CoB LED Modules Deliver Smaller Source Sizes and Increased Flux Density in Retail Applications

With the advent of affordable, high-performance LEDs, retail lighting has become a demanding and complex application. Typical schemes might include ambient lighting to provide a general illumination, colored lighting for decorative or scenic effects and directional or spot lamps to provide focus for specific product areas.

This article will highlight today’s emerging highly integrated chip-on-board (CoB) LED designs which are claimed to achieve flux densities high enough to replace traditional halogen and CDM technologies widely used for spotlights. Reference will be made to Philips Lumileds Luxeon CoB modules, including the CrispWhite versions targeted at the retail sector, and the Cree CXA1310 XLamp arrays.

Further discussions will center on the growing importance of delivering a lighting scheme optimized for a specific application, which can include factors such as faithful and/or enhanced color rendering.

Competitive edge

Of course, the effective lighting of retail spaces and goods on display has long been known to be a key factor in attracting customers. However, with the falling cost of LED luminaires, combined with their extraordinary level of flexibility and control, lighting has become somewhat of a competitive edge among retailers. Cost has arguably been the most significant factor in the rapid growth of LEDs in the retail sector; and that is not only the falling purchase price, but also the energy savings that can be made, particularly for superstores. There is no doubt, however, that the quality of light, and the sheer variety of special effects that can be created with LEDs, is now driving the market forward. Whether it is food, furniture or fashion, megastore or the corner shop, LED technology is flooding into new premises, refits and retrofits.


Figure 1: A cooler, more clinical white light better suits a retail pharmacy area (left), using Cree’s TrueWhite LED technology. Conversely, fashion clothing may require a combination of warm, ambient general lighting plus high CRI spots to best display vivid colors and bright whites simultaneously.

The choice for retail space designers is vast: fixtures and fittings, form-factors including spotlights, downlights, tubes, wall washers, and shelf lights; multi-color modules; cool to warm white color temperatures; plus compelling characteristics such as high CRI, dimmability and tunability. Programmable control and modular construction allow for lighting schemes to change as often as the products on display.

Spotlights are ideal for window displays, creating dramatic effects to draw the eye and the customer into the store. Once inside, accent lighting can be used to create a warm, comfortable feel, perhaps for leather furniture, for example, or in fitting rooms. A cooler, brighter, more dynamic look with directional and/or downlights would be a better choice for sports equipment or DIY stores. Supermarkets already know about selecting high CRI, high R9 value devices for bringing out the natural vibrant colors in fruit and vegetables.

LEDs are now available to meet virtually all these needs. Retrofit lamps continue to be a popular choice to avoid the cost of a total refit, while gaining some of the advantages of energy savings and improved appearance. However, outside the restrictions of traditional luminaire form-factors, LEDs come into their own, particularly for imaginative architectural effects using LED strings to outline shelving, stairs, hand rails, and for concealed lighting for wall washing.

CoBs in the spotlight

Directional lamps or luminaires are favored for highlighting merchandise, and halogen has been a popular choice in the past. However, until recently, LED replacements have struggled to provide satisfactory equivalents for some units, such as the classic MR16. See earlier article: LED breakthroughs for retrofit lamps.¹ For the plug-in replacement market, the challenge has been achieving the necessary flux density, high center beam candle-power (CBCP) and efficacy to fit into the package at an attractive cost.

An alternative approach, now showing promise, is high-density CoB LED arrays that feature smaller source sizes, enabling narrower beam angles, and an increased flux density. Today’s CoB designs are lower cost than earlier solutions, but, importantly, the smaller source size also means more compact secondary optics, resulting in a smaller package size; a crucial factor for the retrofit market.

Philips Lumileds has recently expanded its CoB LED array family with the Luxeon 1202 series, featuring a smaller light-emitting surface (LES), down to 6.5 mm from 9 mm previously. The company has reported that a 30% reduction in the size of the LES can result in a two-fold increase in CBCP. The critical specifications here are a superior CBCP, due to the smaller LES, delivering 65,000 cd at a 10o beam angle. A choice of CCT from 2200 to 5000 K, gives a typical luminous flux range from 525 to 800 lm, and efficacies from 75 to 109 lm/W – impressive for the size of the array.

It is the high CBCP that provides a more uniform and crisp beam, ensuring that it can be used for PAR30/38, halogen and CDM replacement, particularly for directional lighting applications. The arrays are available in 3-step (80 to 90CRI) MacAdam ellipse, to ensure color uniformity and good optical performance.

