Suresh

SATELLITE REMOTE SENSING

DIURNAL HEAT EFFECTS, THERMALPROPERTIES OF OBJECTS, THERMAL SENSORS AND THERMOGRAPHY

Diurnal Heating Effects

It has been implied earlier in this Section that thermal images will vary considerably in appearance depending on whether they are acquired during the warm part of the day or after a night of absence of the Sun and resultant cooling of the atmosphere as well as heat loss from the surface and shallow depths beneath.

This is evident in these two images of central Atlanta, GA. taken during the day (left, or top) and then just before dawn (right, or bottom). The thermal sensor was flown on an aircraft.

Contrasts between heated buildings This is evident in these two images of central Atlanta, GA. taken during the day

Heated Areas In Shadow

In the day thermal image, contrasts between heated buildings and streets and areas in shadow create a scene that resembles an aerial photo. But in the dawn image, differences in temperature have decreased sharply (no shadows), although a part of that image is brighter, representing a local "heat island" effect. Also, in the dawn image, many streets are evident because, being asphalt paved, they absorb more heat and remain warmer through the night.

The "heat island" effect is commonplace over urban areas and towns. Activities of people, such as heating or cooling of homes and operation of autos, add thermal energy to the air. Temperatures can be several degrees (F or C) hotter than rural areas. This holds for both day and night. A pair of Landsat-7 images - the upper true color, the lower a color-coded map of temperature variations as measured by the thermal band on the ETM+ sensor - illustrates this effect for Atlanta, for an observation made on September 28, 2000.

Satellite thermal Image (Landsat)

Unlike remote sensing of reflected light from surfaces in which only the topmost layers (a few molecular layers thick) are involved,

Thermal remote sensing includes energy variations extending to varying shallow depths below the ground surface.

This takes time and is the normal consequence of heating during the day and cooling at night.

The most critical consideration in analyzing and interpreting thermal data and imagery is that of knowing the physical and temporal conditions that heat the near surface layers.

Over the seasons, minor shifts in the mean temperature in bedrock can occur to depths of 10 m (33 ft) or more.

Solar radiation and heat transfer from the air significantly heat materials at and immediately below the surface during the day.

Temperatures usually drop at night primarily by radiative cooling (maximum radiative cooling occurs under cloudless conditions), accompanied by some conduction and convection.

During a single daily (diurnal) cycle, the near surface layers (commonly, unconsolidated soils) experience alternate heating and cooling to depths typically between 50 and 100 cm (20-40 in). The daily mean surface temperature is commonly near the mean air temperature.

Observed temperature changes are induced mainly by changes during the diurnal heating cycle, but seasonal differences in temperature and local meteorological conditions also affect the cycle response from day to day.

Changes in radiant temperatures of five surface-cover types during a 24-hour thermal cycle. From F.F. Sabins, Jr., Remote Sensing: Principles and Interpretation. 2nd Ed., © 1987. Reproduced by permission of W.H. Freeman & Co., New York City.

Changes In Radiant Temperature In Surface

The curves shown here summarize the qualitative changes in radiant temperature during a 24-hr cycle, beginning and ending at local midnight, for five general classes of materials found at the surface.

From these curves we can estimate the relative gray levels that a thermal sensor could record, as a function of the material and the time of day.

Given two thermal images of the same locale, taken 12 hours apart, about noon and about midnight, we might determine the identities of co-registered pixels, based on their temperatures and thermal inertias.

Differences in thermal inertia, reflectance, and emissivity of various materials and variable atmospheric radiance are important factors that modify the measured

surface temperatures. Consider the four sets of curves plotted below:

materials and variable atmospheric radiance

The top curves (A) show temperature variations resulting solely from differences in thermal inertias of materials, with other factors held constant.

Note the distinct crossover points. Values of P much higher than 0.05 (commonly, metallic objects) produce diurnal curves that approach a straight line passing through the crossover points.

This effect is consistent with the earlier statement that materials with high thermal inertias undergo smaller radiant temperature changes during a full heating/cooling cycle.

Curves in B show the effects of different reflectance's the ratio of reflected solar radiation to incident radiation, in percent.

The maximum and minimum daily temperatures and their differences increase with decreasing reflectance of solar insolation.

In C, the curves represent changes associated with different emissivities; where most natural materials have values of ranging from 0.80 to 0.98.

Curves in D indicate the temperature effects introduced by the atmospheric radiances, which are caused by re-emission from water vapor, a common source of error. From these sets of curves, we learn that natural surface materials show considerable modifications in radiant temperatures because of these variables.

cont.…

Regardless of the time of day, temperature profiles in the heating zone converge on a nearly steady value at some depth below 30-50 cm (depending on whether it is rock or soil, and its moisture content), but that convergent temperature slowly increases with depth into the Earth as the geothermal gradient takes over.

