NEURAL ENGINEERING FIELDS OF STUDY

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1 NEURAL ENGINEERING FIELDS OF STUDY Biomedical engineering; computational neuroscience; medicine; neural prostheses; neuroscience; chemistry; neural implants; neural interfacing; bioelectrical engineering; biology; biomaterials; neurosurgery; electrical engineering; materials science; nanotechnology; neural imaging; neural networks; tissue engineering. SUMMARY Neural engineering is an emerging discipline that translates research discoveries into neurotechnologies. These technologies provide new tools for neuroscience research, while leading to enhanced care for patients with nervous-system disorders. Neural engineers aim to understand, represent, repair, replace, and augment nervous-system function. They accomplish this by incorporating principles and solutions derived from neuroscience, computer science, electrochemistry, materials science, robotics, and other fields. Much of the work focuses on the delicate interface between living neural tissue and nonliving constructs. Efforts focus on elucidating the coding and processing of information in the sensory and motor systems, understanding disease states, and manipulating neural function through interactions with artificial devices such as brain-computer interfaces and neuroprosthetics. KEY TERMS AND CONCEPTS Cochlea: Coiled part of the inner ear where the hearing receptors reside. Electrode: Solid conductor through which electrical current enters or leaves a medium. Motor Cortex: Area of cerebral cortex (outer brain layer) that processes motor information and control movement. Photodiode: Semiconductor component with light-sensitive electrical characteristics. Retina: Light-sensitive layer lining the inner eyeball. Thalamus: Mass of neural tissue situated deep in the brain. Vagus Nerve: Tenth and longest cranial nerve, which passes through the neck and thorax into the abdomen. Visual Cortex: Area of cerebral cortex that processes visual information. Definition and Basic Principles Neural engineering (or neuroengineering, NE) is an emerging interdisciplinary research area within biomedical engineering that employs neuroscientific and engineering methods to elucidate neuronal function and design solutions for neurological dysfunction. Restoring sensory, motor, and cognitive function in the nervous system is a priority. The strong emphasis on engineering and quantitative methods separates NE from the traditional fields of neuroscience and neurophysiology. The strong neuroscientific approach distinguishes NE from other engineering disciplines such as artificial neural networks. Despite being a distinct discipline, NE draws heavily from basic neuroscience and neurology and brings together engineers, physicians, biologists, psychologists, physicists, and mathematicians. At present, neural engineering can be viewed as the driving technology behind several overlapping fields: functional electrical stimulation, stereotactic and functional neurosurgery, neuroprosthetics and neuromodulation. The broad scope of NE also encompasses neurodiagnostics, neuroimaging, neural tissue regeneration, and computational approaches. By using mathematical models of neural function (computational neuroscience), researchers can perform robust testing of therapeutic strategies before they are used on patients. The human brain, arguably the most complex system known to humankind, contains about neurons and several times more glial cells. Understanding the functional neuroanatomy of this exquisite device is a sine qua non for anyone aiming to manipulate and repair it. The neuron doctrine, pioneered by Spanish neuroscientist Santiago Ramón y Cajal, considers the neuron to be a distinct anatomical and functional unit. The extension introduced by American neuroscientist Warren S. McCullogh and American logician Walter Pitts asserts that the neuron is the basic information-processing unit 1279

2 Neural Engineering Applied Science cell body dendrites cell body dendrite cell body dendrites Neuron Types Multipolar Neuron motor and interneurons axon Bipolar Neuron less common found in ears and eyes Unipolar Neuron sensory and connective neurons axon axon of the brain. For neuroengineers, this means that a particular goal can be reached just by manipulating a cell or group of cells. One argument in favor of this view is that stimulating groups of neurons produces a regular effect. Motor activity, for example, can be induced by stimulating the motor cortex with electrodes. In addition, lesions to specific brain areas due to neurodegenerative disorders or stroke lead to more or less predictable clinical manifestation patterns. Background and History Electricity (in the form of electric fish) was used by ancient Egyptians and Romans for therapeutic purposes. In the eighteenth century, the work of Swiss anatomist Albrecht von Haller, Italian physician Luigi Galvani, and Benjamin Franklin set the stage for the use of electrical stimulation to restore movement to paralyzed limbs. The basis of modern NE is early neuroscience research demonstrating that neural function can be recorded, manipulated, and mathematically modeled. In the mid-twentieth century, electrical recordings became popular as a window into neuronal function. Metal wire electrodes recorded extracellularly, while glass pipettes probed individual cells. Functional electrical stimulation (FES) emerged with a distinct engineering orientation and the aim to use controlled electrical stimulation to restore function. Modern neuromodulation has developed since the 1970 s, driven mainly by clinical professionals. The first peripheral nerve, then spinal cord and deep brain stimulators