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Lab notes…. Initial results from Inan… note that he has created a ‘picture’ (inverted… like a negative, but a picture) What does this tell us about ‘pixel resolution’?. Pixel resolution. Thermal Imaging. Using ‘Heat’ to create images.
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Lab notes…. Initial results from Inan… note that he has created a ‘picture’ (inverted… like a negative, but a picture) What does this tell us about ‘pixel resolution’?
Thermal Imaging Using ‘Heat’ to create images
The Atmospheric windows for thermal imaging are found at 3-5 microns and 8-14 microns
Thermal Imaging systems are photon detectors… which rely on the particle nature of EMR to record NRG Photon impact is recorded on the sensor. The number of photon impacts is converted to electrical NRG. Various levels of electric current recorded as Digital Numbers (DN) GeHg Mercury doped Germanium InSb Indium Antimony HgCdTe Mercury Cadmium Telluride (MCT or tri-metal) Generally cooled with liquid Nitrogen
Sensitivity curves for sensor components GeHg Mercury doped Germanium InSb Indium Antimony HgCdTe Mercury Cadmium Telluride (MCT or tri-metal)
Kinetic Temp T(kin)actual movement of molecules in a substance…T(kin) is recorded by a device in direct contact with an object. Radiant Temp. T(rad) external reading of internal temp. measured with no contact
Temperature and Emissivity (є) • A theoretically perfect emitter is called a blackbody • Plank’s radiation law… the total amount of EMR given off is directly related to the object’s temp.
Energy distribution curves Wien’s Displacement law: The λ of NRG emitted from an object is directly related to temp. Hotter objects, shorter λ λ max = W/T where W=(Wien’s Constant) and T=Temp Kelvin
Note the peak emission for the sun Note peak terrestrial emission and the associated atmospheric window
Emissivity (є) the measure of an object’s efficiency as an absorber and emitter of NRG. Nothing is a perfect blackbody (a theoretical perfect absorber of EMR and a perfect emitter of EMR at all temps) Nor in anything a perfect ‘whitebody’ (a theoretical perfect reflector of all incoming radiation) Objects are somewhere in between…. Thus the Emissivity (є) of an object/material is an important part of making accurate assessments of temperature.
Kirchoff’s radiation law: (simplified) • Good absorbers are good emitters • є almost equal 1 • Good reflectors are poor emitters • є comes close to 0 • Two materials can have the same kinetic temp, but if their emissivities are different … the radiant temp will be very different. • A metal roof will almost never look ‘warm’ (have a high radiant temp) because it has very low emissivity є
Good absorbers are good emitters • є almost equals 1 • Good reflectors are poor emitters • є comes close to 0
Bright tones indicate warm surfaces (radiant temp) Dark surfaces indicate cool surfaces… or good reflectors…materials with low emissivity e.g. metal The parapets of these buildings are sheathed in metal… making them look ‘cold’
Thermal sensors detect radiation from the surface (the first 50 microns) • The radiant temperature may not be indicative of the internal temp of the whole substance. • e.g. the surface of a body of water can be cooled by evaporation, giving a false reading of the temperature of the entire water column
Once the є of an object is established, it is possible to calculate the kinetic temperature of the material
Other useful terms: Thermal Conductivity- movement of heat through the material Thermal Capacity- measure of a material’s ability to store heat Thermal Inertia- resistance to internal temp change
Thermal Inertia is very important for initial interpretation of thermal images and an important consideration for mission planning. Dry-porous materials- low thermal inertia - rapid temp swings Water- very high thermal inertia- very little diurnal temp swing usually cooler during the day and warmer at night.
Thermal crossover periods: In mission planning, it may be important to avoid the thermal crossover periods. During these times there is little temperature difference between most materials… an image with shades of gray.
Thermal Blooming Often, the sensor becomes ‘saturated’ by very warm spots and can no longer record information… these streaks of no information caused by a saturated sensor are called thermal blooming.
Cross Track Scanner Distortion Many thermal imaging systems are ‘cross track scanners’ The sensor sweeps back and forth beneath the platform… the distortion inherent in this type of sensor is unique.
The’wiggles’ in the roads and other straight features are caused by bouncing and vibration of the platform
Applications of Thermal Remote Sensing: • Intelligence (the ability to see through walls, or to see in the dark) (4th amendment to the Constitution?) • Industrial (thermal plumes, leaks, wastes) • SAR • Looking for water on the landscape (seeps, springs) • Fighting Fires • Stream temps (Salmon habitat) • Residential and commercial heat loss • Hunting ghosts (poltergeists) (http://www.ghosthunterstore.com/raytekphotos.htm)
FLIR: Forward Looking Infra-Red • a simple variant with real time display • video using thermal sensors
Quantitative information is not always necessary (It is not necessary to know the T(kin) for every object for which you have the T(rad) Qualitative analysis is usually sufficient (this is hotter than that).
Temperatures taken with radiant thermometer on 12 Feb 04 29 50 170 115
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