Emissivity and Remote Sensing

Transcription

1 Emissivity and Remote Sensing Introduction: In the last laboratory with reflectivity we touched on the subject of emissivity. In comparison to reflectivity, emissivity is actually a bit more complex from a physics point of view. In the following diagram, the overall concepts of reflectivity and emissivity are presented in simplified form: Where does emissivity come into the diagram? Emissivity is defined as the ratio of absorbed radiation energy to total incoming radiation energy compared to a theoretical blackbody (which absorbs everything), and thus is a measure of absorptivity. In physics, energy is neither created or destroyed and incident photon energy is divided into a lower energy photon, which is re-emitted, and into molecular collisions and movement which is called, interestingly, nonradiative energy. Collisions continue to result in emission of lower wavelengths until motion is dissipated. The measurable part of emissivity is when sensors can pick up on re-radiated wavelengths (IR).

2 A number of atomic substances are identified by individual wavelengths (IR, visible and UV) that are absorbed and then immediately re-emitted in the same wavelength when electrons jump to the next orbital level and then fall back to baseline, with little chance of collision and vibrational (non-radiative) consumption of energy. These patterns are called absorption and emission spectra- the visible emission spectrum of most atoms appears as a set of characteristic color bands at distinct wavelengths. Molecules often do not have the same sharp spectra due to collision incidents or deviated emission pathways, resulting in Stokes shift and colored bands of absorption. courtesy Absorption and emission spectra of molecular perylene

3 Solar radiation is well-documented in terms of wavelength emission, all of which are incident on the earth s surface. boojum.as.arizona.edu/.../spectra/spectra.html The heating of the atmosphere is due to absorption by greenhouse gases of re-radiated IR radiation from the earth s surface. These atmospheric molecules then re-radiate part of this energy as longer wavelength IR in all directions, including back to the earth s surface. IR radiation is often called heat radiation because it results in vibration of molecules and higher temperature (hν = mv 2 /2 = mcδt). atlas.nrcan.gc.ca/.../figure_4.jpg/image_view

4 Note below that earth s continuous radiation spectrum shows mostly radiation in the infrared region, as expected. Measurement of Emissivity: Of interest is the fact that temperatures of different materials in thermal equilibrium in the same location are all the same, but infrared radiation differs by material. As we have seen, substances absorb different frequencies and radiate out lower frequencies due to collision consumption. Emissivity is also dependent on the temperature of the object and the incident wavelength of light on the material. This is of use in our laboratory, where we are looking at emissivity which varies by material properties, not temperature or incident wavelength. In real life, on a global scale, temperatures are not the same for different substances and objects, and since emissivity is also dependent on temperature, satellite images give different values based on type of material and relative temperature. Thermal (infrared) thermometers have detectors that typically measure in the 0.7 to 20 micron range. All are non contact thermometers. Ideally, a good thermal sensor will detect a wide range of wavelengths in the IR and keep its own thermal emissions to a minimum. Internal temperature references allow the sensor to assign a temperature to the incoming IR radiation. courtesy

5 Contact thermometers measure thermal energy by conduction, i.e., vibrational energy passed on to the thermometer, draining energy from the material to set thermometer molecules in motion. The two substances equilibrate close to the original temperature of the material being measured. Each will continue to give off characteristic infrared radiation, however, since vibrational patterns for each will differ. Satellite measured emissivities are translated at times into gray tones, where higher emissivity values are represented by lighter grays and lower values by darker grays, or the entire set of shades are assigned false colors. earth.esa.int/ers/article_archive/etna html (Mt. Etna plume) (Landsat)

6 Laboratory- emissivity of common materials Procedure: 1. Students will use a laser infrared sensor, which when aimed at various materials will give the radiation level, or emissivity of the material. 2. The sensor can be aimed by students at each material at approximately the same height, or set up in a more accurate way by attaching the sensor to a ring stand by clamp. 3. Place each material to be tested on a platform or in a Petri dish on the platform if there is the potential of a mess. 4. The sensor trigger is pulled and a red laser guide should appear on the materialhold until digital readout is stable and release and the sensor will register IR radiation as temperature. 5. Record temperatures and compare to literature values.

7 There are characteristic emissivity values published for a number of materials. Typically, materials with high specific heats tend to be better absorbers/emitters. Emissivity is measured as a decimal value, since 1 represents 100% absorption of incident radiation. A caution about literature values: Incident wavelength and target temperature make a difference. Values listed below did not reference either of these for the most part. The dry and wet soil, skin, snow and electrical tape values were referenced at an incident wavelength range of 3.4 to 5 microns and temperatures well beyond room temperature. Thus laboratory values may differ considerably. We compared our temperatures to a blackbody temperature of our own device- wet charcoal. Literature values: Infrared-USA.com; Material Asphalt Red brick Lime clay brick Glass Clay Literature lab emissivity

8 Clay tiles 0.33 Cloth 0.95 Copper, polished 0.07* Copper, oxidized 0.87 Black elec tape 0.97 Granite-rough 0.90 Grass 0.98 Ice Iron, not oxidized 0.05 Iron, oxidized 0.74 Limestone Paper Plastic 0.95 Soil, dry 0.92 Soil, wet 0.95 Sand 0.90 Sandstone 0.67 Snow 0.80 to 0.90 Straw Water Wood Wood (sawdust) 0.75 Zinc (polished) 0.02* *Shiny metal values must be calculated as the difference in IR thermometer readings between bare metal and metal covered with a thin black coat of paint. Be careful to cover the entire sample of metal with black paint or attenuated reflectance may still reach the IR sensor on the thermometer. Questions 1. How do the laboratory values differ from literature values? If there are differences, an explanation for the differences. 2. How do metals behave in terms of absorption and suggest a reason for their overall Behavior (note the low emissivities). Look up the specific heat of copper and water. How do specific heats seem to correlate with absorption? 3. Note the two images for the westward ocean current off the east coast of the U.S. What do you think the false colors represent in terms of thermoclines?

9 rs.gso.uri.edu/amy/feature.gif Gulf Stream June 1988 Gulf Stream January How can information on temperature of water (currents, etc.) help us? (Hint: Think of fishing, shipping, environmental problems): 5. Is there a difference in the terms emissivity and emittivity? 6. Laboratory experiments for emissivities of materials depend on holding temperature, incident wavelength and area of testing constant. The area is held constant by holding the thermometer at a constant length from the target. The instrument in our lab expresses the following target areas for a corresponding length: 5 cm: cm 2 10 cm: 0.25 cm 2 20 cm: 0.50 cm 2 Is this a linear relationship? Demonstrate how you know this. Project what the area is for the thermometer held at 12 cm from the target.

10 6. Compare the two USDA maps below and speculate as to how this area of North Dakota changed from 2000 to In these maps crop areas are colored red. What could be the cause of the differences? How do land thermal images help us? (Hint: Think of crops, weather, land use): 7. Locate a pair of thermal images for any area and describe the differences and possible causes. You might want to investigate slash and burn agriculture in South America or Africa, desertification in Middle East or western U.S., etc. Here is a list of possible web sites, but use your own if you like: Naturalhazards.nasa.gov Earthobservatory.nasa.gov Science.nasa.gov seamless.usgs.gov ned.usgs.gov soildatamart.nrcs.usda.gov nationalmap.gov county-map.digital-topo-maps.com