3
Optical Sensing, Lighting, and Energy

A major fraction of all information received and analyzed by humans is received through the eyes, whether from reading a newspaper, watching television, or just observing our environment. The ability to optically sense and obtain information in this way is fundamental to our human existence and involves the traditional optical science and technology of the human eye, the vision process, corrective eyeglasses or contact lenses, and the use of lighting to permit the surroundings to be illuminated. Although advances have been made in some of these areas, for the most part the fundamental way we observe and see our immediate surroundings has not changed significantly over the past hundred years, with the exception that now artificial lenses and surgical techniques can improve some vision problems and better eyeglasses and corrective procedures are available.

What is significant, however, is the tremendous advance that has occurred recently in the development and use of new optical and infrared sensors and instruments that can detect and analyze our surroundings and present this information to us visually, thus greatly augmenting our normal visual process and in some cases showing details and information never previously seen. For example, a broad range of newly developed optical sensors and instruments are already used in everyday life, such as those that provide satellite pictures of clouds and weather patterns on TV evening news, infrared night vision scopes used by law enforcement, spaceborne probes to Jupiter that use optical instruments to measure and image the surface temperature of the planet, home security infrared motion sensors, and optical or laser probes to detect and display gas emissions from automobile highway traffic. Related to these advances in optical sensing and imaging technology are associated advances in the development of new, high-efficiency



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Harnessing Light: Optical Science and Engineering for the 21st Century 3 Optical Sensing, Lighting, and Energy A major fraction of all information received and analyzed by humans is received through the eyes, whether from reading a newspaper, watching television, or just observing our environment. The ability to optically sense and obtain information in this way is fundamental to our human existence and involves the traditional optical science and technology of the human eye, the vision process, corrective eyeglasses or contact lenses, and the use of lighting to permit the surroundings to be illuminated. Although advances have been made in some of these areas, for the most part the fundamental way we observe and see our immediate surroundings has not changed significantly over the past hundred years, with the exception that now artificial lenses and surgical techniques can improve some vision problems and better eyeglasses and corrective procedures are available. What is significant, however, is the tremendous advance that has occurred recently in the development and use of new optical and infrared sensors and instruments that can detect and analyze our surroundings and present this information to us visually, thus greatly augmenting our normal visual process and in some cases showing details and information never previously seen. For example, a broad range of newly developed optical sensors and instruments are already used in everyday life, such as those that provide satellite pictures of clouds and weather patterns on TV evening news, infrared night vision scopes used by law enforcement, spaceborne probes to Jupiter that use optical instruments to measure and image the surface temperature of the planet, home security infrared motion sensors, and optical or laser probes to detect and display gas emissions from automobile highway traffic. Related to these advances in optical sensing and imaging technology are associated advances in the development of new, high-efficiency

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Harnessing Light: Optical Science and Engineering for the 21st Century sources of light to illuminate our surroundings and in the use of optics and lasers for development of new energy sources. For example, new lighting sources are being developed that may reduce U.S. energy consumption by tens of billions of dollars per year, and new laser-based nuclear fusion power plants and mass-produced photovoltaic solar cells are being studied for long-range potential as cheap power in the next century. This chapter presents a synopsis of recent advances in optical sensing instruments and techniques, lighting, and energy. The emphasis is on new or revolutionary optical technologies that are expected to significantly impact the future growth and well-being of our society. As such, technical areas of lighting, energy, and optical sensors that either are mature or are not expected to grow dramatically are not covered in as much depth. Although the topics include a rather broad range of optical fields, they are centered primarily on the generation of light (new lighting sources), the conversion of light to energy (solar cells and laser fusion research), and the use of optical and imaging sensors for the measurement and detection of a wide range of physical and chemical parameters (night vision scopes, video cameras, gas vapor sensors, traffic laser radars, bar-code scanners). The topics covered have been divided into four subsections: (1) optical sensors and imaging systems, with application in the environment, global imaging, astronomy, industrial/chemical sensing, video cameras, law enforcement and security, common optical sensors, and scanners; (2) lighting, including new light sources, light-emitting diodes (LEDs), and the use of lasers in entertainment; (3) applications of optics and lighting in transportation, including autos and aircraft; and (4) energy applications, including laser fusion, laser isotope separation, and solar cells. The role that advances in materials have played in many of these fields is also addressed, because the development of new optical materials is often the key factor enabling progress (Box 3.1). Overall, this study finds that the areas of optical sensing, lighting, and energy account for sales, research, and development of about $19 billion per year in the United States. This figure includes about $3.5 billion for optical sensors and imaging instruments, $12 billion for lighting fixtures and lamps, $400 million for light-related energy research and solar cell production, and $2 billion for the use of optics in cars and airplanes. The total world market is estimated to be two to three times as large. Some of these applications have a great impact on other markets and represent key or enabling technologies. For example, the efficiency of lamps has a direct impact on the $40 billion that is spent each year in the United States on electricity for lighting. As such, a 50% change in lighting efficiency can have a $20 billion impact on the U.S. economy and an even larger impact on worldwide energy demand,

