AWEIC Block 3 - Objective 301A

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What is the objective of 301A?

To determine the atmospheric effects on electro-optical systems.

What are electro-optical sensors?

Weather-sensitive.

What is the three-phase problem in forecasting for an EO?

Defining the actual contrast between the target and background, knowing the effect of the atmosphere on the contrast, and the limitations of the sensor to detect the contrast.

What is an EO sensor mounted on an ACFT called?

A target acquisition system.

What is an EO sensor mounted on the bomb or missile itself called?

A precision guided munition.

How do electro-optical systems display images?

They may either display an image of the target or detect energy directly reflected by the target.

What do visual imaging systems depend on?

They depend on the contrast between reflectivities of objects in the target scene.

What do Far infrared (FIR) imaging systems depend on?

They depend on the contrast between emitted thermal energy.

What may electromagnetic radiation be thought of as?

It may be thought of as coupled electric and magnetic waves, both propagating in the same direction and oriented at right angles to each other.

What is another way to think of electromagnetic radiation?

As a discrete parcel of energy called quanta or photon.

Is the amount of energy that is contained in single photon directly or inversely proportional to the frequency?

It is directly proportional; the higher the frequency, the greater the energy per photon.

Which has more energy; an infrared photon or an ultraviolet photon?

The ultraviolet photon has more energy, because its wavelength is shorter and its frequency is higher.

What is the Blackbody Theory?

It is any object that has a temperature above absolute zero that emits (radiates) electromagnetic radiation (EMR). The emitted or reflected energy is the only energy detected by EO sensors.

What laws describe the emission of EMR by a blackbody?

Planck's Law, the Stefan-Boltzmann Law, and Wien's Law.

What is a blackbody?

It is a theoretical object that absorbs all the energy that falls upon it.

How much energy does a blackbody emit?

It emits as much energy as it absorbs (thermal equilibrium). If it didn't, its temperature would change.

What is the proportion of energy emitted for each wavelength?

It is determined by the temperature of the object, not by the wavelengths of energy it absorbs.

What is the type of energy emitted by an object in the blackbody theory?

The type of energy emitted by an object will remain the same, regardless of the type of energy absorbed, as long as its temperature remains the same.

Two blackbodies are at the same temperature; one absorbs ultraviolet light while the other absorbs radio waves. Which will emit energy of a shorter wavelength?

Their emitted energies will be identical, because their temperatures are the same.

What does Planck's Law describe?

Planck's Law describes the amount of energy emitted by a blackbody at a specific temperature for each wavelength of the EM spectrum.

How is the amount of energy emitted at each wavelength in Planck's Law?

The amount of energy emitted at each wavelength is depicted by a Planck curve. Objects at different temperatures will each produce their own distinct curve.

How does each curve work in Planck's Law work?

Each curve has the same distinct shape with a single peak, but the location of the peak differs for each object. There is always a certain amount of energy given off at every wavelength for each object.

What is the Stefan-Boltzmann's Law?

It relates the total amount of energy emitted at all wavelengths of the EM spectrum to the temperature of the blackbody. The total emitted energy is equal to the area under each curve, and increases as the temperature of an object increases.

What is an important consequence of Stefan-Boltzmann's Law?

It is that small differences in the temperatures of two blackbodies result in large differences in the amount of emitted energy due to the fourth power relationship.

Would you expect the emitted energy at 320 degrees K to be 584, 589, or 594 Watts/m(squared)?

594 Watts/m(squared), because of the fourth power relationship, the change in emitted energy increases with increased temperature.

How can the wavelength of peak emission be found?

Wien's Law

Where do electro-optical sensors detect energy?

At the peaks of Planck's curves.

Which window shows a wavelength of maximum temperature near 4 microns?

The Jet engine exhaust is near 750 degrees K.

What are the three types of contrast?

Inherent contrast (Co), apparent contrast (Co(x)), threshold contrast (Co(th)).

What is the definition of visual contrast?

It is the difference in reflectance or albedo between target and background.

What is the contrast for a white target on a black background?

Infinitely large.

What does the comparison against the background take into account?

The difference between light coming from the target and light coming from the background. Light coming from the target provides useful information while light coming from the background provides noise.

What does a perfect reflector require?

It requires the angle of incidence to equal the angle of reflection.

What does a diffuse reflector require?

Light is reflected in all directions and may be detected from all viewing angles.

What is radiometric temperature in infrared contrast?

The radiometric temperature (or apparent temperature) difference between target and background. The radiometric temperature is less than the physical temperature. The radiometric temperature of an object depends on its emissivity.