were introduced in the 1960 s. In 1997, the Food and Drug Administration (FDA) approved deep brain stimulation (DBS) for the treatment of Parkinson s disease. An FES-based device that restored grasp was approved the same year. In the 1970 s, researchers developed primitive systems controlled by electrical activity recorded from the head. The U.S. Pentagon s Advanced Research Projects Agency (ARPA) supported research aimed at developing bionic systems for soldiers. Scientists demonstrated that recorded brain signals can communicate a user s intent in a reliable manner and found cells in the motor cortex the firing rates of which correlate with hand movements in two-dimensional space. Since the 1960 s, engineers, neuroscientists, and physicists have constructed mathematical models of the retina that describe various aspects of its function, including light-stimulus processing and transduction. In addition, scientists have made attempts to treat blindness using engineering solutions, such as nonbiological visual prostheses. In 1975, the first multichannel cochlear implant (CI) was developed and implanted two years later. How It Works Neuromodulation and Neuroaugmentation. Neural engineering applications have two broad (and sometimes overlapping) goals: neuromodulation and neuroaugmentation. Neuromodulation (altering nervous system function) employs stimulators and infusion devices, among other techniques. It can be applied at multiple levels: cortical, subcortical, spinal, 1280

3 Applied Science Neural Engineering or peripheral. Neural augmentation aims to amplify neural function and uses sensory (auditory, visual) and motor prostheses. Neuromuscular Stimulation. Based on a method that has remained unchanged for decades, electrodes are placed within the excitable tissue that provide current to activate certain pathways. This supplements or replaces lost motor or autonomic functions in patients with paralysis. An example is application of electrical pulses to peripheral motor nerves in patients with spinal cord injuries. These pulses lead to action potentials that propagate across neuromuscular junctions and lead to muscle contraction. Coordinating the elicited muscle contractions ultimately reconstitutes function. Neural Prosthetics. Neural prostheses (NP) aim to restore sensory or motor function lost because of disease or trauma by linking machines to the nervous system. By artificially manipulating the biological system using external electrical currents, neuroengineers try to mimic normal sensorimotor function. Electrodes act as transducers that excite neurons through electrical stimulation, or record (read) neural signals. In the first approach, stimulation is used for its therapeutic efficacy, for example, to alleviate the symptoms of Parkinson s disease, or to provide input to the nervous system, such as converting sound to neural input with a cochlear implant. The second paradigm uses recordings of neural activity to detect motor intention and provide input signal to an external device. This forms the basis of a subset of neural prosthetics called brain-controlled interfaces (BCI). Microsystems. Miniaturization is a crucial part of designing instruments that interface efficiently with neural tissue and provide adequate resolution with minimal invasiveness. Microsystems technology integrates devices and systems at the microscopic and submicroscopic levels. It is derived from microelectronic batch-processing fabrication techniques. A neural microsystem is a hybrid system consisting of a microsystem and its interfacing neurons (be they cultured, part of brain slices, or in the intact nervous system). Technologies such as microelectrodes, microdialysis probes, fiber optic, and advanced magnetic materials are used. The properties of these systems render them suitable for simultaneous measurements of neuronal signals in different locations (to analyze neural network properties) as well as for implantation within the body. Applications and Products Some of the most common applications of NE methods are described below. Cochlear Implants. Cochlear implants (CI), by far the most successful sensory neural prostheses to date, have penetrated the mainstream therapeutic arsenal. Their popularity is rivaled only by the cardiac pacemakers and deep brain stimulation (DBS) systems. Implanted in patients with sensorineural deafness, these devices process sounds electronically and transmit stimuli to the cochlea. A CI includes several components: a microphone, a small speech processor that transforms sounds into a signal suitable for auditory neurons, a transmitter to relay the signal to the cochlea, a receiver that picks up the transmitted signal, and an electrode array implanted in the cochlea. Individual results vary, but achieving a high degree of accuracy in speech perception is possible, as is the development of language skills. Retinal Bioengineering. Retinal photoreceptor cells contain visual pigment, which absorbs light and initiates the process of transducing it into electrical signals. They synapse onto other types of cells, which in turn carry the signals forward, eventually through the optic nerve and into the brain, where they are interpreted. Every neuron in the visual system has a receptive field, a particular portion of the visual space within which light will influence that neuron s behavior. This is directly related to (and represented by) a specific region of the retina. Inherited retinal degenerations such as retinitis pigmentosa (RP) or age-related macular degeneration (AMD) are responsible for the compromised or nonexistent vision of millions of people. In these disorders, the retinal photoreceptor cells lose function and die, but the secondary neurons are spared. Using an electronic prosthetic device, a signal is sent to these secondary neurons that ultimately causes an external visual image. A miniature video camera is mounted on the patient s eyeglasses that captures images and feeds them to a microprocessor, which converts them to an electronic signal. Then the signal is sent to an array of electrodes located on the retina s surface. The electrodes transmit the signal to the viable secondary neurons. The neurons process the signal and pass it down the optic nerve to the brain to establish the visual image. Several different versions of this device exist and are implanted either into the retina or brain. Cortical 1281