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Harnessing Light: Optical Science and Engineering for the 21st Century BOX 3.1 DEVELOPMENT OF NEW MATERIALS The development of new optical and semiconductor materials has been a key factor in the recent advances made in optical detector arrays, optical biosensors, digital cameras, lighting sources, and solar cell efficiency. especially in developing countries where lighting and energy production are still expanding. Another example is the real-time global mapping supplied by space-based optical imaging weather satellites. These maps affect a much larger market for weather and crop forecasts and help authorities develop forecasts and emergency plans for storms and hurricanes whose impact in dollars and lives saved is often incalculable. Where practical, these secondary impacts of optics applications are also covered in this chapter. The discussion of each subtopic is based on the results of a workshop held by the committee, as well as on additional written inputs obtained by the committee. The main findings and conclusions, which cover key highlights and challenges, are collected at the end of each major section. Finally, recommendations based on the findings and conclusions are made to the government, academia, and industry, where appropriate. Optical Sensors and Imaging Systems Light reflected from objects has been used by humans for thousands of years as a way to see or remotely sense the presence and composition of the surrounding environment. In most cases, the reflected or transmitted light is seen directly by the eye, and differences in color or intensity over the visible wavelength spectrum are used to detect and differentiate objects and images. Although outside the portion of the spectrum that is visible to the eye, light at ultraviolet (UV) and infrared (IR) wavelengths contains additional information. For instance, absorption and possibly fluorescence at UV and IR wavelengths can be used to detect certain chemicals and pollutant gases, to see objects at night by using IR thermal radiation, and to measure the temperature and composition of a distant object. It is the spectroscopic or wavelength (color) dependent nature of the reflected or transmitted light that allows one to detect a particular feature or the presence of a particular chemical. The use of optical sensors and imaging systems has been enhanced recently with the advent of small, inexpensive video cameras and detectors that operate in both the visible and the infrared; the development of new compact tunable laser sources; and the manufacture of compact

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Harnessing Light: Optical Science and Engineering for the 21st Century optical spectrometer instruments. Although spectroscopic optical instruments have been used for the past hundred years, recent advances in these optical techniques and the reduction in their costs have led to the recent surge in their use in a wide variety of fields. The following sections outline the current use and projected growth of optical sensors in environmental and atmospheric monitoring; Earth and global surface monitoring; astronomy and planetary probes; industrial chemical sensors; imaging detectors and video cameras; law enforcement and security; and common everyday optical sensors, printers, and scanners. Environmental and Atmospheric Monitoring Optical systems can be used for the detection of a number of important gases or pollutants in the atmosphere. In many cases, each chemical has a distinct absorption spectrum in which different wavelengths (or colors) of a transmitted optical beam are preferentially absorbed according to the concentration and presence of the chemical or gas in the atmosphere. Several different optical techniques are used, depending on the substance of interest, its concentration, and the detection range expected from the instrument. An important point is that optical sensing can often be accomplished remotely, because the optical beam can be directed at a distant object and information about the composition and gases surrounding the distant scene can be deduced from backscattered light. In fact, optical remote sensing can be used to detect chemicals (or physical parameters such as speed and dust cloud density) at ranges from a few meters to several hundred kilometers in some cases. This capability has significantly changed the way we measure our environment. For instance, 30 years ago weather balloons were used to carry instruments aloft to sample the upper atmosphere; now, we use laser beams from the ground to make the same measurements. Similarly, where once we measured the severity of air pollution in Los Angeles by measuring the time it took a stretched rubber band to rot, we now use chemical and optical absorption instruments to obtain round-the-clock coverage of the concentration of ozone and other environmental gases. The advances in these areas are covered in the following sections. Open-Path Gas Monitoring Optical gas monitoring uses a beam of light that is transmitted through the open air or through a sample chamber (cell). The beams of open-air systems can cover paths of several hundred meters to several kilometers. Selective absorption of the light allows for detection of the compounds present and quantification of their concentrations. This is usually done by using a conventional optical spectrograph or a Fourier-transform infrared (FTIR) optical spectrometer that directs an

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Harnessing Light: Optical Science and Engineering for the 21st Century optical beam through the atmosphere by means of a telescope. These optical instruments can be used as sensitive real-time monitors of the composition and concentration of environmental gases in the atmosphere or in a plume from a smokestack. They have been used to detect the concentration of organics, refrigerants, carbon monoxide (CO), nitrogen oxides (NOx), ozone, and other gases in the environment and from industrial sources; to sense emission gases from automobiles over a highway; and to detect evidence of the manufacturing of chemical, biological, or nuclear materials. For example, Figure 3.1 shows an FTIR instrument used to monitor the perimeter of an industrial chemical plant to detect the accidental release of a hazardous gas by the plant. Although conventional analytical chemical techniques such as wet chemical analysis or gas chromatography are often used for this purpose, they do not offer real-time remote sensing or on-site capability as easily as optical monitoring does. The advantage of conventional chemical measurements is the longer historical use of these techniques and their lower capital cost, although their operational costs can be higher. Conventional analytical chemical sensors are still dominant, but optical methods now claim about 40% of the market and this fraction is rapidly growing. The current annual U.S. market for optical instruments used in this area is about $500 million (systems cost). The demand is driven by regulatory laws for source ambient air quality usage, although industrial process control is beginning to incorporate these techniques as well. The recent increased acceptance of such optical instruments by the Environmental Protection Agency (EPA) will certainly stimulate their more widespread use. The main technical challenge is for smaller and cheaper laser or optical spectroscopy devices. At present, there is a significant U.S. market, but the market in Europe is somewhat more FIGURE 3.1 An optical-beam FTIR instrument used to measure gas emissions along the perimeter of an industrial chemical plant. (Courtesy of D.N. Hommrich, Essential Technologies, Inc.)

