Remote phosphor LEDs deliver light quality and improved efficacy

Remote phosphor LEDs deliver light quality and improved efficacy

 

Lighting designers today are looking not only for the energy efficiency and long life expected of LEDs, but also for better light quality and colour stability.  Remote phosphor is regarded as delivering this next level of LED lighting technology.  Lance Hemmings, Global Product Group Manager at RS Components explains.

 

The traditional method of producing white light LEDs is to use a blue die (also called a pump), combined with a phosphor layer, usually yellow or amber, directly onto the die.  Phosphor absorbs the light at the blue wavelength and re-emits photons at longer wavelengths.  The result is that monochromatic light is converted to broad-spectrum white light.  Developed commercially in the 1990s, variations of this phosphor conversion process, the substrate, and the phosphor ‘recipe’ are still widely used to produce a broad range of today’s LEDs, as used across the scope of lighting applications.

Manufacturers are continuing to evolve and develop these phosphor conversion techniques, with the aims of improving efficiency and efficacy as well as colour uniformity while reducing die size and manufacturing cost.  A key factor influencing these characteristics is thermal management.

An emerging method now gaining popularity, is ‘remote phosphor’, whereby the phosphor layer is not in direct contact with the die.  Typically, the module incorporates a light-mixing chamber with highly reflective sides, physically separated from the die.  Moving the phosphor away from the die allows for better heat dissipation, which leads to improved efficiency, efficacy and reliability as well as reducing the risk of colour shift over time.

More advanced phosphor recipes are being developed, with two and even three phosphor components, to achieve higher CRI (colour rendition) and high R9 (better rendering of deep, saturated shades of red).

For ruggedness, phosphors such as green aluminate, red nitride and YAG are selected, giving better than 50,000hrs lifetime.  The construction of remote phosphor LEDs is also claimed to simplify packaging, particularly for assembling modules using chip on board (COB) techniques.

 

Cost competitive

Remote phosphor technology can trace its roots back to the 1980s when it was used in the first digital dashboards in cars.  But for lighting applications, companies such as Philips and Intematix introduced remote phosphor LED products in the mid 2000s.  Early products tended to be more expensive than phosphor-converted white LEDs, and despite performance advantages initial take-up was slow.  However, process improvements have since brought prices down to a more competitive level.  Importantly, remote phosphor proponents point out, LED costs become less significant when total cost at the module and system level is considered.  For example, a diffuser is not required with remote phosphor systems, and higher lumen output can mean fewer LEDs required, easier thermal management, as well as simpler driver electronics.

 

 

Remote phosphor LED technology, as delivered by Intematix with its ChromaLit range, can provide better efficacy, yet can still be cost effective.

 

One potential downside of remote phosphor LEDs has been the tendency to appear yellow or orange in the off state.  This is undesirable in some luminaire applications.  However, Intematix has addressed this issue by adding a reflective coating to the remote phosphor optic, designed to increase the reflection of blue light, thereby counteracting the yellow/orange effect.

 

Efficacy

A major benefit of LED lighting in many applications is that it runs cooler, particularly over halogen, for example.  However, passing current through the LED creates heat that has to be dissipated.  Just like other semiconductor technologies, excessive junction temperatures can impact performance over time.  In LEDs, degradation of the epoxy/phosphor layer can lead to reduced luminosity, poor colour stability and reduced operating life.  However, limiting the current reduces the lumen output.  Thus there is always a trade-off between light output and thermal issues.  Manufacturers are constantly seeking processes that maximise lumen output but minimise thermal problems, yet keep packaging compact.  Reduced energy efficiency is another factor to consider when increasing current.

The improved efficacy of the remote phosphor approach is clearly demonstrated through the availability of high lumen output bulbs.  Remote phosphor LEDs are becoming popular for retrofit applications, such as replacing 100W and 150W incandescent bulbs, for example, and enabling much brighter linear luminaires, replacing fluorescent tubes in high brightness applications.

Intematix, for example, offers the CL-840-L152-XT devices, with a typical lumen output of 4300lm, or 210lm per radiant watt.  Surface lumen density ranges from 500 to 2500lm per linear foot.  CRI is 80 and colour temperature is 4000K.  Once integrated into modules, the devices can deliver 163lm/W typically, at system level.  Its latest ChromaLit Linear range further improves performance in terms of uniform luminance over any length, plus the white off-state finish.

Such features now make solid-state devices eminently suitable in applications such as panel lighting, troffers and high bay lighting installations.  Previously, some of these applications have been challenging for LEDs, as the point sources have to be diffused, which reduces system efficacy.

 

Colour stability and uniformity

Colour stability is particularly important to maintain colour uniformity over time.  Higher junction temperatures can cause the phosphor to deteriorate, resulting in colour changes.  The problem is exacerbated when multiple die are used in linear fixtures, for example, as the LEDs will change colour at different rates, giving an uneven appearance and undesirable lighting effects.  The remote phosphor construction, with the heat-generating LEDs not in direct contact with the phosphor, significantly reduces the possibility of colour drift.

 

Colour temperature

By changing the phosphor ‘recipe’, which is the precise wavelength and thickness of the phosphor layer, the colour temperature of the LED can be adjusted.  This is an inherent advantage of both remote phosphor and phosphor-converted LEDs.  A carefully selected mix of red, orange, yellow and green phosphors allows manufacturers to produce LEDs to specific colour temperatures, from warm to cool white.  It also allows multiple die of different CCTs to be mixed and tuned to produce user-programmable colour temperatures.

For remote phosphor processes, specially developed phosphor discs or strips (depending on the shape of the die form) are used.  Once assembled into a module, the user can ‘tune’ colour temperature, such that the light becomes warmer when dimmed, for example, and cooler when brightened.  This is particularly important in dimmable luminaires used in downlighting, accent lighting and spotlights for hospitality, some architectural and consumer applications.

 

Modules and assemblies

With its requirement for a mixing chamber behind the optics, remote phosphor technology LEDs generally require a deeper module construction.  This is not regarded as a significant barrier in many designs, although some applications will require the lowest possible profiles.  Further, with remote phosphor technology, the phosphor disc or strip can be moulded to virtually any shape, providing flexibility of design for lighting designers.

Note that the Intematix product range includes die shapes that can be round, square, candle, panel, elliptical, domed and in strip format.  The ChromaLit Dome series, for example, is ideal for retrofit lamps for pendants, wall sconces and ceiling fixtures.  Typical parts include the CL-930-DOM40-PC with a lumen output of 600 to 1000lm, typical conversion efficacy of 150lm/radiant W, CCT of 3000 and CRI of 90.  Dimensions are 42mm base diameter x 26.1mm height.

For spotlights, modules, downlights and area lighting, the ChromaLit XT/XTS series comprises a broad range of models covering lumen outputs from 500 to 10,000lm and colour temperatures from 2700 to 5000CCT.  At the top of the range is the CL-750-R100-XT, for example, delivering 10,000lm with an efficacy of 230lm/radiant W, CCT of 5000 and CRI of 70.  The CL-927-R23-PC, by contrast, for smaller spotlight applications, has a typical lumen output of 500lm, CCT of 2700 and CRI of 80.  Dimensions are correspondingly smaller, at 22.5 diameter, compared to 100mm for its larger brother.

 

Conclusion

Remote phosphor LED technology has the potential to deliver better efficacy, higher lumen output and improved light quality, together with the expected benefits of energy efficiency and long life.  When considering the module or system cost, rather than just the LED cost, remote phosphor can be a cost effective solution too, for both retrofit and new build lighting applications.