Fundamentals of Measurement Technology (11) Prof Wang Boxiong
Fundamentals of Measurement Technology (11) Prof. Wang Boxiong
4.12 Infrared radiation detection u Infrared radiation, also known as infrared light, is the invisible light outside the red light within the suns light spectrum. Any object, when its temperature is higher than the absolute zero grade (i. e. -273.16C), is said to be in athermal status". The molecules and atoms of a substance in thermal status are in a continuous movement and rotation and an electron transition occurs to produce electromagnetic waves
❑ 4.12 Infrared radiation detection Infrared radiation, also known as infrared light, is the invisible light outside the red light within the sun’s light spectrum. Any object, when its temperature is higher than the absolute zero grade (i.e. − 273 .16C ), is said to be in a “thermal status”. The molecules and atoms of a substance in thermal status are in a continuous movement and rotation, and an electron transition occurs to produce electromagnetic waves
4.12 Infrared radiation detection u Since the wavelength of the electromagnetic wave is outside of that of the visible(red)light, so the light is called infrared ray. when an object loses its thermal equilibrium with the ambient temperature, it emits or absorbs infrared rays. This process is called thermal radiation or infrared radiation The infrared spectrum generally is defined as the range from 0.76 to about 1000um
❑ 4.12 Infrared radiation detection Since the wavelength of the electromagnetic wave is outside of that of the visible (red) light, so the light is called infrared ray. When an object loses its thermal equilibrium with the ambient temperature, it emits or absorbs infrared rays. This process is called thermal radiation or infrared radiation. The infrared spectrum generally is defined as the range from 0.76 to about 1000m
4.12 Infraredradiation detection Gamma and cosmic rays 10 101 102 Ultraviolet Infrared 10 Short radio waves 108 1010三 Commercial TV and FM radio 10 AM radio 1014 Long radio waves 16 1018 f(Hz)×(A)=3X1018A/s=3×108m/s Fig. 4.106 Electromagnetic radiation spectrum
Fig. 4.106 Electromagnetic radiation spectrum 4.12 Infrared radiation detection
4.12 Infraredradiation detection u The relationship between an object's temperature and radiation power are described by stefan -Boltzmann's law W (4.123) where w= net radiant heat transfer or heat flux in w/, o= the Stefan-Boltzmann constant. 5.6697x10-ow.m. K T= thermodynamic temperature in K e- the specific radiance of object (ratio of non-black-body radiance to black-body radiance) The radiant heat flux of an object th Is proportional to the power of its absolute temperatureT
❑The relationship between an object’s temperature and radiation power are described by Stefan-Boltzmann’s law: 4.12 Infrared radiation detection 4 W = T (4.123) where W = net radiant heat transfer or heat flux in 2 W m , = the Stefan-Boltzmann constant, 8 2 4 5.6697 10− − − W m K , T = thermodynamic temperature in K , = the specific radiance of object (ratio of non-black-body radiance to black-body radiance) The radiant heat flux of an object, W , is proportional to the 4th power of its absolute temperature T
4.12 Infraredradiation detection QObject, which absorbs completely radiations of any wavelengths falling on it in any temperatures, is called a blackbody that is, its emissivity 8=I Common objects other than a blackbody have the specific radiance 8<I, in other words, they cannot absorb totally the radiant power on their surfaces. Their emissivities are also smaller than a blackbody and are therefore called gray bodies
❑ 4.12 Infrared radiation detection Object, which absorbs completely radiations of any wavelengths falling on it in any temperatures, is called a blackbody, that is, its emissivity = 1 . Common objects other than a blackbody have the specific radiance 1 , in other words, they cannot absorb totally the radiant power on their surfaces. Their emissivities are also smaller than a blackbody, and are therefore called “gray bodies
4.12 Infrared radiation detection u The plancks law describes the radiation intensity distribution for an ideal radiator (blackbody ) under different temperatures W (4.131) T in which W,the energy emitted by wavelength in Wm.um the 1st radiation coefficient, CI=374.15 MWum/m2 C2-the 2nd radiation coefficient, C2=14388 umK T-the absolute temperature, in K λ- - the wavelength,inm
❑The Planck’s Law describes the radiation intensity distribution for an ideal radiator (blackbody) under different temperatures: 4.12 Infrared radiation detection − = 1 2 5 1 T C e C W （4.131） in which W —the energy emitted by wavelength ，in −2 −1 Wm m ； C1 —the 1st radiation coefficient， 4 2 1 C = 374.15MWm / m ； C2 —the 2nd radiation coefficient，C2 = 14388m K ； T —the absolute temperature，in K； —the wavelength，in m
4.12 Infraredradiation detection If we should heat an ideal radiator to various temperatures and determine the relative intensities at each wavelength, we would obtain characteristic energy-distribution curves such as those shown in Fig. 4.107. Not only is the radiation intensity of the higher-temperature body increased, but the wavelength of maximum emission is also shifted toward shorter waves(from red toward blue)
• If we should heat an ideal radiator to various temperatures and determine the relative intensities at each wavelength, we would obtain characteristic energy-distribution curves such as those shown in Fig. 4.107. Not only is the radiation intensity of the higher-temperature body increased, but the wavelength of maximum emission is also shifted toward shorter waves (from red toward blue). 4.12 Infrared radiation detection
4.12 Infraredradiation detection 90K 1800K 700F 600 500K 0 Fig. 4.107 Graphical representation illustrating the basis for Wiens displacement law
Fig. 4.107 Graphical representation illustrating the basis for Wien’s displacement law 4.12 Infrared radiation detection
4.12 Infrared radiation detection For a nonideal body, the intensity distribution must be multiplied by the emissivity a. the wavelength of peak intensity is given by the Wien's displacement law 2898/T (4.132)
❑ 4.12 Infrared radiation detection For a nonideal body, the intensity distribution must be multiplied by the emissivity, . The wavelength of peak intensity is given by the Wien’s displacement law: 2898/T (m) max = （4.132）