The least variation in temperature The maximum and minimum temperatures at the observed surface

(topmost centimeter or so) depend mostly on the extent of buildup of the heat reservoir and its storage capacity through the affected layers.

For materials that have the same density and albedo but different conductivities and/or specific heats, the difference in predawn (lowest T values) and midday (highest T values) increases with increasing conductivity and decreasing specific heat.

The actual magnitude of the difference decreases with depth, down to the stable (convergent) value at the base.

The effect of increasing the material density (holding other variables constant in P = (ρ Kc)1/2) is to require more thermal energy to heat the additional mass in a given volume, so that less heat transfers to lower layers.

With increasing density, the total added heat (derived from Sun and air) distributes over a decreased thickness of surficial layers. So, in this instance, the maximum T reached at midday is lower, and the minimum at night is greater.

the spread of daily temperature extremes tends to be lower relative to less dense materials.

The corresponding rise in thermal inertia for denser materials simply expresses their increased resistance (sometimes referred to as thermal impedance) to temperature changes (smaller as heating or cooling progresses).

For materials with higher thermal conductivity's, more heat transfers to greater depths, less remains concentrated at the surface, and, again, temperature extremes diminish, as evidenced by lower daytime surface temperatures and higher nighttime temperatures compared with materials having lower Ks (e.g., soils).

cont…

Thermal properties of objects

Emissivity:

• Emissivity(e) of a material is defined as the ratio of radiant flux emitted from the real body to radiant flux emitted by a reference black body at the same temperature.

• The emissivity of a real body means the emitting ability of a real body as compared to that of a black body.

• Emissivity is a measure of material’s ability to absorb and radiate energy. • Emissivity describes how efficiently an object radiates energy compared to a

black body.• Water is the only object whose emissivity is constant under different

conditions.• Emissivity depends upon temperature, emission angel and wave length.• Emissivity of true black body is e=1.emissivity of real object e<1.• Emissivity for all selectively radiating bodies is from 0 to less than 1.It

fluctuates according to the wavelength of energy.

•A grey body is defined as a body with constant emissivity over all wave-lengths and temperatures.•Emissivity of a Non-black body depends on surface geometry and the chemical composition. An object which does not absorb all incident lightwill also emit less radiation than an ideal black body. s•Surface emissivity is generally measured by assuming emissivity =1-reflectivity•A perfect specular surface may reflect 98% of the incident energy absorbing the remaining 2%. This is reverse in black body. emissivity of selective materials

Material Emissivity (mu m)

Granite 0.185Quartz sand, large grains 0.914Asphalt paving 0.959Concrete walkway 0.966Water with thin film of oil 0.972Pure water 0.993Polished metal surface 0.060

Kinetic Heat, Temperature, Radiant Energy And Radiant Flux

•A body in thermal state can be expressed by two temperature

1.internal temperature or kinetic temperature T (kin)2.External temperature or radiant temperature T (rad)

•Internal temperature is due to the motion of its atom and it is measuredby thermometer.•External temperature is measured remotely by a radiometer.•Kinetic heat refers to energy generated due to random motion of Particles of matter. All objects having temperature above absolute 0Exhibits this random motion. Internal kinetic heat of an object is also converted to radiant energyoften called external or apparent energy.Both T (kin) and T (rad) exhibits high correlation.Radiant temperature always being slightly less than the true kinetic temperature of the object due to thermal property called emissivity.Radiant flux is the flow of radiant energy from a square surface area ofA black body per second. It is commonly measured as watts.

The emissivity of a object is influenced by a number of factors which Includes1.Color: Darker colored objects are better absorbers and emitters than the lighter colored ones. 2.Surface roughness The greater the surface roughness of object relative to the size of theIncident wavelength, the greater the surface area of the object and Potential for absorption and re-emission of energy.3.Moisture content: The moisture content object has greater ability to absorb energy and become a good emitter. Ex:: wet soil particles.4.Compaction: The degree of soil impaction can affect emissivity.5.Field of View: The emissivity of a single leaf measured with very high resolution thermal radiometer will have a different emissivity than an entire treeCrown viewed using a coarse spatial resolution radiometer.

Thermal conductivity

Thermal conductivity (k) is the intrinsic property of a material which indicates its ability to conduct heat.Conduction of heat refers to transfer of energy within a material without any motion of the material as whole.Conduction takes place when a temperature gradient exists in a solidmedium.The rate of heat transfer depends upon the temperature gradient and the thermal conductivity of the material.It is measured as calories.Heat flows in the material in the direction of decreasing temperature.Energy is transferred from the more energetic to less energetic Molecules when neighboring molecules collide. K=Q/t*L/A*triangle TQ=quantity of heat transmitted.L=thickness of a materialA=surface area of the materialTri T=temperature gradient

Thermal capacity

tThermal capacity c is the number of calories required to raise the temperature of 1 gm material by 1degree centigrade.

