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).

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.

www.solarobserving.com/halpha.htm

Absorption and emission spectra of molecular perylene courtesy www.grc.nasa.gov

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 = mv2/2 = mCΔT).

atlas.nrcan.gc.ca/.../figure_4.jpg/image_view

Note below that earth’s continuous radiation spectrum shows mostly radiation in the infrared region, as expected.

www.xylenepower.com 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 www.valuetesters.com/Extech 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/etna072001.html (Mt. Etna plume)

www.rsat.com/apps/uhi_app/dcb_thermal.html (Landsat)

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 material- hold until digital readout is stable and release and the sensor will register IR radiation as temperature.

5. Record temperatures and compare to literature values.

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; www.infrared-thermography.com; www.omega.com;www.optotherm.com

Material Literature lab emissivity Asphalt 0.95 Red brick 0.93 Lime clay brick 0.43 Glass 0.85 0.78 Clay 0.95

Clay tiles 0.33 Cloth 0.95 Copper, polished 0.07 0.06* Copper, oxidized 0.87 0.83 Black elec tape 0.97 Granite-rough 0.90 0.87 Grass 0.98 0.68 Ice 0.95- 1.00 0.40 Iron, not oxidized 0.05 Iron, oxidized 0.74 Limestone 0.95- 1.00 0.90 Paper 0.85- 1.00 0.84 Plastic 0.95 0.78 Soil, dry 0.92 0.82 (black) Soil, wet 0.95 0.88 (black) Sand 0.90 0.80(dry) 0.81(wet) Sandstone 0.67 Snow 0.80 to 0.90 0.55 Straw 0.79 Water 0.90- 0.95 0.83 Wood 0.80- 0.95 Wood (sawdust) 0.75 0.78 Zinc (polished) 0.02 0.02*

*Shiny metal values were 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, offer an explanation for the differences.

There are probably significant differences between our laboratory temperatures and the temperatures in the literature-referenced materials, and possibly in incident wavelength. Infrared thermometers can differ in precision and calibration.

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? Metals have a tendency to reflect incoming radiation because electrons do not absorb in very many of the representative wavelengths; however, because of their general looseness, the energy causes a sort of rebound with immediate re-radiation back in a number of directions.

Specific heat of copper is 0.387 J/g K , water 4.19 J/g K. Low specific heat seems to correlate with low absorptivity.

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?

rs.gso.uri.edu/amy/feature.gif

Gulf Stream June 1988 Gulf Stream January 1989 The reds and oranges represent more heated areas while the greens and blues represent colder areas.

4. How can information on temperature of water (currents, etc.) help us? (Hint: Think of fishing, shipping, environmental problems): Fish tend to congregate in cold upwelling areas, with increased oxygen. Location of currents (as seen by temperature) could help in shipping speed. Global warming can be monitored to some extent by watching ocean temperature.

5. Is there a difference in the terms emissivity and emittivity?

Yes, emissivity refers to the absorptive and radiating ability of a substance in general, while emittivity refers to the same property for a specific object.

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: 0.125 cm2 10 cm: 0.25 cm2 20 cm: 0.50 cm2 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. Yes. Δy/Δx for several points = 0.125/5 0.25/10 0.375/15 All are 0.025 for slope, so function is linear. For 12 cm, y/12 = 0.025 y = 0.3 cm2

7. Compare the two USDA maps below and speculate as to how this area of North

Dakota changed from 2000 to 2001. Crop areas in these maps are reddish. What could be the cause of the differences? How do land thermal images help us? (Hint: Think of crops, weather, land use):

This is a Landsat image comparison from Burke County, in northwestern North Dakota, comparing August 7, 2001 with August 12, 2000. Crop conditions are about equal as evidenced from the imagery, but planted area is down, as evidenced by the increased green and light blue fields. Also, this year is much wetter, as evidenced by the increased area of standing water, the black areas.

Land thermal images can differ over time due to deforestation of land, type of vegetation or crops, erosion, urbanization, weather differences.

Thermal imaging can help with weather effects monitoring, land use change, energy balance change, global warming models, deforestation monitoring.

8. 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 www.geodata.gov nationalmap.gov seamless.usgs.gov ned.usgs.gov soildatamart.nrcs.usda.gov www.sdgs.usd.edu/register/index.html county-map.digital-topo-maps.com