적외선이란? (1) 이론적배경임의의광원 ( 물체 ) 에서빛 ( 에너지 ) 이나올때측정량이물리적에너지에관련된양이면단위명앞에 복사 (radiation)" 라는용어를사용하여정의하고, 사람의눈이감지하는빛의양은 광 (light)" 이라는용어를, 파장에따른측정량은 분광 (spectral)" 이란용어를사용한다. 사람의눈이빛을느끼기위해서는 0.38~0.77 μm파장영역내에서충분한세기의전자파가눈에들어와야하며그외의파장영역 의빛은아무리충분한세기로들어와도감지하지못한다. 일반적으로상온정도 (300 K = 27 ) 의물체가발하는에너지영역은가시광선 (visible light) 의스펙트럼 영역과는거의독립적이기때문에눈에보이지않는다. 적외선은 0.77~1,000 μm파장대역으로가시광선과 Radar/Radio 파장영역사이 에존재하는전자파이다. 그림 1 에가시광선및적외선스펙트럼의분포를나타냈 다. 모든물체는적외선을내놓고있다. 그림 1 가시광선및적외선의스펙트럼분포 (Visible and Infrared spectrum.) 영하 273 이상의모든물체는물질을이루고있는기본단위인원자들이미소한 진동을하고있다. 이러한원자들의진동에너지가적외선영역의에너지와동일 www.p-optechs.com 1
하기때문에모든물체는적외선이나오고있는것이다. 그리고온도가높으면높을수록더많은양의적외선이발산하게되는데이러한이유때문에적외선을열선 ( 熱線 ) 이라고도부른다. 그러나모든물체에서동일한적외선이나오는것은아니다. 적외선을보다잘내놓는물질이있는가하면그렇지못한물질도있다. 일반적으로세라믹계열인벽돌, 진흙, 도자기, 황토등에서많이나오며, 금속물질인금, 은, 구리, 철등에서는별로나오지않는다. 이것을방사율 (emissivity) 이라한다. 방사율은 0~1 까지의값을가지는것으로, 이상적인물체의경우방사율은 1이된다. 이러한물체를흑체 ( 黑體, blackbody) 라고부른다. 세라믹계열의재료들의방사율은대부분 0.9 이상의값을가지며, 금속계통은 0.5 이하의값을가진 다. 적외선과온도와의관계를알려면흑체의적외선방사현상을이해해야한 다. 이물리현상은 1900년대초창기에여러물리학자들에의해연구되었으며, Planck라는과학자에의해체계적으로정립이되었다. 흑체란색깔이검다는것을의미하는것이아니라빛이흑체내부로들어가게되면다시는밖으로나오지않아결국흑체내부에서완전히흡수됨을뜻하는것이다. 이것을다른말로풀이하면흑체에열을가하면이열은모두흑체에흡수되어흡수된열은적외선이라는형태로손실없이나온다는것을뜻한다. 흑체와온도의관계는그림 2와같다. 그림 2 특정온도를갖는흑체의분광복사에미턴스 (Radiant existence of blackbody as function of temperature) www.p-optechs.com 2
가로축은파장 ( 波長, wavelength) 을나타내며, 세로축은적외선복사량을나타낸다. 즉, 온도가올라가면적외선이나오는파장의최대값은가시광쪽으로이동하고보다많은양의적외선이나오며, 온도가낮을수록파장의최대값은긴파장쪽으로이동하며나오는적외선의양도작아지는것을의미한다. 그리고 300 K(27 ) 인상온에서는파장의최대값이 10 μm근처이며, 500 K (223 ) 의온도에서는 5 μm근처에서최대값을가진다. (2) 적외선의분류와응용 대개 0.77~3 μm까지를근적외선 (near infrared), 3~6 μm까지를중적외선 (mid infrared), 6~15 μm까지를원적외선 (far infrared) 라고하고 15 μm이상 1 mm 까지 를근원적외선 (extreme infrared) 이라고분류한다. 적외선을여러가지영역으로 나누어부르는데는다음과같은이유가있다. 적외선은전자 (electron) 와같이파동이면서입자인성격을가지고있기때문에, 물체에서나온어떤영역의적외선은공기중의입자와충돌하거나흡수된다. 그림 3에흡수및충돌을일으키지않고대기를무사히통과하는대기투과창을나타내었다. 그림 3 대기투과특성의예 (1.8 km 거리 ) (Fig. 3. Atmospheric transmission through 1.8 km at sea level) www.p-optechs.com 3
그림 3 에서가로축은파장을의미하며, 세로축은대기를투과하는정도를의미한 다. 그림에서보듯이바로근적외선, 원적외선그리고근원적외선영역이대기를 무사히통과함을알수있다. 그리고중적외선은온도가 500K 에해당하는적외선 이최대로나오는파장영역이며, 원적외선은 300K 에해당하는적외선이최대로 나오는영역이다. 우리생활과밀접히관련된파장영역은바로원적외선이며민 수용으로많이응용되고, 중적외선은비행기나미사일의연기에서나오는온도와일치하기때문에군수용으로사용되고또한근적외선영역은광통신에응용되고있다. 불이났을때불에탄후연기가자욱한가운데서는현장에생존자나상황판단을할수없지만적외선센서를통해서보면연기가없는것과같은효과로현장의상황을판단할수있기때문에신속한응급조치가가능할것이다. 집적회로기판의이상여부를판별하기위해적외선으로보면이상이있는부위는열이나기때문에잘못된위치를쉽게찾을수있다. 온도가매우높은전기로또는용광로등에서온도를읽고자할경우기존의열전대 (thermocouple) 를용광로에넣는순간 높은온도때문에녹아내린다. 그러므로비접촉식인적외선이외에는방법이없 다. 그이외에도전기배선의이상유무등열이발생하는모든분야에는응용이 가능하다. 표 1 에적외선응용분야를크게군수분야, 산업분야, 의학분야, 과학분 야로쓰이는목적에따라나누어열거했다. 표 1 적외선응용분야 (Table 1. Infrared application matrix-from Intrared System Engineering, by Richard D. Hudson Jr., John Wiley & Sons, 1969) Military Industrial Medical Scientific Search Track Range Bomber defense Missile guidance Navigation and flight control Proximity fuses Ship, aircraft, ICBM, and mine detection Fire control Aircraft collision warning Forest fire detection Guidance for fire-fighting missiles Fuel ignition monitor Locating hidden law violators Monitoring parking meters Detect fires in aircraft fuel tanks Radiometry Target signatures Detection of hot boxes on railroad cars Noncontact dimensional determination Process control Measurement of the temperatures of brake Obstacle detection for the blind Measurement of skin temperature Early detection of cancer Monitor healing of wounds and onset of infection without Satellite detection Space vehicle Navigation and flight control Horizon sensors Sun followers for instrument orientation Studies of the optical structure of the horizon Measurement of lunar planetary and stellar temperatures Remote sensing of weather conditions Study of heat transfer in plants www.p-optechs.com 4