4 Neural Engineering Applied Science visual prostheses could entirely bypass the retina, especially when this structure is damaged from diseases such as diabetes or glaucoma. Retinal prostheses, or artificial retinas (AR), could take advantage of any remaining functional cells and would target photoreceptor disorders such as RP. Two distinct retinal placements are used for AR. The first type slides under the retina (subretinal implant) and consists of small silicon-based disks bearing microphotodiodes. The second type would be an epiretinal system, which involves placing the camera or sensor outside the eye, sending signals to an intraocular receiver. In addition to challenges related to miniaturization and power supply, developing these systems faces obstacles pertaining to biocompatibility, such as retinal health and implant damage, and vascularization. Functional Electrical Stimulation (FES). Some FES devices are commercialized, and others belong to clinical research settings. A typical unit includes an electronic stimulator, a feedback or control unit, leads, and electrodes. Electrical stimulators bear one or multiple channels (outputs) that are activated simultaneously or in sequence to produce the desired movement. Applications of FES include standing, ambulation, cycling, grasping, bowel and bladder control, male sexual assistance, and respiratory control. Although not curative, the method has numerous benefits, such as improved cardiovascular health, muscle- mass retention, and enhanced psychological well-being through increased functionality and independence. Brain-Controlled Interfaces. A two-electrode device was implanted into a 1998 stroke victim who could communicate only by blinking his eyes. The device read from only a few neurons and allowed him to select letters and icons with his brain. A team of researchers helped a young patient with a spinal cord injury by implanting electrodes into his motor cortex that were connected to an interface. The patient was able to use the system to control a computer cursor and move objects using a robotic arm. Brain-controlled interfaces (BCIs), a subset of NP, represent a new method of communication based on brain-generated neural activity. Still in an experimental phase, they offer hope to patients with severe motor dysfunction. These interfaces capture neural activity mediating a subject s intention to act and translate it into command signals transmitted to a computer (brain-computer interface) or robotic limb. Independent of peripheral nerves and muscles, Fascinating Facts About Neural Engineering Even though neuroengineering is still in its infancy, ethical questions are already arising. Will it affect human identity? Could it be used in the future to control thought processes? This is just the beginning. Cochlear implants are a great achievement of modern medicine and represent the most successful of all neural prostheses developed to date. Cell-containing polymer implants that release therapeutic factors hold promise for treating retinal disorders. An exciting new development in antiepilepsy therapy, closed-loop devices record electroencephalograph (EEG) signals, process them to detect imminent seizures, and deliver stimuli to stop them. The limb prostheses of the future will be equipped with multichanneled sensors that send tactile and proprioceptive feedback to the brain, continuously informing it about the effector s function. This approach will improve the patient s sense of ownership of the artificial limb. Scientists developed neuroprostheses that restore urinary bladder function by stimulating the spinal cord or nerves controlling the lower urinary tract. Advances in miniaturization and biosensors are expected to facilitate noninvasive monitoring of neuronal signaling and intracellular environment, thus greatly improving the diagnosis and treatment of nervous-system disorders. In a quest to replace the conventional, inadequate brain stimulation methods, scientists developed neural cells that become active when exposed to light and implemented carbon nanotube-based stimulators. BCI have the ability to restore communication and movement. This exciting technological advance is not only poised to help patients, but it also provides insight into the way neurons interact. Every BCI has four main components: recording of electrical activity, extraction of the planned action from this activity, execution of the desired action using the prosthetic effector (actuator), and delivery of feedback (via sensation or prosthetic device). 1282