It also represents the ability of a material to store heat.

Water has the highest thermal capacity of 1.01. The temperature of a lake usually varies very little between night and day. water has a veryhigh thermal capacity compared to other materials.

C=rou c

Rou= density of materialc = product of the density

Thermal inertiaThermal inertia p is a measure of the thermal response of a materialto temperature changes.

P=(kpc)1/2[J m-2 k-4 s -1/2]K=thermal conductivityRou=densityC=thermal capacity

The higher the density, higher is the thermal inertia. Materials havinghigh thermal inertia are cooler in day time and warmer at night.Materials having low thermal inertia are warmer in day time andCooler at night.Thermal inertia represents the ability of a material to conduct and store heat. Thermal inertia predominantly depends upon the physical propertiesof the near surface materials such as particle size, degree of Indurations, rock abundance and exposure of bedrock.Thermal inertia is a measure of the geological properties of the surface on the same depth scale.

Lowest thermal inertia probably represents loose, fine surface dustand very few rocks.

Medium thermal inertia probably represents a combination of coarseloose particles, crusted fines, a fair number of scattered rocks, andPerhaps a few scattered bedrock outcrops.

Higher thermal inertia is a combination of coarse sand, dune sand,Strongly-crusted fines, abundant rocks, and scattered bedrockExposures.

Thermal sensors use photo detectors sensitive to the direct contact of photons on their surface, to detect emitted thermal radiation. The detectors are cooled to temperatures close to absolute zero in order to limit their own thermal emissions.

Thermal sensors….

thermal sensors…. Quantum and photons are used for this purpose. These detectors are capable of very rapid (less than 1nano

seconds) response. They operate on the principle of direct interaction between

photons of radiation incident on them and the energy level of

electrical charge carriers within the detectors. For maximum sensitivity detectors must be cooled to

temperatures approaching absolute zero to minimize their

own thermal emission.

Thermal imagers are typically across-track scanners that detect emitted radiation in only the thermal portion of the spectrum.

Thermal sensors employ one or more internal temperature references for comparison with the detected radiation, so they can be related to absolute radiant temperature.

The data are generally recorded on film and/or magnetic tape and the temperature resolution of current sensors can reach 0.1 °C.

Thermal sensors essentially measure the surface temperature and thermal properties of targets.

Because energy decreases as the wavelength increases, thermal sensors generally have large IFOVs to ensure that enough energy reaches the detector in order to make a reliable measurement. Therefore the spatial resolution of thermal sensors is usually fairly coarse, relative to the spatial resolution possible in the visible and reflected infrared.

Thermography

Thermography, thermal imaging, or thermal video, is a type of infrared imaging. Thermographic cameras detect radiation in the infra red range of the electromagnetic radiation (roughly 900–14,000 nanometres or 0.9–14 µm) and produce images of that radiation.

Since infrared radiation is emitted by all objects based on their temperatures, according to the black body radiation law, thermography makes it possible to "see" one's environment with or without visible illumination.

Image of a small dog taken in mid-infrared ("thermal") light (false color)

Finally, let's take a quick peek at two aspects of military use - here the appearance of a tank and a supply truck operating actively at night during maneuvers.

Individuals in everyday clothing appear as distinct temperature variants. Their faces usually are warmer (reds) than their outside clothes (in cooler greens and blues):

Thermographic image of a traditional building in the background and a ‘passive house' in the foreground

Difference between IR film and thermography

IR film is sensitive to temperatures between 250 °C and 500 °C while thermography is sensitive to approximately -50 °C to over 2,000 °C.

So for a IR film to show something it must be over 250 °C or be reflecting infrared radiation from something that is at least that hot.

Night vision goggles normally just amplify the small amount of light that is available outside like starlight or moon light and can't see heat or work in complete darkness

Advantages of Thermography

You get a visual picture so that you can compare temperatures over large area It is real time capable of catching moving targets Measurement in areas inaccessible or hazardous for other

methods

Limitations & disadvantages of thermography

Quality cameras are expensive and are easily damaged Images can be hard to interpret accurately even with experience Accurate temperature measurements are very hard to make

because of emissivities Training and staying proficient in IR scanning is time consuming

Condition monitoring Medical imaging Research Process control Non destructive testing Chemical mapping

Applications….

References lillisand t.m and r.w. kiefer remote sensing and image interpretation(1994) anand p.h remote sensing &gis www.rst.gsfc.nasa.gov https://en.wikipedia.org/wiki/Thermography Lillisand T.M And R.W. Kiefer Remote Sensing And Image

Interpretation(1994 Anand Ph. Remote Sensing &Gis www.rst.gsfc.nasa.gov Remote Sensing a Tool for Environmental Observations Steven M. de Jong

(Ed.) Raymond Sluiter, Maarten Zeijlmans ,Elisabeth Addink January 2005