To provide the contrast between the 6.5 mm and 9 mm LES versions, consider the LHC1-4090-1202. This is the larger, 9 mm LES array in a 19 x 16 x 1.5 mm package. The neutral/cool white 4000 K (3-step MacAdam Ellipse) array features a CRI of 90, typical luminous flux of 725 lm and efficacy of 102 lm/W. The 400 K version has a minimum CRI of 80, a typical luminous flux of 800 lm and typical efficacy of 113 lm/W.

Cree, meanwhile, is on the same track; having introduced a range of high density CoB arrays with 6, 9 and 12 mm LES, destined for those retail spotlight applications that require a more focused beam, and especially for the halogen replacement market. The company claims that its 6 mm LES arrays reduce the size of halogen replacements by half and deliver twice the CBCP of CMH at 30 to 50% less power.

The CXA1310, 6 mm LES array can be used to deliver 31,000 cd with a 9o beam at 15.4 W. For comparison, a mid to high range 44 W PAR20 CMH lamp delivers 13,220 cd CBCP and a 10o beam. The lumen output range across the series is quoted at 1400 to 2700 lm, with luminous flux at 85oC, in the 780 to 1290 lm range, depending on forward voltage and current. There is a choice of 18 and 36 V forward voltage and a current range from 500 to 1050 and 350 to 525 mA respectively. Devices are available in color temperatures from 2700 to 6500 K, with CRI options in the 70 to 80 band for the cooler whites and 80 to 95 for the warmer CCT versions. Efficacy is specified in the 87 to 100 lm/W range.

To give a couple of examples, the CXA1310-0000-000F00K240F, is a 4000 K neutral white (4-step MacAdam Ellipse) array, while the CXA1310-0000-000F0HK250F is a 5000 K cool white (4-step MacAdam Ellipse) array. Both deliver 1245 lm (typical) luminous flux at 700 mA/18 V. Efficacy is 100 lm/W, CRI is 80, and package size is 13.35 x 13.35 x 1.55 mm.

Cree has produced a Reference Design/Application Note,² demonstrating the use of a single CXA1304 or CXA1310 LED array to design equivalent replacements for 35 W and 50 W GU10 lamps. The guide outlines the design goals in terms of light output, CBCP, beam angle, efficacy, power, CCT and CRI, plus optical, thermal and electrical efficiency, and finally mechanical considerations such as standard form-factor and secondary optics. The analysis concludes that light outputs are exceeded using less than 22% of the energy, and lifetime is typically sixteen times longer.

True colors

CoB LEDs not only deliver energy savings, but importantly, high-quality color rendering. In the retail sector, both whites and colors need to be presented in the best possible way. As the choice of suitable retrofit LED luminaires increases, so does the emphasis on light quality. In the true sense of the word, light quality does not necessarily mean the highest lumen output, or CRI approaching 100, but the level appropriate for the application. Just as important are color consistency, efficacy, long lifetime and reliability.

Philips Lumileds developed its CrispWhite version of the CoB LED array specifically for retail applications. The LED closely mimics the spectral power distribution of ceramic metal halide (CMH) lamps in order to deliver both vivid color rendering and bright whites simultaneously. The issue with whites is white paper and textile products that include optical brightening agents (OBAs). The LED construction technique uses additional deep blue (410 nm) LED chips that excite the brightening elements.

A high CRI is important for color rendering, and the CrispWhite versions achieve CRIs in excess of 90. There is a slight compromise on efficacy. The 9 mm LES, LHC1-3090-1202CRSP, for example, is specified at 90 lm/W typical, 92 CRI, compared to 113 lm/W for the 80 CRI 3000 K equivalent, or 93 lm/W for the 90 CRI version. The 6.5 mm version has an efficacy of 85 lm/W (3000 K, 92 CRI) compared to 88 lm/W for the 3000 K 90 CRI equivalent.

Interestingly, the R9 value (measure for rendering saturated reds) for the Philips Lumileds CrispWhite arrays is quoted at 55, which is relatively low. However, the company maintains that this is high enough, and is higher than the 39 figure typically quoted for the CMH lamps, which have traditionally met customer expectations. A very high R9 value, in the 90 range, could give a red/yellow tint to whites and light colors, it is claimed.

Conclusion

For the halogen and CMH replacement market in the retail sector, the new generation of high density, small LES CoB arrays appear to be delivering the expected light output and light quality in the required form-factor, with the added advantages of energy savings, long lifetimes and lower total cost of ownership.

References:

1. DigiKey article: LED Breakthroughs for Retrofit Lamps
2. Cree GU10 Reference Design/Application Note