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Harnessing Light: Optical Science and Engineering for the 21st Century advanced. The slower U.S. development is due to the U.S. regulatory agencies' longer acceptance times for new environmental monitoring technology (currently about 5 years or more). Lidar Remote Sensing Laser radar (lidar) has been used for more than 25 years to detect from afar a wide range of atmospheric or environmental characteristics, such as temperature, gas concentration, and wind velocity. Lidar uses a laser beam to probe a remote target, aerosol layers, or gas clouds at ranges from 10 m to several kilometers and deduces the range and composition of the cloud or target from the detection of backscattered light. Combined with spectroscopic wavelength control, tunable lidars have detected and mapped ozone, water vapor, methane, and other pollutant gases in the atmosphere or in smokestack plumes. In the effort to understand global climate change, lidars have been used to monitor gas concentrations and temperatures in the upper atmosphere and the concentration of ozone, water vapor, and methane over the Amazon jungle. If their sensitivity is high enough, range-resolved lidar returns can be used to map in three dimensions the physical extent of a plume or haze region; this was done to map the global movement of volcanic ash clouds from the eruptions of Mount St. Helens and Mount Pinatubo. Airborne lidar systems have been used to make range-resolved maps of the density of haze over the Los Angeles basin. Figure 3.2 shows a plume of ozone detected and mapped using a differential-absorption lidar; this ozone plume was found over the mid-Pacific near Tahiti and was part of a smoke plume produced by biomass (trees) burning in Africa and transported thousands of miles by global winds. Also of FIGURE 3.2 A plume (layer) of ozone detected by a differential-absorption lidar near the mid-Pacific (Tahiti) that had been transported by global winds from biomass (tree) burning in Africa. (Courtesy of E.V. Browell, NASA-Langley Research Center.)

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Harnessing Light: Optical Science and Engineering for the 21st Century importance is the recent use of a airborne range-resolved precision lidar by the National Aeronautics and Space Administration (NASA) to measure the canopy height and depth of foliage to determine the biomass coverage of Earth. This latter use will significantly increase our knowledge of the density of forests and jungle growth in remote sites, which is crucial for accurate predictions and understanding of the production of oxygen and uptake of carbon dioxide (CO2) by plants on Earth. A potentially significant new application for lidar will be its joint use with an open-path optical spectrometer instrument, since lidar can measure and map cloud or aerosol movements in three dimensions while the open-path instrument can determine the integrated gas concentration. Such measurements would yield gas flux values, which are most vital for environmental and gas emission regulatory detection. Lidar instruments are still rather expensive and one of a kind; they are used more for research than commercial applications, with government funding for lidar still greater than private or commercial funding. The total U.S. market is on the order of $10 million to $20 million per year, but significant growth is expected as the required laser sources become available in more compact, less expensive forms, especially diode-pumped solid-state lasers and optical parametric oscillator (OPO) lasers. Thus, the growth potential for lidar remains dependent on developments in lasers. One important use now being evaluated is for aircraft wind shear and wake vortex detection at airports. Such a device would be an important enhancement to aircraft safety. Airlines, air cargo companies, and the U.S. Air Force are also interested in the use of on-board lidars for measurement of wind profiles below, above, and ahead of aircraft. Significant fuel savings could result from the use of such data. At present, significant work in this area is being done in Japan and Europe, as well as in the United States. Another growing lidar market is for police laser radars to detect traffic speeds. The current annual world market for this application is on the order of $10 million to $40 million. Laser devices have the advantage over radar that the small laser beam can select a single automobile from a group of vehicles and can measure the range to the vehicle with an accuracy of better than a few centimeters. The type of traffic lidar shown in Figure 3.3 costs about $4,000, compared with about $1,500 for conventional microwave radar. Several U.S. states now each have several thousands of these lidars in use by local police agencies. A related instrument, the laser range finder, is used by land surveyors to map distances to an accuracy better than 1 cm and for the detection of Earth's crustal movement (earthquakes) over fault lines or volcanic sites. Since a laser beam is very directional and small, it can be used for the precision determination of angular and distance measurements in both construction and land surveying, where the traditional

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Harnessing Light: Optical Science and Engineering for the 21st Century FIGURE 3.3 Laser radar used to measure traffic speed. The narrow laser beam can select one car from a group of vehicles, unlike conventional microwave radar. (Courtesy of the Institute of Police Technology and Management, University of North Florida.) transit for survey work essentially has been displaced by an invisible, infrared laser transmitter and a retroreflecting mirror on a pole. The question of eye safety is always of concern with lidar since the laser beam can be directed toward an urban population in some cases. Usually, this concern is handled by increasing the size of the laser beam transmitted into the atmosphere so that its intensity falls below the allowable eye safety value for direct ocular viewing (i.e., for a beam aimed directly into the eye). This value is about 10 mJ/cm2 per pulse for an IR wavelength greater than about 1.4 microns. For lasers at visible wavelengths, the limit is several orders of magnitude lower, since the eye focuses visible light onto the retina, whereas infrared wavelengths are not focused but absorbed in the cornea and interior portion of the eye. Optical Environmental Biosensors A new type of optical biosensor, developed during the past decade, uses the combination of an optically active bioreceptor and a photodetector as an ultrasensitive sensor (Rogers and Gerlach, 1996; Vo Dinh et al., 1994). Biosensor materials change color or other optical properties in the presence of trace amounts of a known chemical or biological substance; they provide excellent specificity and sensitivity for a wide variety of environmental chemicals and biological agents. Most of these optical biosensor materials use an enzyme, DNA, and an antibody-based fluorescence label or other bioreceptor or an optically active bioagent that changes color or fluoresces in the presence of a specific substance. Most place the bioreceptor on the end of a fiber-optic probe or waveguide as the sensing end of the instrument, although some techniques use an optical microcavity with a chemically permeable membrane. These techniques are already being employed in the pharmaceutical and medical laboratory industries in the form of test kits and are used in polymerase chain reaction (PCR) applications as DNA probes. The market for such optical instruments was about $400 million in 1991 and is growing rapidly. Future markets are predicted to be about $1 billion annually for monitoring and bioremediation (making harmless) of hazardous waste dumps and $300 million annually for environmental sensing of water and air quality. Table 3.1 shows a list of some bacteria and viruses that are being detected using DNA-sensitive optical biosensors.

