By Adil Sidiqi
Vertical Segment Manager, Future Lighting Solutions, a division of Future Electronics
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From around the middle of the last decade, the display industry deployed a magic recipe that was at first little understood by lighting industry professionals.
TriGain® technology, originally developed by GE Lighting and now a property of Current Lighting, is a patented potassium fluorosilicate (PFS) phosphor formulation which produces a narrowband emission profile in the red part of the spectrum, resulting in higher quality of light and more vibrant rendition of red colors than traditional phosphors used in white LEDs provide (see Figure 1). The technology is sometimes called a red-line emitting phosphor.
The display industry was the first to embrace the benefits of TriGain technology, deploying it in LED backlighting systems to produce superior image quality.
Fig. 1: The emission spectrum of LEDs from Nichia that feature TriGain phosphor. The plot shows a pronounced peak in the red part of the spectrum, and almost no wasted emission outside the visible spectrum. (Image credit: Future Lighting Solutions)
But TriGain phosphor superior color rendition at high efficiency is a valuable combination of benefits for lighting applications too: a reliable rule of thumb is that TriGain technology provides 90 CRI color quality at 80 CRI typical efficacy, as well as making reds appear more vivid.
It has taken several years for TriGain technology to find its way into the broader LED industry because of highly restricted licensing. Now, however, Current is providing access to the technology widely, both by providing TriGain phosphor directly to LED manufacturers, and by licensing the recipe to enable LED manufacturers to produce TriGain phosphor.
Manufacturers that have taken advantage of the new licensing arrangements to make or start development of TriGain-based LEDs include Bridgelux, Lumileds, Luminus, Nichia, and Seoul Semiconductor.
So should lighting equipment manufacturers be rushing to replace the conventional white LEDs in product designs with new TriGain-based 90+ CRI LEDs for an instant uplift in quality of light without sacrificing efficacy? As we shall see, the commercial opportunity is compelling, but care must be taken to ensure that implementation does not risk disappointing customer expectations for color point accuracy and color consistency.
OEMs can improve value of luminaire portfolio
The basic numbers leave no room for doubt: TriGain-based light sources with a minimum 90 CRI provide better quality of light than 80 CRI conventional LEDs, while matching them for efficacy, as shown in Figure 2.
How might this affect a lighting manufacturer’s product strategy?
Fig. 2: Comparison of efficacy, with lm/W on left axis, of TriGain-based F90 emitters with conventional emitters at 90 CRI and 80 CRI. (Image credit: Future Lighting Solutions)
Most obviously, in any tender that requires the high efficacy normally provided by 80 CRI LEDs, OEMs can substitute TriGain-based 90 CRI LEDs, giving customers an increase in quality of light without entailing increased power consumption. This approach can be particularly attractive in settings that place a high value on well-being and the health and comfort of users, such as educational establishments, hospitals, and offices. The provision of superior quality of light potentially justifies a price premium, helping OEMs to raise the average selling prices.
The use of TriGain-based LEDs can also enable OEMs to streamline inventory and production operations. Today, lighting manufacturers typically stock separate inventories of 80 CRI LEDs, for applications requiring higher efficacy, and 90 CRI LEDs for applications requiring higher quality of light. A single TriGain-based LED can replace both sets of 80 and 90 CRI LEDs, enabling the OEM potentially to satisfy both sets of application requirements with a single stock-keeping unit (SKU).
The supply chain for TriGain technology is already well populated, and TriGain-based LEDs at all the important ANSI CCT values are widely available, making them suitable for use in both single-color and tunable white lighting equipment.
Implement red-line phosphor LEDs with care
So the commercial case for using TriGain-based LEDs in luminaire designs that would previously have used conventional 80 CRI or 90 CRI LEDs is clear.
Nevertheless, OEMs need to take care over the implementation of LEDs with PFS red-line phosphor to ensure that they meet the customer specification for color point and color consistency. The reason for this is the marked variance in color in PFS phosphor as temperature and drive current vary. This is illustrated in Figure 3.
Changes in humidity also affect the color point of PFS phosphor-based LEDs. Since LEDs also tend to fail faster in more humid the operating conditions, control of humidity in the application is always an important design consideration when using TriGain LEDs.
Fig. 3: The color of PFS phosphor-based LEDs shifts away from the blackbody curve as the LED junction temperature changes. (Image credit: Bridgelux)
The high quality of light provided by TriGain-based LEDs is a feature required in interior rather than exterior lighting. In interior lighting, where ambient temperatures are controlled by heating and ventilation systems, it will normally be possible to specify the LED junction temperature within a narrow range for any given application.
But operating temperature will vary widely between applications: a downlight densely populated with LEDs and intended for mounting in a ceiling void will typically run hotter than a pendant linear light, for instance. OEMs therefore need to be aware that LEDs from a single manufacturer’s single bin might emit light at different points in CIE 1931 color space in a downlight compared with a linear light. This color shift behavior of the PFS phosphor is particularly troublesome for OEMs responding to tenders that require tight color consistency between different fixtures in a single lighting scheme.
So how should the OEM implementation of TriGain-based LEDs account for the color shift behavior?
There are in fact information resources and tools that OEMs can use to accurately model the color behavior of their chosen LED in the application conditions. The first step is to specify precisely the operating conditions application by application. This calls for accurate testing or modeling of each intended application’s drive current and junction temperature. If humidity is expected to vary widely in the application, this parameter should also be built into the design models.
The second step is to survey the product offerings to find the right selection or combination of LEDs to meet the OEM requirement. The manufacturers of TriGain-based LEDs supply the products, and the test data which characterize them, in color bins for various junction temperature values, typically 25°C, 65°C and 85°C.
OEMs can manually research and sort the datasheet read-outs for every PFS LED in each manufacturer datasheet. This is a laborious task, but helpfully, Future Lighting Solutions has created a set of proprietary tools which automate the product search function. The tool enables the OEM to specify a color point, junction-temperature range and drive current. The tool then generates a list of the PFS LEDs that most closely match the specification. The tool can tell which LEDs’ output stays within a 3 Macadam ellipse over the specified junction-temperature range.
The third step is to review the proposed product selections for each fixture type and application that the OEM intends to build. At this stage, the OEM might need to trade off color consistency, which might call for multiple LED SKUs for different applications, against inventory cost and production efficiency, which would normally call for one rather than many SKUs.
Since the Future Lighting Solutions tool looks across the market at LEDsfrom Bridgelux, Lumileds, Luminus, Nichia, Seoul Semiconductor and more, OEMs can expect to find LED products that meet any color point requirement in any application. The portfolio of TriGain-based LEDs on the market today includes mid- and low-power LEDs in industry-standard footprints, CoB LEDs, and LED modules.
So the LEDs are available to enable lighting manufacturers to take advantage of the high quality of light of TriGain-based LEDs without sacrificing efficacy. With prudent use of resources such as the tools provided by Future Lighting Solutions, OEMs can at the same time maintain the required color consistency between fixtures and applications.
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