ver the last several years, LEDs have been the “it” technology in the lighting world. LIGHTFAIR is seemingly dominated by LED companies; the federal government under the auspices of the DOE has an extensive program in place to support LED applications; and even the mainstream media has devoted major coverage to LEDs as a potential replacement technology for incandescent.
LEDs, however, are not the only emerging light source. Another technology—light emitting plasma (LEP)—is gaining traction as a viable system for high-illuminance applications, specifically as a replacement for 400-W-plus metal halide and high-pressure sodium systems, by offering the same delivered lumens using about 40 percent less energy. It’s not only the lighting industry that’s taking notice. Forbes magazine included an article last year on LEP, while the Wall Street Journal recognized Luxim, a LEP developer based in Silicon Valley, as one of its “Top 10 Clean Tech Companies” in the U.S.
Aside from the energy savings, LEP has a number of other important attributes, ranging from dimmability, to expected lifetime, to color. LEP can be dimmed to 20 percent output allowing further energy savings. In addition, LEP has a projected 50,000-hour life at 70 percent lumen maintenance. This compares favorably to 18,000 hours for metal halide. LEP lamps are available today at a color temperature of 5,200K with warmer color temperatures expected shortly (Luxim, for example, demonstrated 4,800K systems at LIGHTFAIR). Finally, LEP delivers a continuous spectrum ensuring high color quality; systems are available today at 75, 80 and 95 CRI.
HOW LEP WORKS
The term plasma is used in the industry to describe sources with a continuous spectrum. The LEP system (Figure 1) comprises three components: emitter, driver and power supply. The emitter consists of a quartz capsule embedded in a ceramic puck. Inside the capsule is a blend of gasses and halides designed to emit a certain spectrum. A highly reflective material placed between the capsule and the puck, causes the light to emit in a forward pattern.
The driver, which is essentially a solid-state RF amplifier, creates electrical energy that is fed into the puck by a coaxial cable. The ceramic puck then focuses that energy onto the capsule, energizing the mixture inside and causing the LEP lamp to emit brilliant white light.
A quartz capsule is at the heart of the LEP system. This simple construction has no electrodes, no glass-to-metal seals and no alien materials inside the capsule. This simplicity and purity of construction gives the LEP emitter its efficiency and ruggedness. This simple design has the following benefits:
Elimination of energy wasted as heat in the electrodes themselves
Elimination of glass-to-metal seals which are typically the weakest link of an HID lamp
Elimination of quartz wall darkening, a source of lumen depreciation and failure, as electrode material evaporates and deposits on the transparent surface of lamp.
Elimination of molybdenum foils allowing faster warm-up and restrike times
As a result, over the years, there has been a great deal of interest in plasma light sources. The wireless revolution has provided cost effective, efficient and reliable solid-state amplifiers that make LEP light sources possible today.
DIRECTIONALITY AND POINT SOURCE
On the applications front, a single LEP source, only a few millimeters in size, can produce the light needed for a complete luminaire. Because the source is compact and directional, light can be harvested from it more effectively than from induction, LED or HID. The directional source prevents light from being trapped and wasted in the luminaire. The point source optics effectively and uniformly map the source to an illuminated area preventing unwanted light spill which can cause glare and light pollution.
A number of fixture companies have designed reflectors that capitalize on the compactness of the LEP source. These reflectors direct light to the work plane and offer much better uniformity and a more even distribution than traditional HID. These improvements allow streetlight pole spacing, for example, to be expanded from 100 ft (typical of HPS) to 150 ft while maintaining IES Recommended Practices. On new construction, this reduction of fixtures and poles allows LEP to have lower capital costs and lower operating costs compared to legacy technologies (Figure 2).
“Real-world” lighting efficiency is another important consideration with any emerging light source. As these new technologies become available, lighting designers and end users learn how to assess the technology’s benefits. A hard-learned lesson from LED implementations is that it is not the light on the datasheet that matters but the light on the ground. For many reasons light on the ground can be less than the datasheet might lead one to believe (operating temperature, fixture efficiency, measurement discrepancy, binning variation). There are also reasons why light on the ground may be higher than quoted on the datasheet but these cases seldom intrude on real-life experience.
Today LEP fixtures deliver system efficiencies above 70 lumens per watt with excellent backlight, uplight and glare control. Furthermore, LEP fixtures in development can deliver circa 90 lumens per watt. These figures compare favorably with best-in-class LED fixtures on the market and on the drawing board today.
Significantly, for outdoor applications, LEP has excellent mesopic and scotopic lumens. In fact, LEP had the highest scotopic/photopic ratio of all sources tested in preparation for the new IES Lighting Handbook (10th Edition). For the first time, this new edition includes guidelines to account for the improved nighttime visibility that comes with full spectrum sources. This allows further energy savings using LEP in outdoor applications.
COMPLEMENT NOT COMPETITOR
LED and LEP is not an either/or proposition. In high-illuminance applications in particular, LEP can serve as high-output complement to LED. Both technologies offer efficiency, life and digital control. The basic building block of LED systems is the LED chip with an output of circa 100 lumens. The basic building block of LEP is the quartz emitter with an output of 20,000 lumens. Given this starting point, one can envisage a family of fixtures that uses LED for low and medium illuminance and expands to LEP for high illuminance.
Inside this broad positioning, LEP has unique advantages where small form factor is important. These include high-mast systems (compactness reduces wind load), portable systems (compactness simplifies transportation and setup) and beam systems (compactness simplifies optical design).
Finally, there is the question of cost. Because its architecture is based on a single emitter and a single driver, LEP has lower cost than LED in high-illuminance applications. While costs vary by vendor, quantity and terms broadly speaking we see LED and LEP systems having similar costs at about a 5,000-lumen output level. As output increases, however, LED costs scale proportionately while LEP costs remain relatively flat. This provides LEP with a significant cost advantage at the 20,000-lumen level. Furthermore, LEP costs are falling rapidly and are projected to match the costs of mainstream HID luminaires by FY 2013.
LED does have an advantage over LEP in its ability to scale down. If a job calls for a 3,000 lumens, the LEP system would still require an emitter, driver and power supply and would therefore have approximately the same cost as a 20,000-lumen LEP system. In this particular case, LED would be a more cost-effective solution because it scales down well. On the other hand, if the job called for a 45,000 lumens, LEP still has the same three components and similar cost. By contrast, for LED to scale up, the manufacturer would have to add more arrays of LEDs, bigger heat sinks and a larger fixture. In short, LED scales down well. LEP scales up well.