5 Applied Science Neural Engineering Brain-controlled interfaces rely on four main recording modalities: electroencephalography, electrocorticography, local field potentials, and singe-neuron action potentials. The methods are noninvasive, semiinvasive, or invasive, depending on where the transducer is placed: scalp, brain surface, or cortical tissue. The field is still in its infancy; however, several basic principles have emerged from these and other early experiments. A crucial requirement in BCI function, for example, is for the reading device to obtain sufficient information for a particular task. Another observation refers to the transparency of action in brain-machine interface (BMI) systems: Upon reaching proficiency, the action follows the thought, with no awareness of intermediate neural events. Deep Brain Stimulation (DBS) and Other Modulation Methods. Deep brain stimulation of thalamic nuclei decreases tremors in patients with Parkinson s disease. It may alleviate depression, epilepsy, and other brain disorders. One or more thin electrodes, about 1 millimeter in diameter, are placed in the brain. An external signal generator with a power supply is also implanted somewhere in the body, typically in the chest cavity. An external remote control sends signals to the generator, varying the parameters of the stimulation, including the amount and frequency of the current and the duration and frequency of the pulses. The exact mechanism by which this method works is still unclear. It appears to exert its effect on axons and act in an inhibitory manner, by inducing an effect akin to ablation of target area, much like early Parkinson s treatment. One major advantage of DBS over other previously employed methods is its reversibility and absence of structural damage. Another valuable neuromodulatory approach, the electrical stimulation of the vagus nerve, can reduce seizure frequency in patients with epilepsy and alleviate treatment-resistant depression. Transcutaneous electrical nerve stimulation (TENS) represents the most common form of electrotherapy and is still in use for pain relief. Cranial electrotherapy stimulation involves passing small currents across the skull. The approach shows good results in depression, anxiety, and sleep disorders. Transcranial magnetic stimulation uses the magnetic field produced by a current passing through a coil and can be applied for diagnostic (multiple sclerosis, stroke), therapeutic (depression), or research purposes. Impact on Industry Neural engineering is a fast-developing bioengineering specialty that is expected to grow tremendously. The increasing societal burden of neurological disorders, and the demand for more sophisticated medical devices, will drive an increase in new careers and employment. A global industry, with cutting-edge research under way in the United States, Europe, and Asia, neural engineering concentrates talent and capital in a network of neurotechnological innovation. Government and University Research. Research in this field is funded through universities and various organizations such as National Science Foundation (NSF) and National Institutes of Health (NIH), including National Institute of Biomedical Imaging and Bioengineering (NIBIB). The Whitaker International Fellows and Scholars Program awards funds to emerging biomedical engineering leaders to conduct projects worldwide. Neurotechnology industry also supports some parts of academic research. Therapeutic approaches approved by the FDA include spinal cord stimulation for pain, DBS for Parkinson s disease and essential tremor, and vagus nerve stimulation in epilepsy and depression. Techniques still at the investigational stage include DBS for depression, epilepsy, headache, Tourette s syndrome, and pain; cortical stimulation in Parkinson s disease, tremor, pain, depression, and stroke rehabilitation; and peripheral nerve stimulation for headache and tinnitus. Industry and Business. With the promise of new treatments for billions of people suffering from nervous-system disorders, neurotechnology is fast becoming the leading recipient of life science venture capital worldwide. The NE and neurotechnology industry includes firms that manufacture neuromodulation devices, neural prostheses, rehabilitation systems, neurosensing devices including electroencephalograph (EEG) systems, magnetic sensing systems, sleepmonitoring equipment, neurosurgical monitoring equipment, and analytical tools. According to Neurotech Reports, the industry revenue is expected to grow to about $8.8 billion by Examples of prominent companies include Medtronic, Cyberonics, Neuro- Pace, and Trifectas Medical. Research endeavors at universities and clinical institutions frequently lead to start-up firms, such as Cyberkinetics Incorporated, a manufacturer of BCI devices. 1283

6 Neural Engineering Applied Science The cochlear implant industry has progressed significantly in the first decade of the twenty-first century, achieving hundreds of millions of dollars in revenue. As hearing devices become more and more affordable and socially acceptable, the industry is expected to develop rapidly. Key players in this industry are Advanced Bionics, Cochlear, and Med-El. Careers and Course Work Most careers in NE require a bachelor s degree in engineering, biology, neuroscience, physics, or computer science. Neuroengineers often undergo formal training in mechanical or electronics engineering, combined with biomedical training. Educational programs in bioengineering, including NE, are growing rapidly in the United States. Typical course work integrates engineering and life sciences studies. Students may receive instruction in neuroengineering fundamentals; chemistry; fluid mechanics; engineering electrophysiology; diagnostic imaging physics; and drug development. In addition to core courses, students take electives related to their ultimate career goals. Subsequently, many pursue a master s or doctoral degree, sometimes followed by one or more years of postdoctoral training. As in other biomedical engineering fields, some graduates go on to obtain a medical, law, or business degree. Neuroengineering