What is Kirchoff's Law in infrared contrast?

Gray bodies emit as much energy as they absorb, but do not absorb all of the energy that is incidental upon them. If they absorb less, they emit less.

What does Kirchoff's Law tell us that absorptivity equals?

Kirchoff's Law tells us that absorptivity equals emissivity.

If emissivity is very low, what can we say about an opaque object's reflectivity?

Low emissivity indicates high reflectivity, since the total of emissivity, reflectivity, and transmissivity added together equals 1.

What is thermal response in infrared contrast?

It is the rate of heating or cooling of an object.

What does thermal response depend on?

It depends on absorptivity, thermal conductivity, thermal capacity, and SFC to mass ratio.

What is absorptivity?

It is the measure of the proportion of energy absorbed by the skin of an object. Objects with high absorptivities have greater thermal response: they heat up and cool off faster than objects with low absorptivities.

What is thermal conductivity?

It is the measure of how rapidly heat is transferred within a material. The surface of objects with low thermal conductivity have greater thermal response: they heat up and cool off faster.

What is thermal capacity?

It is the measure of how much heat an object can store. It depends on the object's specific heat (energy required to raise a unit of mass by 1 degree C). Objects with low thermal capacity have greater thermal response: they heat up and cool off faster.

What is the SFC to mass ratio?

The higher the surface to mass ratio, the greater the thermal response.

Which relationships to thermal response are direct?

Absorptivity and SFC to mass ratio.

Which relationships to thermal response are inverse?

Thermal conductivity and thermal capacity.

What is extinction in relation to contrast transmission?

The apparent contrast (Co(x)) is always less than inherent contrast (Co) due to effects of the intervening atmosphere.

What is scattering in relation to extinction?

It is the predominant cause of extinction for visible wavelengths.

What is absorption in relation to extinction?

It is the predominant cause of extinction for infrared wavelengths.

What kind of scattering and visibility is associated with blue skies?

Rayleigh/Isotropic Scattering - >7sm.

What kind of scattering and visibility is associated with dust/haze?

Mie/Anisotropic Forward Scattering - 3-7sm.

What kind of scattering and visibility is associated with fog?

Geometric/Anisotropic Backward Scattering - <3sm.

What is the significant cause of infrared contrast degradation?

Absorption.

What are the two major "windows" of low atmospheric absorption?

3.5 - 4.2 microns & 8.5 - 13.0 microns.

What will partially "close" the window to IR EMR?

High absolute humidity (dewpoint) and RH > or equal to 90% in the presence of aerosols

What is Beer's Law?

The relationship between distance (x) and contrast transmission.

What is path radiance?

It is non-image "noise" scattered into the path between the object and the sensor that is not very significant for IR wavelengths & can cause contrast degradation in the visual wavelengths. (AIRLIGHT)

Are radar systems affected by clouds or precipitation?

Precipitation, such as large cloud droplets.

What do laser systems require to be useful?

Cloud-Free Line-of-Sight (CFLOS)

What is beam scintillation?

It occurs when microscale turbulent eddies, much smaller than beam width, cause fluctuations of energy intensity within the cross-section of the beam.

What is beam spread?

It occurs when microscale eddies are smaller than or about the same size as beam width. This increases the cross-sectional area of the beam, thereby decreasing its intensity.

What is beam wander?

It is caused by microscale eddies larger than the beam width which bend the beam, making it difficult to aim.

A laser beam appearing to jump around the viewing screen would be affected primarily by what refractive effect?

Beam wander.

What are the two key relationships that describe the effectiveness of using a particular wavelength of EMR to image a target scene?

As wavelength decreases, resolution increases. As wavelength decreases, weather sensitivity increases.

What are two reasons for using 8-13 micron portion of the IR spectrum for imaging?

It is an open atmospheric window and it is the range of values expected from the background.

If a tree has a radiometric temperature of 300 K, at what wavelength would it re-emit energy and can we see this energy?

Approximately 10 microns - not directly with our eyes but, we could see it with an infrared sensor.

How would a tobacco shed in NC with a smooth metal roof appear during a summer day to an IR sensor if this target absorbed 10 percent of the incoming radiation?

Cold - The shed is opaque so the low absorptivity and emissivity, 10%, is associated with a cool source.

When you turn on your high beams while driving in fog, which type of scattering occurs?

The most significant visibility reducing scatterers will be geometric although Mie and Rayleigh scatterers could also be present.

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