Medical Device Link.

Originally Published MEM Spring 2002

LIGHTING TECHNOLOGY

Metal halide offers a new alternative to xenon and halogen in medical lighting applications.

Ian K. Edwards

A key illumination application in the medical setting is in the field of endoscopy. Supporting instruments cover myriad configurations for arthroscopy, urology, sinuscopy, laryngoscopy, bronchoscopy, thoracoscopy, laparoscopy, and plastic surgery, along with many related diagnostic and surgical procedures.

The increasing complexities of diagnostic and surgical environments, and the resulting scarcity of space therein, require greater degrees of system integration and miniaturization for all components. This and the never-ending dictate for cost containment in all aspects of healthcare delivery are two significant trends that continue to affect medical equipment designers. Until recently, attempts to address these trends have been further complicated by the limited range of lighting technology choices. At one end of the spectrum is xenon technology, with its attendant high performance at a premium cost. At the other end is halogen technology, which is available at an attractive price point but is lacking in color quality and precision. Traditionally, there has been no middle ground in this range.

The choice of lighting options has now expanded to include solutions that fill important gaps in the technology range. These expanded choices can help integrators meet endoscopic product design requirements derived from application needs clearly set forth by hospitals and medical professionals. New technology choices also help satisfy the cost models necessary for the integrator's commercial success.

Design Challenges

The main goal of endoscopic illumination design is to provide the best possible light quality—white daylight at a correlated color temperature (CCT) of 5600 K—at the tip of the endoscope. This yields the best tissue color rendition to support accurate medical diagnosis and properly lights the visible field for study and surgery. Illumination designers are also challenged by other engineering and economic factors, many of which are interdependent.

Precision. Because endoscopes depend on a thin fiber-optic cable or bundle to transmit light to the tip, precision at the arc gap of the light source (lamp) is critical. The smallest possible arc gap is ideal because that would create a smaller spot diameter. This maximizes the light passing through the aperture that couples with the fiber optics, optimizing the lumen level through the fiber and on to the field of view. Precision in the light path also helps minimize the overall diameter of the endoscope, which is also affected by other factors such as cladding that may be constructed of various materials. In addition, having a minimal-diameter endoscope contributes to patient comfort.

Size. Space is at a premium throughout the diagnostic and surgical environment. The light source must be packaged as compactly as possible. As the level of instrumentation integration increases, light sources and supporting electronics must be designed to fit into enclosures that may also house video monitor circuitry, pumps, or other equipment. The space constraints are further compounded with the continuous introduction of new technologies and instrumentation into the surgical environment. The need for compact packaging also applies to existing installations where the light source is being upgraded in a stand-alone enclosure.

Weight. Equipment portability is an important requirement in the increasingly crowded surgical suite. Light sources, along with other mechanical systems and test or inspection equipment, must be sufficiently lightweight to be quickly and conveniently deployed or relocated by any assigned staff person in the room.

Power Requirements. Depending on the lamp selection, power requirements for the light source can vary widely. Filament (halogen) lamps require more power to provide the same luminous flux as a xenon lamp. Smaller filaments demand more power to produce an equivalent amount of light. Increased power requirements generally mean more components and complexity in the power supply. This, in turn, increases both power supply cost and weight.

Power supply design for xenon sources has traditionally been more complex than for other technologies. Whereas a halogen lamp requires a simple ac power supply, a xenon lamp requires a high-voltage starting circuit and dc power regulation.

Metal halide is three times more efficient than xenon and halogen, so it only requires a third of the wattage to provide the same amount of light. A 60-W metal-halide lamp is comparable to a 180-W xenon lamp. Metal-halide power supplies are as technically complicated as those for xenon, but being a third of the wattage makes them intrinsically smaller and cheaper.

Heat Dissipation. As the power requirements increase or become more complex, heat dissipation requirements also increase. In addition to reducing the safety risk of having a hot box among all the instruments, cooling is necessary for the optimum performance of the lamps and other components. None of the lighting technology options allows for convective (air) cooling. Some sort of cooling fan arrangement is required to move the heat away from the light source, and the chosen cooling option will affect system weight, size, and construction.

Noise. Cooling fans add noise to the surgical environment. Minimizing the size of cooling fans and their resulting noise is an important consideration to the personnel in the area. Light-source cooling fans are but one of several noise sources in instrumentation and analysis equipment.

Electromagnetic Interference. Xenon arc and metal-halide lamps give off electromagnetic interference (EMI). The starting pulses emitted from the lamp can reset digital electronics, and in today's surgical suite, there are myriad digital circuits within the equipment configuration that must be protected from EMI. Designers must be sure that the lamp and its associated electronics and wiring are enclosed by a satisfactory metallic (Faraday) cage.