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Harnessing Light: Optical Science and Engineering for the 21st Century TABLE 3.1 Bacteria and Viruses Detected Using DNA-Sensitive Optical Biosensors Disease Causative Agent Sample Source Food poisoning Salmonella bacteria Food processing Pneumonia Legionella bacteria Water samples Diarrhea Giardia lamblia bacteria Water samples Hepatitis Hepatitis virus Shellfish Work is progressing to make optical biosensors more rugged and cheaper over the next 2 to 3 years, which would greatly expand their utility and commercial use for applications in medicine and public health (e.g., glucose sensors; see Chapter 2). In addition, current research is directed toward producing a complete optical biosensor on a chip, using techniques similar to those used to manufacture silicon integrated circuits. Earth and Global Surface Monitoring Optical sensors and television imaging systems based on high-altitude aircraft, balloons, and satellites have been used for more than three decades to detect and map weather patterns, mineral resources, ocean currents, and land topography on Earth's surface. Recently, more sophisticated optical instruments have been used that can detect the concentration of important greenhouse gases related to the study of global climate change. As such, there is both a commercial and a scientific use for high-altitude aircraft (U2), balloon, and satellite-based optical instruments. Atmospheric and Global Climate Change Remote sensing from satellites or high-altitude aircraft or balloons is a cost-effective way to obtain homogeneous, global measurements of critically important weather and climate variables such as atmospheric temperature and humidity profiles, cloud properties, stratospheric and tropospheric aerosol amounts, sea surface temperature, ocean color, sea ice coverage, stratospheric and tropospheric ozone, and other important trace gas concentrations. A wide variety of techniques are used, including passive microwave, infrared and visible spectroscopic imaging, solar and lunar occultation, and radar and laser ranging. Of these systems, about half are optics based, including those used for cloud properties, ocean temperature, trace gas measurements, and humidity profiles. For climate monitoring, long-term precision in the measurement of properties is required. Such measurements have been

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Harnessing Light: Optical Science and Engineering for the 21st Century used to monitor the Antarctic ozone hole and to measure trends in cloudiness, Earth's radiation balance, and air temperature. Since the first weather satellite was flown in 1960 there has been a continuous program of improvement along with the introduction of techniques to measure new physical variables from space. In the United States, NASA has its Mission to Planet Earth—Earth Observing System (EOS) program to collect a benchmark series of important visible and infrared global climate observations in the late 1990s and the early twenty-first century. The Upper Atmosphere Research Satellite (UARS), placed in Earth orbit earlier this decade, incorporates a wide range of optical spectroscopic instruments to measure freon and ozone-related chemicals in the upper atmosphere. A vigorous program of innovation is under way to develop smaller, more capable, less expensive instruments. The National Oceanic and Atmospheric Administration (NOAA), the Department of Defense (DOD), and NASA are cooperating in developing the National Polar Orbiting Environmental Satellite System, a more efficient and capable system for operational weather and climate observations from satellites. The European Space Agency (ESA) and the Japanese space agency (NASDA) also have active Earth remote sensing programs for weather and climate research and are developing new tunable-laser lidar and long-path absorption technologies for this purpose. Earth's Resources and Weather Earth remote sensing satellites typically have optical or infrared instrumentation included in the satellite sensor package, sometimes in addition to microwave or radar sensors. The optical sensing instruments provide extensive knowledge of the global weather, agricultural resources, and land topography of Earth's surface (Office of Technology Assessment, 1990). Since the 1960s, satellite systems such as the Geostationary Orbital Environmental Satellite (GOES) system have provided near real-time photographs and digital images of clouds and weather patterns. For example, Figure 3.4 shows the image of a hurricane and its associated weather pattern off the coast of Florida in 1996. Since 1972, spectroscopic (or multispectral) wavelength bands in the visible to near-infrared region have also been used to detect agricultural parameters such as plant stress, plant density, and growth rates and to produce resource maps showing the location of minerals and sediment flow in rivers. For example, satellite-based multiwavelength optical imaging sensors have been used to map Earth's green biomass, i.e., plant density. An estimated $28 billion has been invested in remote sensing satellites that include optical imaging systems (see Figure 3.5). Of this total, a relatively smaller amount, on the order of several hundred million