researchers are employed by universities, medical institutions, industry (medical devices, pharmaceutical, biotechnology), and governmental agencies. They work as physicians, clinical engineers, product engineers, researchers, managers, or teachers. Social Context and Future Prospects Bioelectrodes for neural recording and neurostimulation are an essential part of neuroprosthetic devices. Designing an optimal, stable electrode that records long-term and interacts adequately with neural tissue remains a priority for neural engineers. The implementation of microsystem technology opens new perspectives in the field. More than 200 million people around the world suffer from hearing loss, mainly because sensory hair cells in the cochlea have degenerated. The only efficient therapy for patients with profound hearing loss is the CI. Improvements in CI performance have increased the average sentence recognition with multichannel devices. An exciting new development, auditory brainstem implants, show improved performance in patients with impaired cochlear nerves. Millions of Americans have vision loss. The need for a reliable prosthetic retina is significant, and rivals the one for CI. Technological progress makes it quite likely that a functioning implant with a more sophisticated design and higher number of electrodes will be on the market soon. The epiretinal approach is promising, but providing interpretable visual information to the brain represents a challenge. In addition, even if they prove to be successful, retinal prostheses under development address only a limited number of visual disorders. Much is left to be discovered and tested in this field. The coming years will also see rapid gains in the area of BCI. Whether they achieve widespread use will depend on several factors, including performance, safety, cost, and improved quality of life. The advent of gene therapy, stem cell therapy, and other regenerative approaches offers new hope for patients and may complement prosthetic devices. However, many ethical and scientific issues still have to be solved. Implanted devices are changing the way neurological disorders are treated. An unprecedented transition of NE discoveries from the research to the commercial realm is taking place. At the same time, new discoveries constantly challenge the basic tenets of neuroscience and may alter the face of NE in the coming decades. People s understanding of the nervous system, especially of the brain, changes, and so do the strategies designed to enhance and restore its function. Mihaela Avramut, M.D., Ph.D. Further Reading Blume, Stuart. The Artificial Ear: Cochlear Implants and the Culture of Deafness. New Brunswick, N.J.: Rutgers University Press, Historical study of implant development and implementation. DiLorenzo, Daniel J., and Joseph D. Bronzino, eds. Neuroengineering. Boca Raton, Fla.: CRC Press, Essential review of neuroengineering developments written by leaders in the field. Durand, Dominique M. What Is Neural Engineering? Journal of Neural Engineering 4, no. 4 (September, 2006). Written by the editor in chief of the journal, who defines NE and its scope. 1284

7 Applied Science Neural Engineering He, Bin, ed. Neural Engineering. New York: Kluwer Academic/Plenum Publishers, Introductory overview of research in neural engineering. Katz, Bruce F. Neuroengineering the Future: Virtual Minds and the Creation of Immortality. Hingham, Mass.: Infinity Science Press, Fascinating introduction to this field, describing the state of the art and speculating on long-term developments. Montaigne, Fen. Medicine By Design: The Practice and Promise of Biomedical Engineering. Baltimore: The Johns Hopkins University Press, Bioengineering (including neuroengineering) applications made accessible to the nonspecialist through vignettes and portraits of researchers. Web Sites Engineering in Medicine and Biology Society International Functional Electrical Stimulation Society (IFESS) National Institute of Biomedical Imaging and Bioengineering Whitaker International Fellows and Scholars Program See also: Biomechanical Engineering; Bionics and Biomedical Engineering; Cell and Tissue Engineering; Computer Science; Electrical Engineering; Nanotechnology; Neurology. 1285

8 NIGHT VISION TECHNOLOGY FIELDS OF STUDY Vision science; optics; photonics; physics. SUMMARY Night vision technology is used to allow for better night vision than is possible with the human eye alone. Night vision technology uses light amplification and thermal-imaging components incorporated into goggles, cameras, binoculars, and other devices to improve vision under low-light conditions. KEY TERMS AND CONCEPTS Cones: Photoreceptors in the human retina responsible for color vision and vision in bright light. Infrared: Electromagnetic energy with wavelengths of 750 nanometers to 1 millimeter, not visible by the human eye. Mesopic Vision: Vision under medium-light conditions, such as twilight. Microchannel Plate: Device made from coated glass that consists of an array of tiny glass tubes used to accelerate electrons while maintaining the entering pattern or image of the electrons. Phosphor: Fluorescent coating used in night vision technology that emits green light when electrons strike the coating. Photocathode: Coated metallic electrode that emits electrons in response to light. Photoelectric: Phenomenon of light in the form of photons striking a metallic surface, which causes electrons to be released. Photonics: Science of light and light particles. Photopic Vision: Vision under bright-light conditions. Photoreceptor: Cell in the human retina that responds to light. Rods: Photoreceptor cells in the human retina responsible for