Why LED shortcuts won’t work

Why LED shortcuts won’t work

Trying to make legacy sockets and fixtures accommodate SSL is a losing battle

BY DAN JACOBS
TT electronics OPTEK Technology
Carrollton, TX
http://www.optekinc.com

Most implementations of LEDs for general lighting currently involve tailoring them to fit existing socket form factors. As examples, there are LED “bulbs” to directly replace incandescent light bulbs in Edison screw sockets and LED “tubes” to directly replace fluorescent lamps in linear tube fixtures.

In each case, multiple subsystems are forced to coexist in nearly the same space — primarily optics, thermal management, and electronics —sacrificing the efficiency of each. Retaining old form factors provides upfront savings and allows direct retrofit with minimal effort, but doing so curtails LED performance and stigmatizes LED products as dim or unaesthetic.

To fully realize the potential of LEDs as a general lighting solution, new form factors must be developed, accepted, and adopted. LED-centric form factors will enable designs that capture the full effect of LEDs by segregating or centralizing lighting subsystems and by enabling versatile luminaire concepts. As flaws in retrofit solutions are recognized, it will be easier to accept the initial costs necessary to gain all the benefits from bringing LED form factors to bear.

What’s wrong with old?

Legacy socket form factors provide an immediate path to market, but sacrifice optical performance. The various subsystems making up an LED lamp are ill served by such form factors because fixtures as they are currently do not fit LEDs well.

Fixtures employing Edison sockets or fluorescent tubes are designed for an isotropic source, so they either take advantage of that isotropy and permit emission in all directions, or they use wide reflectors to gather and direct the light. LEDs can’t adequately generate isotropic light since LED technology inherently directs light emission, and so LEDs perform poorly in open space.

In nearly all cases, lighting is designed to enhance aesthetics, and the beam pattern from LEDs in open space does not meet aesthetic standards. In targeted fixtures such as recessed cans and troffers, the reflector and diffuser form an enclosure that captures heat from LEDs installed within the fixture and limits its dissipation (see Fig. 1 ). Airflow is poor and the only nonconvective path for heat is the socket, and the Edison screw socket is a poor thermal conductor. Whether the fixture is open or uses reflectors, LEDs do not fit or are hindered in their performance.

Fig. 1. In targeted fixtures such as recessed cans (top) and troffers (bottom), the reflector and diffuser form an enclosure that captures heat and limits its dissipation from LEDs installed within the fixture.

Additionally, electrical efficiency is sacrificed because of the counterproductive overlapping of rectifying circuit and light sources. Since LEDs require low dc-voltage inputs, extra circuitry is required within the LED bulb or tube to step down and rectify supply voltage. The amount of power transformed is relatively small and the change in voltage is typically large, neither of which makes ultra high-efficiency conversion feasible.

Some don’t like it hot

Since the circuit is enclosed within the bulb or tube, the heat generated from these components must be dealt with by the same thermal management mechanics as are used for the LEDs. The reliability of the LED components and the rectifying circuit are negatively impacted by each other, an effect that is often exacerbated because the fixture forms an enclosure around them.

In LEDs, extra heat works against performance overall by causing elevated junction temperatures, which lower the efficacy of the emitters. Either the brightness is insufficient as a result, or excess power must be used to compensate. With all the trapped heat, typical general lighting enclosures work against using LED systems for retrofit because of poor thermal management.

It’s true that Edison bulbs and T8 or T10 fluorescent tubes each occupy significantly more physical space than comparable LEDs require, making it possible to use that space for heat sinks. And having a good thermal path as part of the bulb or tube helps. But many applications for the light require the light source be installed within an enclosure. The ambient temperature around the bulb will be elevated far above what it would experience in open air.

Further, most enclosures are made of plastic or sheet metal and are painted or coated for high reflectivity. This material combination provides very little thermal transfer and little venting. The net thermal resistance, once you account for the enclosure, will push junction temperature well above rating. Ultralong life with high brightness becomes unattainable when the LEDs degrade to 50% output in 10,000 hours or less of accelerated, high-temperature stress.

Though there are reasons to stay with the same socket form factors that other technologies employ, the penalties paid in performance, and ultimately cost, add up. Resistance to change stands above all else, but the biggest reason integrators attempt to retrofit without replacing fixtures is that this shortcut allows LEDs to penetrate the existing market most seamlessly.

Unfortunately the term “retrofit” is not generally accurate. A retrofit typically occurs for two reasons: either the legacy version is no longer available, or the new version updates and improves the old one to justify replacement. The first does not apply—except for some incandescent bulbs as laws banning them go into effect this decade—and there are non-LED alternatives, particularly compact fluorescent lamps.

As for the second reason, given all the limitations that current fixtures with legacy sockets and form factors place on LED light sources, the justification for retrofitting, versus redesign/replacement, is not there. Electrical efficiency may be improved somewhat, but the increase in initial cost and general reduction in aesthetic performance override the electrical savings (see Fig. 2 ).

Fig. 2. Aesthetically speaking, lighting is paramount to a pleasant environment. LED retrofit solutions make it difficult for architects to achieve these effects.

The well-lit road ahead

The future in LED lighting is creation and adoption of new form factors and lighting systems that overcome the poor efficiency of mimicry. Lamp subsystems first need to be separated and centralized.

First and foremost, the entire fixture must be part of the thermal solution. A direct, efficient thermal path must replace the socket. There is nowhere else for a standard interface for thermal transfer—the space around the lamp must be open for versatile design.

Second, electrical management should be separated from the LEDs and centralized. There is precedent for lamps impacting electrical standards upstream. Lighting originally drove the parameters of centralized electrical power. Early carbon filament lamps operated at 100 V, so 110 V was selected for regional grids. (In Germany, 230 V was chosen to accommodate two bulbs.)

This time the electrical grid is not going to bend to lighting, but when transforming and rectifying line voltage, there are efficiency, performance, and cost gains in centralizing the process at the room or building level; LED fixtures need not handle 110 Vac.

Third, optics suited for LEDs should replace the reflectors and diffusers currently used for incandescent and fluorescent sources. Plastic lenses are possible for LEDs because of the lack of intense, destructive heat, and diffusers need to eliminate much smaller hot spots than before.

Fourth, serviceability is important. A belief that LEDs will last forever is a logical and practical fallacy. Lamps fail for reasons beyond their manufacturer’s control, including vandalism and electrical surge. If the entire fixture must be replaced upon failure, it will reduce acceptance of the solution.

Finally, industry-wide consensus on form factors will establish the foundation for wider acceptance and use.

The LED industry has responded with many potential solutions. In most cases, the components are permanently integrated to the fixture. This approach makes the solution impractical, since each product is non-standard. When serviceability is necessary, permanent LEDs force the end user to remove and replace the entire fixture, which is difficult and time consuming. In the long run, the expected savings from enhanced efficiency may be lost when the entire fixture is compromised by a lamp failure, or if the user decides to change the lighting scheme.

In legacy sockets, poor economics and aesthetics trump environmental benefits, generating negative perception of solid-state lighting with LEDs. Upon installing an LED bulb in a lamp or recessed can, the typical reaction is often, “Wow, that’s dim. Maybe they warm up like CFLs.” Or “Boy, LEDs sure are blue. I paid $30 for this?!” Before long, the public will conclude that LED products are dimmer than their predecessors, cost way too much, and are meant for consumers who don’t care what their lighting looks like, but want to be able to say something good about themselves vis-à-vis the environment. The shortcut to market will turn the market off and away. ■