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Harnessing Light: Optical Science and Engineering for the 21st Century FIGURE 3.4 GOES-8 weather satellite image of Hurricane Fran in the Atlantic Ocean off the coast of Florida. (Courtesy of NASA Goddard Space Flight Center.) dollars, represents the actual optical components and instruments used in the satellites. There are currently five operational optical imaging satellites in orbit: SPOT 3, Lansat 5, JERS-1, OFEQ 3, and IRS-2C. The majority of such systems are funded by governments including costs from satellite and downlink station design through deployment. Eight commercial remote sensing systems are scheduled to be launched by the year 2000: EarthWatch, Inc.'s Earlybirds (2) and Quickbirds (2); Space Imaging (1); and Orbital Science Corporation's SeaStar (1) and Resource 21 (2). The U.S. government plans to launch a series of Earth observing sensors starting in 1999. Other countries are planning to deploy an additional 10-15 satellites by 2000. Anticipated (and demanded) lower future launch costs are driving the systems toward lighter and cheaper packages. The trend is therefore toward an increase in the number of commercial satellites at lower costs ($50 million to $100 million each) for all applications, including agricultural and forest remote sensing. Optical systems capable of generating detailed digital elevation models (DEMs) have been identified as one of the biggest markets for this type of data and will be used for the generation of precision land contour and elevation maps for precision farming, watershed flow prediction, and land surveys. U.S. government policy regarding remote sensing data is contained in the Land Remote Sensing Act of 1992 and relates to the market for monitoring information, which is estimated to be on the order of $300 million per year for environmental uses. Approximately $2 billion per year for all applications (including

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Harnessing Light: Optical Science and Engineering for the 21st Century Optoelectronic sensors using laser, LED, or microwave radar have the potential to be a major growth area for use in collision avoidance, laser ranging, and blind-spot warning of nearby vehicles or obstacles. New optics-based chemical sensors are being used to monitor catalyst efficiency by measuring the concentrations of such exhaust gas species as unburned hydrocarbons, carbon monoxide, and oxides of nitrogen using miniature spectrometric real-time sensors. An important concern for these optoelectronic systems is that they must operate over a wide temperature range, from -40 to +85°C; they must be vibration tolerant; and they must last the life of the vehicle (typically 10 years or 10,000 hours of operation). Energy Advances in the efficient generation of electricity can have a significant impact on the energy consumption of our society. It should be noted that the energy (including electricity, gas, oil, and coal) used in the United States each year amounts to about $600 billion, of which $200 billion is used in business and residential buildings. This section covers several optics technologies that may have significant impact in this area, including the use of lasers to produce inertial confinement fusion as a future source of energy and for new basic science; the use of lasers to enrich uranium for reactor power plants; and recent developments in solar cell technology. Inertial Confinement Fusion Using Lasers Laser-induced inertial confinement fusion (ICF) is an approach to producing controlled nuclear fusion on earth (Lindl et al., 1992). As an integral part of the pursuit of a zero-yield nuclear test ban treaty, the United States has committed to the design, development, and construction of the glass-laser-based National Ignition Facility (NIF). The NIF is intended to use inertial confinement fusion for weapons studies, but it will also provide insight for future energy applications. Figure 3.15 shows a drawing of the NIF facility, which is based on flashlamp-pumped neodymium-doped glass technology and frequency conversion. It is scheduled to be completed in 2002 and to produce 1.8 MJ at 350 nm, with a total project cost of approximately $1.1 billion. From 1995 to 1998, the United States will also invest about $170 million in this project for the development of laser technology, large-scale precision optical components, and low-cost advanced optics manufacturing methods. NIF will be the largest optical system in the world and will develop and employ state-of-the-art adaptive optics systems on each of its 192 laser beam lines. To further control the beams'

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Harnessing Light: Optical Science and Engineering for the 21st Century FIGURE 3.15 Drawing of the National Ignition Facility (NIF), to be used for laser-controlled fusion. The system will have 192 laser beams and will be the world's largest laser and optical system. (Courtesy of Lawrence Livermore National Laboratory.) temporal and spatial coherence, NIF will use diffractive optics and phase modulation technology to produce optical bandwidths up to 0.5 THz. Recent scientific and technological advances in efficient, powerful semiconductor laser diode arrays (as laser pump sources), in specialized crystalline laser gain crystals, and in sophisticated gas flow cooling techniques now permit the conceptualization of an efficient, multimegawatt, all solid-state laser suitable for driving a central electric power plant based on inertial fusion energy (IFE). The development and demonstration of a highly modularized 1-kJ unit beam line for such a laser system over 5 to 10 years represents a grand challenge that would drive the envelope of diode-pumped solid-state laser technology and lay the foundation for the timely pursuit of IFE after laboratory ignition is achieved at NIF early in the next century. A major advance required for IFE is to reduce the cost of laser diode arrays to less than 10 cents per watt. Overall, ICF now amounts to about a $400 million per year project worldwide ($240 million in the United States in 1996). All funding for ICF currently comes from governments. Although ICF has not yet been demonstrated, NIF is the next critical step. The spin-off value in its optics and laser advances is great, and the potential payoff as a new energy source is enormous, although still uncertain and many years in the future. The consequences are also wide-ranging for astrophysics and other sciences.