vision under dim light. Scotopic Vision: Vision under low light. Definition and Basic Principles Night vision technology is the use of light-amplifying and thermal-imaging devices to enhance human vision performance in low light. These devices can take the form of cameras, goggles, binoculars, and spotting scopes. This technology takes ambient light and amplifies it through photoelectric techniques or thermal imaging that takes advantage of the energy released in the infrared spectrum in the form of heat. Night vision devices use a photocathode that collects photons, which are light particles present even in dim light. These photons strike a photocathode, which then emits electrons. Photocathodes can be made of a variety of coated metallic materials. These electrons are multiplied by a microchannel plate and then transformed back into green light using a phosphor screen. Green light works well because of the sensitivity of the human eye to these wavelengths. There are variations on this technology, including early night vision systems that project infrared light and then amplify the reflected light. Background and History The groundwork for the development of night vision technology was laid by early scientists such as Heinrich Hertz who described the photoelectric effect in The discovery that electrons are emitted when light strikes metal was further developed by German physicists Max Planck and Albert Einstein in the early twentieth century. Their work confirmed the particle nature of light and provided the foundation for future applications, which included night vision technology. William E. Spicer was a cofounder of the Stanford Synchotron Radiation Lightsource and was instrumental in the development of light amplification. His work paved the way for the first generation of night vision goggles and had applications in medicalimaging technology. Spicer s work provided the basis by which light in the infrared spectrum, which is not visible to the human eye, can be detected, amplified, and transformed into visible green light. All of the night vision devices rely on this basic technology. As a result of the research done by Spicer night vision goggles were developed for use by the military in World War II in the 1940 s. England, Germany, and the United States all developed sniper scopes using infrared cathodes. These devices used an infrared beam to generate reflected light from the 1294

9 Applied Science Night Vision Technology surroundings that were then amplified by the scope. These devices had the disadvantages of low range and the ability of the enemy to detect the infrared beam. Early devices using an infrared beam to create reflected light are called active night vision devices and are referred to as generation zero. Militaries around the world continued to work on improved night vision technology. Generation one devices, the next iteration, improved on the light amplification so that ambient light could be used without the need to use an infrared beam. These systems did not work well on very dark or cloudy nights. Early night vision devices were large and created distortion of images. The Starlight scope used in Vietnam is an example of this generation of devices. Generation zero and generation one night vision devices are now available to the general public. As technology advanced, the next generation of night vision devices became more sensitive by the addition of microchannel plates, which further amplified the signal. A microchannel plate is manufactured from lead oxide cladding glass. Generation two devices have less distortion and increased brightness. Generation three night vision technology incorporates gallium arsenide cathodes, which further increases sensitivity. Generation four devices, which are typically used for military applications, incorporated changes to the microchannel and added gating. Gating is a system that switches on and off to allow for rapid response to changes in light. For example, if night vision goggles are on and then a light is suddenly switched on, the user will be then able to see under the lighted conditions. Thermal imaging has been made possible with improved sensitivity and light amplification and also creates images using infrared wavelengths that are emitted as heat. Not all night vision devices are able to detect thermal energy. How It Works To understand how night vision technology works, it is important to have a basic understanding of light and of how the human eye responds to light. Before the twentieth century, there was an ongoing debate as to whether light was a wave or a particle. Sir Isaac Newton favored a particle theory, which was later substantiated by Henrich Hertz, Max Planck, and Albert Einstein. However, modern understanding of light is that it behaves like both a wave and a particle. For the purpose of understanding night vision technology, it is the photoelectric effect that forms the basis for these devices. When particles of light called photons strike metal, electrons are emitted. Specialized photocathodes are coated with various materials to make them more sensitive. The technical specifications of the photocathodes have improved over the generations since the 1940 s, in part because of the use of different materials and coatings. The function of the photocathode in a night vision device is to convert the light into electrons. In low-light conditions the night vision devices are able to detect infrared light that is not detectable by the human eye. The electrons are then converted into visible light by a phosphor screen, which then converts the electrons back into green light visible to the human eye. Later devices added a microchannel plate, which serves to amplify the electron energy while preserving the pattern or image. The microchannel plate is an array of tiny glass tubes. The electrons enter and are confined in each tube as they travel through, which results in the preservation of their entering pattern. While traveling through the microchannel plate, the electrons are further amplified by the application of voltage across the microchannel plate. This allows for more energy entering the phosphor screen and a subsequently brighter image. Infrared light travels from the environment to the Military organizations have used night vision technology since World War II. In fact, the wars in Iraq and Afghanistan led to a shortage of goggles available to American medical pilots. (AP Photo) 1295

