Metal Halide Arc Lamps

Outperform Halogen Lamps in Specialized Medical Instruments

By Jim Kiesa, Welch Allyn Lighting Products Division

Minimally invasive surgical procedures require very special light sources and until recently, manufacturers had to make do with less than perfect solutions. The nature of these procedures dictates the need for a high-intensity, low-wattage lamp with a bright, white output that can be focused into the small aperture, fiber optic bundles used in endoillumination, without the problems of heat and exposure to harmful UV rays. Although past applications have used high-wattage halogen lamps for illuminating fiber optic bundles, there is a move toward low-wattage metal halide arc lamps as a more effective means of illuminating the surgical field.

The use of miniature, metal halide arc lamps in medical applications is rapidly increasing for a variety of reasons. Short arc gap, metal halide arc lamps, a newer version of the traditional arc lamp, can operate at much lower wattages (18, 21, 24 and 50 watts) and provide significantly whiter (5500K, as opposed to 3200K for halogen) and brighter light for improved diagnosis.

Metal halide arc light lamps are available from some manufacturers with a 1.2mm arc gap, giving it the ability to precisely pinpoint its light output beyond any comparable incandescent or arc lamp. Combined with elliptical reflectors, metal halide arc lamps have a bright, white light that can be focused with extreme precision into a fiber optic cable or bundle. This results in higher efficiency of light energy coupled into the bundle -- leading to brighter, crisper images.

Escalon Medical Corporation, New Berlin, WI, is one example of a company that recognizes the benefits of using metal halide arc lamps versus halogen lamps in their light source. Escalon's VitLite 1, a light source for intraocular visualization, photography and videography, combines a low-energy metal halide arc lamp with Trek (a division of Escalon) technology to make illumination appear white, similar to that of daylight.

The VitLite 1 is used in specialized applications, such as intraocular surgery, that have stringent requirements. Here, a white light is paramount for accurate diagnosis of the retina. Furthermore, a small spot size is needed to illuminate the 750-micron single-fiber optic bundle used in this procedure.

In the past, manufacturers of intraocular light sources and related instruments have used 150 watt halogen lamps for their light source. Halogen lamps produce a yellowish light, much of which is wasted before being focused into the fiber bundle.

"Since the metal halide arc's light creates a white light, it improves the definition of red tissue images by three times that of a halogen light running at the same power level," said Roger Edens, engineering manager, Escalon Medical Corporation.

By projecting light that appears bluish/white, the metal halide arc light source causes less detrimental UV energy than halogen and even daylight, due to a filter that is used. The metal halide arc's spectral output is 240-700 nanometers compared to halogen's spectral output which is <400-700 nanometers (depending on the IR filter used). However, the halogen light contains more output in the higher wavelength regions, causing it to appear more yellow; therefore, more total light is required for an effective visualization of retinal tissue.

Since the metal halide arc lamp is operating at a lower wattage than halogen lamps in the same application, less heat is generated. As a low power lamp, the metal halide arc lamp runs much cooler than halogen and uses less total light energy to visualize retinal tissue, due to the whiteness of the light.

Surgical microscopes are another example of a product that benefits from using metal halide arc lamps. Using the short-arc gap found in today's metal halide arc lamps, a greater amount of light is focused into the fiber optic bundle. Additionally, the blue/white output provides true sunlight illumination, as opposed to the yellow output of a halogen lamp.

Metal halide arc lamps are being used in several other applications, such as projection displays. Their compact size, small arc gap, high light output, precise focus and low consumption make them ideal for projection display engineers. And by emitting 5500K color temperature light, it ensures that each image projected has excellent white balance, as well as true color rendition and color balance.

Metal halide arc lamps also are used in remote/portable lighting products, such as electronic news gathering camera lights and bike lights. Metal halide arc lamps provide more light in a smaller size and can be battery powered. The lamp is as safe to operate as a halogen lamp, but provides three times the light output, as well as sunlight-caliber illumination.

The potential applications of low power, short arc gap, metal halide arc lamps are increasing their acceptance and they are becoming the light source of the future. The ability of halide lamps to have a lower wattage, provide significantly whiter and brighter light, and to create an output three times that of a halogen lamp is rapidly being realized by light source manufacturers as they create the products of tomorrow.

For more information, contact:

Welch Allyn, Inc., Lighting Products Division, 4619 Jordan Road, Skaneateles Falls, NY 13153-0187. 315-685-4358. Circle 464.

Escalon Medical Corporation, 2440 South 179th Street, New Berlin, WI 53146. 800-433-8197. Circle 454.

Originally published in the March 1999 issue of Medical Equipment Designer.
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