Another EMI consideration affects power electronics design. The high-frequency electronics inside the cage can generate radiation back through the input line cord to the branch power line of the surgical suite. From there, it can travel to another piece of equipment connected to the same power line, emitting interference within that circuitry. This can be addressed by isolating the lighting power electronics from the input power line and is usually accomplished by input line filtering.

Cost-Structure Considerations. Designers and integrators are under pressure to pay close attention to all of the previously mentioned factors, while adhering to a cost structure that makes sense to them as a supplier, as well as to the end customers in the healthcare delivery environment. Cost models include technology considerations directed at the cost to build and cost to acquire the lighting source and endoscope, as well as the cost to operate the device over its life span. Cost to build and cost to acquire are generally integrator issues, whereas cost to operate is a pass-through consideration that affects the final user. The level of light-source integration with other equipment also affects the integrator's cost structure. Technologies can be acquired from the lighting provider at packaging levels that allow the integrator to take advantage of the engineering work already applied to the lighting system.

Lighting Technology Options

Halogen and xenon arc lamp designs have traditionally been the viable options for light sources in endoscopy, but there is a wide gap between the two.

Halogen filament-lamp technology, introduced 30 years ago and widely used in commercial and medical applications, has dominated the fiber illumination market. Halogen lamps are inexpensive, readily available, and use relatively simple power supplies that involve only stepping down from 120 or 240 V to 21, 15, or 12 V for endoscopy.

However, there are many drawbacks in using halogen lamps as illuminators for endoscopes. They do not provide the same color light as daylight, under which doctors have learned to identify tissue disease and damage. Instead, halogen lamps radiate as a blackbody at a CCT of 3200 K, considerably less than daylight. For reference, the sun, also a blackbody radiator, has a color temperature of 6500 K, which reduces to 5600 K after it passes through the atmosphere.

The large filament inside a halogen lamp makes it difficult to design a spot size smaller than about 8 mm. This does not allow enough of the light to be directed into the thin fibers of the medical instruments. Increasing the fiber bundle size to accommodate this larger spot size results in a very large diameter for the endoscope, which is undesirable. To correct this problem, smaller lamp filaments have been used. However, these generate less light, which in turn increases the power requirement. The resulting heat can actually damage the end of the optic fibers. Higher power also creates heat problems within the lamp that reduce lamp life to typically about 50 hours.

Xenon lamps, introduced about 15 years ago, addressed many of halogen's technical problems. As an arc lamp, its light source is only about 1–2 mm across, reducing spot size and allowing it to more precisely illuminate smaller-diameter fibers. Its color is much closer to daylight, 5600 K, and there is no filament to burn out, resulting in a lifetime of 500 hours, which is 10 times longer than that of halogen lamps. However, xenon arc lamps require a more complicated power system than halogen lamps, increasing component count and cooling requirements. This increases the illumination system cost for the integrator (and ultimately the end user). For an original equipment manufacturer (OEM) implementation, xenon system costs are typically about $6000, which is six times greater than that of halogen systems. Ongoing costs—the cost of light, a ratio of lamp cost to lamp lifetime—is approximately six times higher for xenon than for halogen lamps. The cost of xenon lamp replacement is approximately $500, whereas halogen lamp replacement is between $15 and $20.

In the marketplace, budget considerations seem to determine the choice of xenon or halogen. On a worldwide basis, halogen illumination systems outsell xenon by a factor of about five or six to one. However, in developed countries, the ratio is actually inverted, with deployments in favor of xenon. Recent data from Frost & Sullivan provide an example: "for the United States, xenon shipments for the year 2001 are forecast to be just over 15,800 units, compared with nearly 2,800 units for halogen."¹

Recently, another technology choice has evolved, and it helps both the integrator and the end-user deal with the technology gap dilemma. There have been major refinements made to metal-halide technology, and the new generation of lamps bears little resemblance to metal halide as designers know it.

The new-generation metal halide incorporates an arc gap comparable to xenon, 1.2 mm. When combined with precise reflector designs, the narrow gap permits efficient targeting for today's thin optical fibers. Furthermore, the design produces near-daylight quality, yielding a CCT of 5500 K. Typified by the Welch Allyn (Skaneateles Falls, NY) Solarc system, the new lamp is physically smaller and three times more efficient than both halogen and xenon lamps. As a result, the lamp requires only one-third the power for the same brightness. For example, a 60-W daylight metal-halide lamp emits the same luminous flux as a 180-W xenon lamp.

