Scientific Method / Science & Exploration

New OLED lighting panel hopes to outshine fluorescent bulbs

At the very least, the future looks bright for OLED lighting.

A helical arrangement of Philips Lighting's GL350 OLED panels, on display at Light + Building 2012 in April.
Philips Lighting

With the arrival of its OLED lighting panel, the Lumiblade GL350, Philips Lighting is attempting to quietly usher in the era of practical OLED lighting. The diminutive squares, 3.3mm thick with edges not even five inches (they're precisely 124.5mm) in length, put out 120 lumens each. As organic light-emitting diodes go, that's really rather punchy. It's this higher output that has led the Dutch electronics giant to declare the GL350 "the first OLED that is suitable for general lighting purposes" in its product catalog.

But can output alone justify the claim? Ars delved into the specs of the OLED panel, which Philips unveiled to dramatic effect at the Light + Building trade show in Frankfurt in April. We spoke to Iain Macrae, president of the Society of Light and Lighting and head of Global Lighting Application Management at light fitting manufacturer Thorn Lighting, about the broader state of play in OLED illumination.

Empty the contents of a pack of GL350s, and you'll find three OLED panels which, if placed end to end, would stretch 373.5 mm (14.7 inches) in length and deliver a total output of 360 lumens. Compare that to a 300mm (foot-long) fluorescent tube which puts out 400 lumens, and it's clear that Philips's claims of viable performance are serious. OK, our improvised OLED strip is longer, wider, and emits less light, but it's in the same ballpark of usefulness.

But Philips isn't relying on output alone to make the case for the GL350, which boasts other impressive specs. First among these is its color temperature of 3250K. If you're not familiar with the concept of color temperature (or, strictly, correlated color temperature), it's an absolute expression of the color of a lamp that uses a black-body radiator (an ideal emitter) as a yardstick. The hotter a black body radiator, the bluer and cooler it appears to glow. The color of a lamp can simply be expressed by quoting the temperature (in kelvin) of the black body radiator it resembles.

Though ideal color temperature is highly subjective (and varies with application), 3250K is a shrewd choice. Being just on the yellow side of white, the GL350 is nearly identical in hue to a halogen lightbulb, which has been a mainstay of interior lighting, and one people are very used to. But achieving warm colors with an OLED hasn't been the technical hurdle it was with LED technology. Really, the 3250K boils down to a marketing decision. The GL350 actually appears cooler than OLED alternatives. Osram's ORBEOS OLEDs are a positively toasty 2800K, similar to other Philips Lumiblade OLEDs.

Perhaps more impressive is the GL350's Color Rendering Index (CRI), which Philips quotes as better than 90 out of a possible 100. Though the color appearance of a light source does affect its ability to accurately render color, color appearance and CRI should not be conflated. For light sources with a color temperature (under 5000K), the color rendering performance of a light source is directly compared to a black body radiator at that temperature. For light sources with color temperature above 5000K, daylight is the basis for comparison.

Truly ready to shine?

An incandescent light bulb has a CRI of 100 because it is basically as good as a black body radiator at rendering the eight specified colors stipulated by the International Commission on Illumination (CIE). The fact that it's not good at rendering different shades of blue is neither here nor there. It's as good as the yardstick.

"You have to be careful on the CRI scale and new technologies," The Society of Light and Lighting's Iain Macrae told Ars. "Where white light OLED and LED are concerned, there is already a question if the use of RGB color mixing is less suited to the current measure. The CIE are currently looking at more appropriate measures based on research. CRI is of course important for domestic and workplace lighting; generally, a CRI above 80 is considered minimum good practice. New technology doesn't change the way we see. The eye hasn't evolved in line with technology. Color rendering is still important to being able to see well, just the measure may need to change."

Ideally, for a lamp to accurately render color, it will have both a high CRI and the color temperature of daylight—5000 to 6000K. The upshot of the GL350's CRI combined with its color temperature is that its color-rendering performance should be much the same as halogen lighting.

But there is a problem with Philips's claims of the practicality of the GL350. It has a power consumption of 7.2W. That may not sound much, but the light source emits a meager 120 lumens, giving a lumen efficacy of only 16.7lm/W. This suffers in comparison to energy-saving light sources such as fluorescent lighting and LEDs, where certain products achieve efficacies over (and in LED's case, well over) 100lm/W. The latest compact fluorescent light bulbs weigh in at between 50 and 70lm/W.

The GL350's performance is poor even by OLED standards. Philips's own Lumiblade PLUS OLED panel achieves 45lm/W, though its output per unit area is only one quarter that of the GL350's. In fact, an efficacy of 16.7lm/W is firmly within incandescent territory, and had international lighting legislation focused more on performance targets than on banning specific technologies, the GL350 might never have emerged. It certainly falls well short of the minimum efficacy of 45lm/W for general-purpose lighting, set to come into effect in 2020 in the US.

Looking through the lens of efficiency, it's hard to justify the claim that the GL350 is the first OLED suitable for general lighting. "I would have still put [OLED] in its innovation stage. It is not stable enough, efficient enough, or low enough cost to become mainstream yet," Macrae told Ars, when asked for his take on the general state of OLED lighting today. "Our brand specifically would not adopt a technology until it can outperform the existing technology on a number of levels," he added, speaking with his Thorn Lighting hat on. "I think energy efficiency has to be balanced with sustainability, where OLED will have benefits, but also on cost, useful light, and on practicality. Compared to LED and to fluorescent, OLED technology is simply not efficient enough for our customers, but there will be early adopters who see other values, making it worthwhile."

Reason for optimism

Having said that, Macrae shares the industry's broader optimism for the possibilities OLED technology presents, which we reported on last year. "The technology could allow the window glass to be lit, the walls too, the ceiling even, but as complete surfaces or decorative panels," he said. "It should push the market away from the ceiling-mounted regular arrays of luminaries we currently see." Though Macrae points out that for such developments to occur, some innovations in power supply are necessary.

If the future of OLED lighting really is glowing walls and ceilings, then Philips would appear to be barking up the wrong tree by producing small, bright panels that compete for the same market niche as traditional light sources. Though the GL350 is doubtless a noteworthy evolutionary step in the development of OLED lighting, it is larger, lower output surfaces we're waiting for (and besides, LEDs have the small-bright thing covered).

Whatever form OLED lighting eventually takes, more efficiency is required. Philips is optimistic. "Moving forward, we expect efficacy of our Lumiblade OLED panels in our decorative line to reach 35 lm/W by 2018," a Philips spokesman told Ars. "For our Lumiblade OLED performance line we expect to attain an efficacy level of 130 lm/W by 2018." (It's interesting that by this metric, Philips would seem to be positioning the GL350 as decorative rather than general purpose lighting.)

In the near future, Macrae's preference is for polymer light-emitting diodes (PLEDs), a subset of OLED technology in which the electroluminescent layer is a polymer, such as a polyfluorene derivative. "OLED potentially offers colored lighting options, but requires a difficult manufacturing process, so it's less sustainable," he told Ars. "PLED can effectively be printed with less impact on the environment, but is generally a white light source. I prefer the latter as it's more applicable to most applications, and long-term sustainability is vital."

Though it seems likely that OLED will join LED as the predominant light sources of the future, it appears that, in OLED's case at least, the future isn't here just yet.

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