Retailers Pulling for LEDs with Specialty Spectra

Retailers Pulling for LEDs with Specialty Spectra

Following the quest for LEDs that mimic halogen in every positive way, the retail lighting industry is starting to miss a few things. Brick-and-mortar retailers – with their intense awareness of how lighting impacts product appearance and therefore sales – find themselves pining for the cool whiteness of metal halide, the yellow baked goodness of incandescent and maybe even that touch of UV that makes merchandise pop. Manufacturers are working hard to bring back some of those “quirks” of legacy light sources while maintaining energy efficiency. They are developing modules and replacement lamps with specialty spectra to meet specific needs.

No, retailers don’t actually miss the UV in their lighting; they’re just looking to achieve that whiter-than-white appearance. To meet this demand, LED manufacturers are employing two basic methods. The first is to shift the chromaticity of the source slightly down from the blackbody line to provide a less-yellow white than halogen renders; more like ceramic metal halide (CMH), which was traditionally weak in the reds. Though CMH chromaticities appeared on the blackbody line, CRI was poorer. Other disadvantages of CMH included slow start and expensive dimming.

“We really got into this whole thing based on what some customers were asking for: track lights and lighting for retail. High-end, chain retailers around the globe are looking for something to replace metal halide; to provide the same kind of punch as narrow-beam ceramic metal halide PAR lamps,” said Tom Jory, vice president of illumination marketing at Luminus Devices. He described customers that are receptive to testing, which results in a pull for below-blackbody-line chromaticities.

A below-the-blackbody-line (BBL) LED can provide high color rendition across the spectrum. Here contrasted with a halogen source that produces more-yellow whites.

Violet spike

A second path to whiter-than-white takes advantage of fluorescent whitening agents (FWAs) used in plastics, paper, textiles, detergent and cosmetics. These “optical brighteners” absorb UV and violet light (which are available in sunlight, halogen and CMH) and re‑emit in the blue region of the spectrum. This fluorescence can counter a yellowish or dull cast in white merchandise, giving the appearance of a luminous white. (It technically is a luminous white.)

LED modules based on violet chips can exploit this high-energy spike in the spectrum to excite FWAs in materials. Some manufacturers use this strategy in speciality products, but Soraa now uses it throughout its product lines.

This violet spike can lead to LEDs with lower efficacies, due to Stokes loss. When converting high-energy (short wavelength) light to low-energy (long wavelength) light – which occurs in a red, green or blue phosphor – energy is lost. The greater the wavelength differential, the greater the losses. Therefore, using a blue chip creates a more-efficacious conversion in RGB phosphors than a violet or UV diode can. “That is why most of the industry uses blue LEDs,” Aurelien David, senior principal scientist at Soraa explained in an email. Manufacturing and product design can offset these losses.

Some retailers are seeking LEDs with a familiar warm color temperature, plus whites that “pop.” LEDs including a violet spike in the visible range can take advantage of “optical brighteners” found in many products.

“This improvement was not specifically added in response to retailers. However, we did get feedback from some retailers that they had previously been refusing to upgrade to LED technology because it didn’t have the same visual ‘pop’ as halogen lights (obviously not a well‑defined quantity),” wrote David.

Spectral Power Distribution vs Wavelength graph
This white-light LED based on a violet diode, rather than a more-conventional blue diode, emits more light in the higher-energy wavelengths. But UV emissions remain low.

Violet is closer to UV wavelengths, which are notorious for fading and damaging merchandise. But these white LED spectra spike within the visible light range. Depending on duration and light intensity, any light source could potentially cause harm over the long term. “The amount of UV light below 400 nm is only a fraction of a percent – much less than any standard incandescent bulb,” David wrote.

Stretching color gamut

Beyond whites, retailers displaying a wide variety of merchandise demand excellent color rendition, far beyond what initial generations of LED provided. According to Luis Acena, senior manager, LUXEON Stylist Series at Lumileds, many retailers prefer very saturated colors, which is why some are still using halogen.

In specialty-spectra product lines, a simple CRI number does not sufficiently describe the appearance of colors. TM‑30 also measures gamut index (Rg) compared to a reference illuminant, e.g., halogen. An Rg greater than 100 indicates vivid colors. “For fashion you want all the colors to be saturated,” said Acena. “Most of the colors in our fashion line are more saturated, compared to products that exist in the market today.”

Retail fashion presents a demanding color rendering task.
Retail fashion presents a demanding color rendering task. An LED might have a high CRI score but still under-render pinks and reds. Here a specialty fashion LED shows saturated colors across the board (Rg=101).

Like Luminus, Lumileds pushes their fashion products below the blackbody curve. “We want to balance saturation of colors and also the white that is very neutral and clean. We don’t want pinkish or greenish white. That’s why we play with this color point and the spectra at the same time,” Acena said. He cautioned that because TM-30 metrics are based on blackbody reference sources, they are unhelpful in evaluating products that wander too far from the blackbody line.

Grocers have long used specialty light sources for different merchandise: metal halide for produce, red-filtered fluorescent for meat and incandescent to highlight baked goods. Each of these spectra enhances its product category, but with inherent disadvantages. LED has reached efficacies high enough to challenge all these legacy sources. “Now we can do it all with the same platform, with the same footprint. With LED you can provide those types of color temperatures or color points that suit better each of the merchandise,” Acena explained.

In the fish case the solution is to pump up the blue but include enough red such that expensive tuna and wild‑caught salmon remain vivid. Bread and pastries solutions shift toward yellow and orange; a creamy white on the blackbody line is desirable. In produce, greens and yellows and oranges and reds must all render well, so a more continuous spectrum is required.

Spectral Power Distribution vs Wavelength graph
Grocers know that lighting that renders fresh foods in a more appetizing light reduces waste in the form of unsold merchandise. Here a saturated spectrum enhances all the colors of fresh produce (Rg=103) and a “marbled meat light” emphasizes reds while rendering a “clean” white (Rg=115).

LED sources that enhance the colors of meat and reduce discoloration of the product have the most potential to boost profits for grocers. UV and infrared are the main culprits in lighting-induced discoloration (graying). Though it’s not a health hazard, unappetizing meat most often remains unsold, representing steep losses. Lumileds estimates that increased sales and less waste will pay back a lighting upgrade in less than a year, completely aside from any energy savings.

Lois I. Hutchinson

About Lois I. Hutchinson

Lois I. Hutchinson is a freelance writer specializing in lighting and energy issues. She is also the content marketing mastermind behind Inverse Square LLC, a Los Angeles–based consultancy. Contact her via [email protected] with your comments and any article ideas that concern the lighting community here in the Southwest.

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