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Illumination & Displays

Advances in microplasma array lamps

Extremely thin, flat plasma lamps can illuminate broad areas and have potential applications in medicine and other fields.
19 December 2007, SPIE Newsroom. DOI: 10.1117/2.1200712.0850

Plasma lamps that are less than 1mm thick yet able to radiate areas up to 900 cm2 have been demonstrated and are undergoing further development and optimization. Aside from general purpose illumination, potential applications for this relatively inexpensive, mercury-free lamp technology include treatments for medical disorders (such as psoriasis and actinic keratosis), photocuring of polymer films, and sterilization.

These lamps comprise large arrays of microcavity plasma devices in dielectric barrier structures. Figure 1 shows a cross-section of the lamp design. Fabricated in commercial grade aluminum (Al) foil with overall (finished) thicknesses under 1mm, the lower of the two foils has a thickness of 127μm or 250μm. The upper electrode consists of a 35-50μm thick Al screen with microcavities that contain diamond−shaped cross sections. The photograph in Figure 1 (lower right) shows the screen, prior to processing, with a magnified view of the microcavities.

Figure 1. Generalized cross-sectional diagram (not to scale) of a flat lamp structure based on aluminum foil encapsulated in nanoporous Al2O3 and a thin glass coating (details shown at lower left). The lower right portion of the figure presents photographs at two magnifications of an electrode screen with diamond cross-sectional microcavities. The smallest gradation of the scale in this photograph is 1mm.

Anodizing the electrodes under carefully controlled conditions encapsulates the foils and the interior of all of the microcavities with a film (typically 10−30μm thick) of nanoporous aluminum oxide (Al2O3). The nanostructured Al2O3 offers a dielectric breakdown strength approximately quadruple that of bulk Al2O3, thereby allowing thinner metal oxide films to serve as the dielectric. This wet chemical process yields Al2O3 films of uniform thickness over large areas and produces nanopores that are virtually free of defects. Preparation of the foils culminates in application of a thin glass coating produced by firing glass paste at 500°C for 30 minutes.

Fabricating the lamp structure illustrated in Figure 1 involved bonding the Al2O3-encapsulated screen with the foil and sealing the assembly between thin sheets (<600 μm) of glass, quartz (if ultraviolet transmission is desired), or a polymeric packaging material. Having an overall thickness less than 1mm, these lamps were subsequently evacuated, backfilled with the desired gas or gases, and driven by a sinusoidal voltage with a nominal frequency of 20kHz.

In general, microcavity array lamps possess three distinctive assets. First, they are lightweight and require no ballast. In addition, microplasmas prefer operation at pressures up to one atmosphere and beyond, thereby minimizing or eliminating the pressure differential across the lamp packaging. Finally, design and fabrication are inexpensive.

A photograph of an unsealed lamp with a radiating area of 100cm2 and operating in 500 Torr of neon (Ne) is shown in Figure 2. The uniformity of the optical emission across the entire array is evident. Lamps with radiative areas as large as 900cm2 have been fabricated and tested to date. The luminous efficacy (radiating efficiency) of these lamps has been measured after first spin-coating a 15μm thick film of a commercial phosphor onto the inside surface of the output (glass) window and illuminating the phosphor with ultraviolet emission from the microplasma array. Figure 3 is a photograph of a 15cm2 sealed lamp with an internal green phosphor excited by an ultraviolet (UV)-emitting Ne/xenon (Xe) gas mixture.

Figure 2. Photograph of a 10×10 cm2 array operating in a Ne pressure of 500 Torr.

Figure 3. Photograph of a 15cm2 lamp with an internal green phosphor and operating in a Ne/30% Xe gas mixture at 600 Torr.

Although no aspect of the lamp structure has yet been optimized, the luminous efficacy of ∼15 lumens/watt measured with a green phosphor (Mn:Zn2SiO4) and a Ne/30% Xe gas mixture is comparable to or exceeds that for incandescent lighting. Further development of this technology is expected to yield efficacies in excess of 20-30 lumens/watt from radiating areas of thousands of square centimeters.

Aside from general purpose illumination, biomedical and other applications may be anticipated for this mercury-free lamp technology. Examples include treatments for psoriasis and actinic keratosis, photocuring of polymer films, and sterilization. In recent publications we offer further details concerning Al/Al2O3 lamp construction and performance, and the general characteristics of microcavity plasma devices.1,2

This work was conducted in the Laboratory for Optical Physics and Engineering at the University of Illinois Urbana-Champaign by S.-J. Park, J. D. Readle, A. J. Price, J. K. Yoon, J. Putney, and J. G. Eden, and was supported by the U.S. Air Force Office of Scientific Research.

J. Gary Eden  
Laboratory for Optical Physics and Engineering
University of Illinois Urbana-Champaign
Urbana, IL