One company that has commercialised this into a product (KPT of Korea) and it has impressed all that have viewed it.

The early-to-market RPT products such as the Philips Fortimo and the much advertised alphaLED products need to be effectively tested for CCT and CRI consistency between fixtures and their lifetime and performances monitored especially if they are using standard blue LEDs illuminating a single phosphor plate.

It is very difficult to ensure the phosphor materials are placed across a 25mm to 100mm plate with a homogeneous density and consistent thickness. If either of these variables change significantly then the CCT and CRI of the light fixture will vary and although it is possible to replicate one or two samples for research it is much more difficult to scale up for higher volumes.

This said, the possibility of significantly improved white LED luminaire efficacy and lifetime which certain remote phosphor technologies have shown, paves the way towards the 150 lumen per watt LED fixture using today’s LED technology.

It will be a matter of time before high quality, consistent and efficient RPT LED fixtures will reach the market with the majority of research required being in the production capability of producing thin consistent layers of phosphor on plates.