10 Night Vision Technology Applied Science photocathode, where it is translated into electrons, which in turn enter the microchannel plate. The amplified signal then strikes the phosphor screen, which turns the energy into green light that the viewer can see. The human eye is most sensitive to visible light with wavelengths of about 400 to 700 nanometers (nm). Infrared light is in the 700 nm to 1 millimeter (mm) range. Infrared is further divided into nearinfrared IR-A with 750 to 1400 nm wavelength range, medium wavelength IR-B of 1,400 to 3,000 nm range, and long wavelength or far IR-C with wavelengths of 3,000 nm to 1 mm. The long wavelengths are used in thermal-imaging devices. Infrared light is not detected by the human eye, so night vision devices are used to transcribe this light into visible green light. The human eye is particularly sensitive to green light. For example, watt of green light will appear bright, while watt of blue light will appear dim. Applications and Products Military Applications. Military organizations used night vision technology in World War II and continue to be at the forefront of new developments. Military applications include night vision goggles for military personnel, sniper scopes, reconnaissance, and vehicle navigation. The advances that led to thermal imaging were on display in the media during the Gulf War in Those who may have watched the coverage of this war on television will remember the pictures with greenish images and periodic flashes of bright green corresponding to tracers and explosions. Thermal forward-looking imaging (FLIR) devices are installed on vehicles and helicopters. Night vision devices are available to personnel for survival purposes even in a downed aircraft. This technology has continued to be employed in weapons-aiming devices. Data collection and communications technology have been added to some night vision devices in order to improve military communication and reconnaissance. Law Enforcement. Law-enforcement applications are similar to military applications and include surveillance, weapons aiming, recording, and identification of suspects in situations of low light. Thermal imaging is used to identify illegal marijuana-growing operations, which are sometimes located in ordinary urban neighborhoods. The heat lamps used in growing the plants make it possible for law enforcement to identify these operations by air. A helicopter equipped with thermal-imaging equipment can detect an increased heat signature coming from the roof of the house that contains the growing operations. FLIR is also used on law-enforcement vehicles. Night vision technology is also used in searchand-rescue operations by law enforcement and other agencies. Photography. Some photographers are using night vision cameras to create artistic images. To address the green images created by this technology, the photographers employ digital-editing techniques. The resulting images are unique works of art. Recreational Use. Recreational use of night vision technology has expanded as the older generation of devices has become less expensive. Newer generations are still mostly used by the military and law enforcement because of the higher costs of these advanced devices. Spotting scopes, binoculars, and cameras are used by hunters, campers, hikers, and fisherman. Night vision devices are used for wildlife viewing and photography. A unique activity that makes use of night vision goggles is dining in the dark. The servers use night vision goggles to provide a meal for diners who do not have the night vision goggles. The idea is to make the meal more of an adventure and to enhance the dining experience. Some companies use this as a team-building activity. Scientific Research. Scientists use night vision devices to study nocturnal animals and other phenomena that might not otherwise be visible to the human eye. This has opened up a new area of study for wildlife biologists. In some parks, night vision technology is used to study wildlife and vehicle collisions in order to determine ways to reduce these incidents, which are dangerous to both humans and animals. Astronomical research has also benefited from the use of night vision technology. The National Aeronautics and Space Administration (NASA) has used night vision technology to acquire images with the Hubble Space Telescope and the Mars Rovers. This technology is also being offered to amateur astronomers to enhance the images that can be acquired. Impact on Industry The invention of night vision devices has led to an industry of device manufacturers and researchers. 1296