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Harnessing Light: Optical Science and Engineering for the 21st Century Laser Isotope Separation of Uranium for Nuclear Energy Atomic vapor laser isotope separation (AVLIS) is an economical, environmentally improved method to enrich natural uranium for light-water reactor fuel. AVLIS is based on technology demonstrated over the past 20 years and uses a precisely tuned laser to selectively excite and photoionize uranium-235 (235U). The selectively ionized 235U is then collected to generate a product enriched in this isotope. It should be noted that the United States controls about 40% of the world's uranium enrichment market (currently several billion dollars annually) using technology developed 50 years ago. AVLIS will be able to produce enriched uranium at a much lower cost and will enable the United States to capture a significantly larger fraction of the world market. The U.S. Enrichment Corporation, a government corporation formed in 1992 with plans to privatize, is refining the technology and designing a large AVLIS system for this purpose. Key plant systems consist of separators to vaporize the uranium and collect the selectively photoionized 235U. There will be several identical separator lines. Dye lasers generating 50 kW of process light are optically energized by 160-kW copper vapor lasers (or possibly solid-state lasers) distributed via a fiber-optic network. The overall goal is to construct an AVLIS enrichment facility and bring it to full production early in the next century. The AVLIS project may become the largest technology transfer effort from DOE to the commercial sector. Space Solar Cells Solar cells, which convert light to electricity, have been used as the primary power in communication, defense, and weather satellites for the past 35 years and can be considered part of the satellite and spacecraft manufacturing industry. The worldwide space solar cell business is approximately $150 million per year, with about two-thirds of the total dominated by two American companies. The United States thus has a strong international position in this field, with a total annual market of about $100 million. In the past 5 years, however, there has been a technological and materials revolution in this field. For 30 years before 1994, the technology was stable, using standard crystalline silicon solar cell technology. Then in 1994, gallium arsenide (GaAs) cells grown on germanium substrates became mature for space use and the market switched largely to GaAs cells. The inherent leverage in reduced area and weight associated with the higher-efficiency GaAs cells influenced the launch booster design and the overall system. Therefore, the added cost of the new technology was more than warranted, to the degree that it is now difficult to sell any but GaAs-based solar cells for space use. In fact, by

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Harnessing Light: Optical Science and Engineering for the 21st Century early 1996 the space market had moved to begin production of dual-junction and triple-junction InGaP-GaAs cells. Because of the great demand for new space-based communications technologies, this market is booming. Telephone systems requiring 800-satellite constellations have been proposed, and several communications systems using up to 66 satellites have already been started. A major concern for the industry is the capital investment required to meet a demand surge that may not last long enough to amortize the equipment. This industry is playing a leadership role in optoelectronics and optical materials, being the first to apply many technologies that will become more widespread later. For example, there is an opportunity to apply new high-speed MOCVD equipment, which is likely to allow low-cost, high-speed epitaxial growth. This equipment will eventually become standard in many other areas of the semiconductor manufacturing industry, eventually making high-quality epitaxial films much cheaper and creating new opportunities for their use in many fields. Terrestrial Solar Cells The terrestrial uses of solar cells can be classified into two general categories: on the power grid and off the power grid. Off-grid applications include supplying power for hand calculators, remote instruments, stand-alone communication gear, remote mechanical pumps, and refrigerators. Other important off-grid uses are for industrial and residential general-purpose power, particularly in remote locations. On-grid solar cell systems make up only a tiny fraction of the available electric power capacity of the United States. The fraction is expected to become much larger as the cost of solar cells declines, the conversion efficiency of sunlight to electrical energy increases, nonrenewable fuel becomes scarcer and more expensive, and greenhouse gases begin to severely impact the environment. To quote the senior managing director of the Royal Dutch/Shell Group concerning renewable energy usage in general (Herkströter, 1997): . . . our various scenarios suggest that renewables could provide some 5% of the world's energy by 2020. They also suggest this could rise to over 50% by mid-century—a shift as fundamental as that from coal to oil in this century. The United States currently spends approximately $600 billion per year on energy—about 8% of the $7 trillion annual gross domestic product (GDP). By the middle of the next century, renewable energy is expected to meet a significant fraction, perhaps 50%, of U.S. energy needs. Solar energy, particularly photovoltaic solar cells, could play a central role in renewable energy, especially for electric power generation. It is reasonable to expect that the U.S. solar cell industry could reach $100 billion per year by the middle of the next century.

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Harnessing Light: Optical Science and Engineering for the 21st Century Although a number of competing sources of renewable energy exist, there are obvious advantages to photovoltaics if costs can be reduced. Solar cells can produce electrical energy at a rate of about 200 kWh/m2 per year (Zweibel, 1990). The current electric energy usage of the United States is 3 billion kWh per year, requiring solar cells covering a land area 120 km x 120 km. With a cost of $150 per square meter of installed solar energy panels and associated infrastructure, including power conditioning and storage, the capital investment in solar energy required to meet current needs is $4.5 trillion. By spreading conversion over 30 years, which is also the approximate lifetime of a panel, the cost would be $150 billion per year. The United States currently spends approximately $200 billion a year on electrical energy; this is about 3% of the $7 trillion GDP per year. For comparison, shipments of photovoltaic modules in 1996 were at the worldwide level of $500 million per year (Strategies Unlimited, 1996). The industry grew 10% in 1996 and was expected to grow 20% in 1997. There have been steady, incremental reductions in the cost of photovoltaic power over the past 15 years. In quantity, terrestrial solar panels can now be purchased for $4.50 per watt (this figure is for conventional crystalline silicon-based panels.). This represents a decline by a factor of 33 from the 1970 cost of $150 per watt. There have been suggestions that certain manufacturers have an internal cost of production of $2.75 per watt, which is a credit to their streamlining of the manufacturing process. The $4.50 per watt figure translates into a price of about $500 per square meter for solar panels. To provide solar electric power at a cost comparable to the present cost of electricity, silicon solar panels must drop in price by a factor of 5 to 10, if the remaining systems cost is equal to the panel cost (Zweibel, 1990). This is a reasonable expectation in the next 10 to 15 years, given the steep decline noted above and the even more remarkable decline in the cost of silicon electronic devices. The power output of photovoltaic modules shipped in 1996 was 82.5 MW, divided according to technology as indicated in Table 3.2. About 4 MW of the thin film figure went to consumer products such as calculators. There have been a number of attempts over the years to replace single-crystal silicon with lower-cost forms such as polycrystalline or amorphous silicon, but the cost advantages are somewhat offset by the reduced efficiency. Through the 1980s, the single-crystal market share dropped from 70 to 40%; it has since recovered to the level shown in Table 3.2. Silicon photovoltaics are still in a fairly early stage of development as a practical power technology. A number of complex issues surround the various technologies (Partian, 1995; Zweibel, 1990). These include the cost of materials, growth and fabrication of wafers or films,