Metal-halide design efficiencies also affect lighting characteristics such as power requirements and heat dissipation. Because the technology generates significantly less heat, dissipation requirements are reduced, allowing the use of smaller cooling fans. Smaller fans mean less noise; fans needed for such designs are essentially inaudible.

Metal-halide lamps have a much more favorable cost-of-light ratio than xenon lamps. They provide about twice the life for half the cost, which represents a fourfold advantage in cost of light. Their lifetime is 1000 hours, twice that of xenon arc lamps, and approximately 20 times that of halogen lamps. This results in dramatically reduced life costs in terms of both unit downtime and technical labor cost for lamp changes. Table I compares cost of light for the three lighting technologies, along with the maintenance implications as defined by lamp change cycles.

The refinements made to the metal-halide platform now give integrators and end-users a daylight lighting technology choice that fills the gap between xenon performance and halogen economy. Metal halide lamps offer low voltage, long life, and daylight illumination at a cost structure close to that of halogen lamps.

Figure 1. A performance comparison of xenon and new-generation metal-halide lamps. (click to enlarge)

Performance Considerations

All light sources are subject to some variation during operation. Even xenon lamps, with their near-daylight color characteristics, will lose intensity over their lifetime. Typically, xenon lamp intensity will degrade to approximately 50% by end of life, usually 500 hours. Daylight metal-halide lamps, in contrast, typically retain about 60% of their intensity by end of life, at 1000 hours. Figure 1 illustrates the performance comparison (lumen maintenance) of xenon and new-generation metal-halide lamps.

Figure 2. The color-temperature drift of metal-halide lamps. (click to enlarge)

Daylight metal-halide lamps exhibit a CCT of 5500 K, which is very close to xenon and substantially better than halogen at about 3200 K. Over the life of the lamp, however, metal-halide lamps have the potential for some slow color-temperature drift. As Figure 2 illustrates, the color could shift to the range of 6500 K at some point over the 1000-hour lifetime. Although this represents some deviation from xenon light characteristics, the CCT values for metal-halide lamps vastly exceed those of halogen alternatives.

Figure 3. A typical light module, shown with optional ballast.

Technology Integration Options

Designers and integrators can adapt a metal-halide lighting platform at several levels of integration to suit application and cost requirements.

The simplest form of integration is the light module, or source. This configuration comprises the lamp and cooling block. Approximately 50% of the necessary engineering work needed for an application is included at this level. The assembly would need mechanical, electrical, and physical integration to be included in a larger system or subsystem. This level would provide the OEM the greatest opportunity to add customization and, therefore, value. A typical module is shown in Figure 3.

Figure 4. A typical light engine.

An intermediate level of integration would be the light engine. This consists of a tray-mounted assembly with a lamp, ballast electronics, and a fiber-optic-bundle adapter system for a variety of interfaces, such as ACMI, Wolf, Storz, Olympus, and others. Requiring only mechanical and voltage integration, approximately 80% of the engineering work for integration is provided at this level. This configuration could be readily adapted into an all-in-one or multifunction box containing video monitor circuitry, pumps, or other equipment. Figure 4 shows a representative light engine.

Figure 5. A typical illuminator, a self-contained package with fiber connectors.

The highest level of integration is the illuminator (see Figure 5). This is a self-contained, stand-alone package with industry-standard fiber connectors that needs only to have the endoscope or probe plugged in (plug and play). Virtually all of the engineering work is done at this level. A typical application for the illuminator is the replacement of a stand-alone lighting package with earlier-generation light technology.

Conclusion

Advances in lamp design have resulted in a daylight metal-halide technology that fills an important gap between xenon technology, with its cost premium, and halogen technology, with its economic advantage. Endoscope designers and integrators should investigate metal halide's combination of excellent light-color quality and realistic cost structures as a new alternative. In turn, this new option will produce lighting solutions that help healthcare delivery end-users with a range of budget structures to reap the benefits of daylight-quality illumination and long lamp life.

Ian K. Edwards, MS, is the R&D manager at Welch Allyn's Lighting Products Division (Skaneateles Falls, NY). He has 22 years of experience in the electro-optics industry and has authored numerous industry articles. He can be reached at edwardsi@mail.welchallyn.com.

Reference

1. "Light Source Market for the Endoscopic Visualization Equipment Industry, Unit Shipment Forecasts (U.S.), 1997–2007," U.S. Endoscopic Visualization Equipment Market, Frost & Sullivan (New York), Report 7883-54.

Copyright © 2002 Medical Electronics Manufacturing