11 Applied Science Night Vision Technology Fascinating Facts About Night Vision Technology William E. Spicer, who is credited with the development of light amplification, suffered from speech difficulties and dyslexia as a child. He credits his dyslexia to his later success, since he believed it allowed him to think about problems in a different way. In some of the Spider-Man comics, this superhero has night vision capabilities. Night vision goggles have also been featured in movies such as Iron Man (2008) and Watchmen (2009). Thermal imaging can be used in accident reconstruction since thermal images can be detected even when there are no visible skid marks. Cosmic rays can be detected by astronauts as flashes of light. Cosmic rays are subatomic particles that originate in space and resemble particles produced by particle accelerators. The dark-adapted human eye undergoes physiological and biochemical changes to become 100,000 times more sensitive than when in bright light. Exposure to bright light reverses these changes and will reduce night vision until the eye is dark adapted again. The retinal cells responsible for night vision are rods, which become most sensitive after thirty or forty minutes. Infrared video cameras can detect the infrared signal from TV remote controls and may also reveal images behind tinted glass. Thermal cameras are also used for home-energy audits and for detection of mold spores. Although a large part of the market continues to be in military applications and law enforcement, night vision technology is being marketed widely to the general public. In addition to the creation of an industry based on the manufacture and sales of night vision devices, the technology of the photocathode and light amplification has impacted numerous fields, including television, astronomy, and medicine. NASA has used infrared imaging to explore the solar system. There are images available online from the Hubble Space Telescope and from the Mars Rovers that were generated using the same techniques that are used for night vision devices. Careers and Course Work Manufacturers and distributors of night vision technology utilize a variety of personnel. For technical jobs in this field, a solid mathematics background is necessary. Understanding of physics, electronics, optics, photonics, and software is important for some of the career paths. Automatic data processing equipment technicians (ADPE) are used in the night vision technology industry. This type of technician will require two years of technical training. ADPE technicians work with engineers and other personnel in a range of activities including assembly, design, and troubleshooting. Software engineers should have a bachelor of science degree in a field such as physics, mathematics, or engineering. Some jobs require a master s degree. Strong computer programming skills are also required. Engineers in a variety of areas are employed in the night vision technology industry. Electrical engineers and manufacturing engineers are two examples. A bachelor s degree and a strong understanding of mathematics are requirements. Fields of study will vary between the various engineering programs. Researchers in the night vision technology will often have a master s degree or doctorate in physics, engineering, mathematics, or other related field. Research in this field is done by universities, industry, and by the military. Social Context and Future Prospects The development of night vision technology has changed the way wars are fought. Before this technology was available most militaries avoided night operations. Militaries have competed to stay on the forefront of night vision technology research in order to maintain a tactical advantage. Night vision technology has been credited with the success of Desert Storm in 1991, giving the U.S. military an advantage in the conflict. As this technology advances into solid-state formats, additional communications and analysis features will be added to allow real-time communication between soldiers. Remote surveillance and reconnaissance using thermal imaging is becoming more widely used. Night vision technology is already being used in the acquisition of astronomic images. NASA is already using thermal and infrared imaging in their missions to Mars. 1297

12 Night Vision Technology Applied Science Thermal-imaging systems are now being marketed for night driving, heavy equipment operators, maritime applications, and pilots. As the costs of these systems decline they will be more widely available for the general public and possibly may eventually become a standard option in passenger vehicles. Ellen E. Anderson Penno, M.D., M.S., F.R.C.S.C., Dip. A.B.O. Further Reading American Academy of Ophthalmology. Clinical Optics. San Francisco: American Academy of Ophthalmology, This volume covers the fundamental concepts of optics as it relates to lenses, refraction, and reflection. It also covers the basic optics of the human eye and the fundamental principles of lasers. Hobson, Art. Physics: Concepts and Connections. 5th ed. Boston: Pearson Addison-Wesley, Includes chapters on light, geometric optics, wave nature of light, and a section on night vision imaging. Kakalios, James. The Physics of Superheroes. 2d ed. New York: Gotham Books, Uses comic-book references to cover basic physics theory. Includes chapters on mechanics, energy (heat and light), and modern physics. Newell, Frank W. Ophthalmology:Principles and Concepts. 5th ed. St. Louis: Mosby, Covers basic eye anatomy, optics, and retinal physiology and biochemistry. Tipler, Paul A., and Gene Mosca. Physics for Scientists and Engineers. 6th ed. New York: W. H. Freeman, Paul Tipler s physics text has been a staple for introductory university physics courses for many years. Chapters cover basic physics concepts including the basic physics of optics and the dual wave and particle nature of light. Web Sites HubbleSite Jet Propulsion Laboratory Mars Exploration Rover Mission Night Vision and Electronic Sensors Directorate See also: Optics; Photonics. 1298