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Harnessing Light: Optical Science and Engineering for the 21st Century TABLE 3.2 Market Share of Photovoltaic Technologies for Terrestrial Use, 1996 Shipments   Power (MW) Percentage Single-crystal 45 54.5 Polycrystalline including ribbon 26 32.5 Thin film 11.5 14   Source: Strategies Unlimited. manufacturing technology, and process control. The industry is quite contentious and competitive, which are both healthy signs. The use of silicon by the solar cell industry is approaching 10% of its use by the electronics industry. There have recently been shortages of silicon for solar cells. The sale of electronic-grade silicon is much more profitable for silicon manufacturers than sale of the lower-grade silicon needed for solar cells. There is concern that the portions of boules discarded by chip makers may no longer provide an adequate supply for solar cell manufacturers. The cost of single-crystal compound semiconductor films, such as GaAs epilayers, is expected to come down in the future. Advanced MOCVD reactors for growing high-quality epitaxial material are now available with growth platters 40 cm in diameter and cycle times of 2 minutes. Several techniques are available for reusing the compound semiconductor growth substrates from these reactors. There are great opportunities for research on solar cells that combine the best of both worlds, high-efficiency and low cost. Research is being carried out on other solar cell materials such as Culn Se2, CdTe, GaAs, and GaSb. Depending on the material, questions remain concerning issues such as stability and the feasibility of low-cost, large-scale manufacturing. The development of new materials has been and will continue to be a major driving force in solar cell advancements. Solar Thermal Energy Of the other forms of solar energy (e.g., wind, hydropower, and so forth) only solar thermal energy uses optics. Solar thermal energy is widely used to provide hot water for domestic and commercial use, process heat for industry and agriculture, and space heating and cooling. With sun-tracking parabolic concentrator mirrors, sufficiently high temperatures can be reached to drive a turbine generator. Using this technology, more than 350 MW generating capacity was installed in the Mojave Desert in the 1980s.

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Harnessing Light: Optical Science and Engineering for the 21st Century World leadership in optical science and engineering is essential for the United States to maintain its dominance in energy-related technologies such as laser-enhanced fusion, laser uranium enrichment, and solar cells. The Department of Energy should continue its programs in this area. Summary and Recommendations Optical Sensors and Imaging Systems Optical gas sensors are beginning to make a major impact in the field of air quality and pollution emission monitoring and offer real-time quantititative advantages (remote, in situ) over standard chemical analytical techniques. Open-path air monitoring is often used to measure environmental emission levels for compliance with environmental regulations. There are several lidar research programs for global mapping of greenhouse gases and environmental emissions. Lidar is starting to be practical as a commercial instrument, but commercial uses at present are mostly in traffic laser radars, wind shear sensors, and precision range finders and mappers. Industrial optical chemical sensors are just starting to be used for process control and have shown significant potential in several cases. Optical methods are used in only a minority of chemical sensors, but the fraction is growing as sensors and lasers become smaller and cheaper. Optical biosensors are important trace detectors in the pharmaceutical and medical laboratory industries and are the basis for a wide range of sensitive medical diagnostic tests and DNA sequencing instruments. New photooptical materials, sensitive to specific trace chemicals or biological species, will enable the development of new families of optical sensors. Satellite-based optical spectroscopic instruments have been used to detect the ozone hole and gases involved in global climate change. Optical and infrared camera sensors in Landsat and weather satellites are used to provide important agricultural and weather data on a daily basis. Future weather and Earth-viewing satellites will be cheaper, smaller, more numerous, and more often commercially financed. Ground-based telescopes using atmospheric compensation and optical interferometric techniques will revolutionize ground-based astronomy at visible and IR wavelengths. Planetary and space probes use optical and microwave sounders to detect and image heretofore unknown chemical species on different planets and have discovered water on the moon. New high-resolution (high pixel count) optical imaging arrays and CCD video detectors are increasingly used in commercial digital cameras. The most advanced digital cameras have a resolution comparable

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Harnessing Light: Optical Science and Engineering for the 21st Century to photographic films and are expected to revolutionize and computerize the photographic film and printing industries. New infrared detector arrays are providing improved high-resolution detection of thermal images and will be increasingly used in industry for real-time monitoring of manufacturing lines. Sophisticated optical and laser sensors are not used to a large degree in forensics and law enforcement because of their cost and lack of portability, but video surveillance and infrared motion security sensors are used extensively. Infrared LED and laser spotlights coupled to IR video cameras are being developed for night vision surveillance. Law enforcement will greatly benefit from advances in materials and systems for IR lasers and room-temperature IR cameras, while decreased size and cost of lasers will increase the use of optical and laser spectroscopic sensors. Optical sensors are used in many everyday devices, including bar-code readers, proximity switches for water faucets, safety shutoff beams for elevator and garage doors, and new ear-type thermometers for children. Laser printers and fax machines use optical sensors and imaging systems. Optical sensors are employed in a wide range of important industries and fields and will be an important factor in future industrial growth and competitiveness. However, at present, there is little coordination or information exchange of optical sensor technology between industrial sectors. DOE, NIST, and industry, in cooperation with the technical and professional societies, should pursue a program to enhance the coordination and transfer of optical sensor technology among industry, academia, and government agencies. Lighting New light sources and delivery systems will offer a large improvement in lighting efficiency. ''White-light LEDs" made by coating a blue LED with a phosphor are now as efficient as incandescent lamps and are expected to become competitive with fluorescent lamps within 5 years. High-efficiency, room-temperature red LEDs are being used in red traffic lights and are expected to save $175 million per year in electrical costs. New light delivery systems, such as plastic fiber cable light bundles and light pipes, are being used in advertisements and other neon-sign-type applications. New materials for light sources, light delivery, and controlled reflectivity have led to increases in lighting efficiency, lifetime, and utility. There is a need for light output measurement standards that account for the response of the human eye and the delivery efficiency of the lighting system. Laser light shows and optical staging are used in many live performances, although they are often considered secondary in importance