Spectroradiometry Thermal imaging Reflected flux Cooperative source Terrain analysis Poison gas detection Target and background signatures Fuel vapor detection Detection of contaminants in liquid oxygen piping Reconnaissance and surveillance Thermal mapping Submarine detection Detection of underground missile silos, personnel, vehicles, weapons, cooking fires, and encampments Damage assessment Night driving Carbine firing Area surveillance Camouflage detection Station keeping Docking and landing Terrestrial communications Command guidance for weapons Countermeasures for infrared system Drone command link linings, power lines, cutting tools, welding and soldering operations, and ingots Detection of clear-air turbulence Analysis of organic chemicals Gas analysis Determination of alcohol in the breath Discovery of leaks in pipelines Detection of oil in water Control of oxygen content in germanium and silicon Nondestructive testing Inspection Locating piping hidden in walls and floors Inspection of infrared optical materials Detect and display microwave field patterns Industrial surveillance and crime prevention Examination of photographic film during manufacture Detection of diseased trees and crops Traveling matte photography Automatic focusing of projectors Automobile collision prevention Traffic counting Radiant heating and drying Data link Intervehicle speed sensing Aircraft landing aid Cable bonding removing bandages Remote biosensors Studies of skin heating and temperature sensation Detection and monitoring of air pollution Determination of carbon dioxide in the blood and in expelled air Early detection and identification of cancer Determination of the optimum site for an amputation Localization of the placental site Studies of the efficiency of Arctic clothing Early diagnosis of incipient stroke Measurement of papillary diameter Location of blockage in a vein Monitoring eye movements Study the nocturnal habits of animals Examination of the eye through corneal opacities Monitor healing processes Ranging and obstacle detection for the blind Heat therapy Measurements of earth's heat balance Determination of the constituents of earth and planetary atmospheres Detection of vegetation or life on other planets Terrain analysis Monitor spacecraft atmospheres Zero-G liquid level gauge Measurement of magnetic fields Earth resources surveys Locate and map the gulf stream Detect forest fires from satellites Study volcanoes Detect and study water pollution Locate crevasses Sea-ice reconnaissance Petroleum exploration Detection of forgeries Determine thickness of epitaxial films Determination of the surface constituents of the moon and the planets Gem identification analysis of water quality Detection of diseased crops Space communications Understand the mechanism of animal communication Peripheral input for computers Study the nocturnal habits of animals Terrain illumination for night photography www.p-optechs.com 5
참고문헌 Robert E. Fischer/Biljana Tadic-Galeb. Optical System Design. New York: McGraw-Hill Company, 2000 Rudolf Hartmann/Warren J. Smith et al. Infr ared Optical Design and Fabrication, Critical Reviews of Optical Science and Technology, V 38, 1991 Max J. Riedl, Optical design fundamentals for infrared systems, SPIE PRESS, Washington, 2001 정용택, 적외선센서의응용과원리, 국방과학연구소 홍석민, 열상장비개요, 물리학과첨단기술, 1998 www.p-optechs.com 6