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Harnessing Light: Optical Science and Engineering for the 21st Century to the performance itself. The annual market for lasers and optics for laser light shows and live performances is steady. New high-efficiency lighting sources and light distribution systems will have a significant impact on the country's electricity use. There is only moderate coordination of research and standards setting on new, efficient, and effective lighting sources among government, academia, and industry. DOE, EPA, EPRI, and NEMA should coordinate their efforts and create a single program to enhance the efficiency and efficacy of new lighting sources and delivery systems, with the goal of reducing U.S. consumption of electricity for lighting by a factor of two over the next decade, thus saving about $10 billion to $20 billion per year in energy costs. Optical Sensors and Lighting in Transportation With the exception of fiber-optic gyroscopes, optical sensors are not used extensively in aircraft, where they do not yet offer advantages over conventional mechanical or electrical sensors. Optical sensors and lighting are used to a great extent in automobiles, where they play an integral and important role. New high-efficiency headlamps, LED taillights, and optical collision avoidance systems are being introduced into automobile lines. Energy The world's largest laser and optical systems are being developed for nuclear energy-related programs. The National Ignition Facility will be the largest sophisticated optical system in the world and a major new research tool for the United States. The AVLIS program will provide economical separation of uranium reactor fuel. Solar cell efficiency has increased and cost may have decreased to $2.75 per watt. A decrease of a factor of 5 to 10 in solar cell cost would make the price of solar photovoltaic electrical energy comparable to that for nonrenewable sources. If the cost of solar photovoltaic cells continues to decline, solar cells could begin to impact the electric power industry by 2020 and could provide as much as half the world's electric power by 2050. As a source of renewable energy with low environmental impact, efficient, low-cost solar energy could have a great impact on world energy consumption. If successful, such programs could have a significant effect on future energy programs and the cost of energy in the United States. World leadership in optical science and engineering is essential for the United States to maintain its dominance in energy-related technologies such as laser-enhanced fusion, laser uranium enrichment, and solar cells. The Department of Energy should continue its programs in this area.

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Harnessing Light: Optical Science and Engineering for the 21st Century References American Chemical Society. 1996. Technology Vision 2020: The U.S. Chemical Industry. Washington, D.C.: American Chemical Society. Bruton, T.M., et al. 1997. Multi-megawatt upscaling of silicon and thin film solar cell and module manufacturing. Paper presented at the European Community Photovoltaic Conference, Barcelona, June. (Author's address: BP Solar, P.O. Box 191, Sudbury-on-Thames, Middlesex TW16 7XA, United Kingdom.) Herkströter, C.J.A. 1997. Speech at Erasmus University, Rotterdam, March 17. Available from Shell Oil Company, 712 Fifth Avenue, New York, NY 10019. Janata, J., M. Josowicz, and D.M. DeVaney. 1994. Chemical sensors. Anal. Chem. 66:207. Lindl, J., R. McCrory, and E.M. Campbell. 1992. Progress toward ignition and beam propagation in inertial confinement fusion. Phys. Today (September):32. Menzel, R. 1989. Detection of latent fingerprints by laser excited luminescence. Anal. Chem. 61:557a. National Research Council. 1993. Counterfeit Detection Features for the Next-Generation Currency Design. Washington, D.C.: National Academy Press. Office of Technology Assessment. 1990. Technology for a Sustainable Future. Washington, D.C.: U.S. Government Printing Office. Partian, L.D., ed. 1995. Solar Cells and Their Applications. New York: Wiley. Rea, M., ed. 1993. The Illumination Engineering Society Lighting Handbook. New York: Illumination Engineering Society of North America. Rogers, K.R., and C.L. Gerlach. 1996. Environmental biosensors: A status report. Environ. Sci. Technol. 30:486A. SPIE. 1996a. Physics Based Technologies for the Detection of Contraband. Conference No. 2936. Boston: SPIE Conferences. SPIE. 1996b. Chemistry and Biology Based Technologies for the Detection of Contraband. Conference No. 1937. Boston: SPIE Conferences. Strategies Unlimited. 1996. Five-Year Photovoltaic Market Forecast: 1995-2000. Report PM-43. Strategies Unlimited, 201 San Antonio Circle, Suite 205, Mountain View, CA 94040. Vo Dinh, T., K. Houck, and D.L. Stokes. 1994. Surface enhanced Raman gene probes. Anal. Chem. 33:3379.

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Harnessing Light: Optical Science and Engineering for the 21st Century Warner, I.M., S.A. Soper, and L.B. McGown. 1996. Molecular fluorescence, phosphorescence, and chemiluminescent spectrometry. Anal. Chem. 68:73. Zweibel, K. 1990. Harnessing Solar Power: The Photovoltaics Challenge. New York: Plenum.