The Value of Low-Observable Technology

In this section we compare narrow-band and broad-band low emissivity coatings.  Narrow-band low emissivity coatings are understood to be coatings that have a low emissivity in a desired spectral range, and a high emissivity outside this range.  Broad-band low emissivity coatings, on the other hand, have a low emissivity throughout a spectral range that significantly exceeds the desired range.
For example, consider Band II low emissivity coatings.  For these coatings, the spectral range where the emissivity is to be reduced is from 3 to 5 mm.  Outside this range, low emissivity is not considered desirable.  Hence a narrow band low emissivity coating for such an application would be one whose emissivity is low between 3 and 5 mm, but whose emissivity is high outside this range, as indicated schematically by the red curve in the figure at left.  A broad-band low emissivity coating would have a low emissivity not only between 3 and 5 mm, but outside it as well, as indicated by the blue curve in the figure at left.
With this understanding of narrow- and broad-band low emissivity coatings, we can address the question of why a narrow-band low emissivity coating is superior to a broad-band low emissivity coating.
Due to its broad-band nature, the broad-band low emissivity coating reduces the radiative channel for thermal exchange with the environment over a large range of wavelengths.  An analogy may be made with putting a blanket over oneself at night which reduces the convection channel for thermal exchange.  Since our bodies continue to generate heat at a (relatively) constant rate, the result of putting a blanket over oneself is that our bodies becomes warmer.  The broad-band low emissivity coating acts in a analogous fashion, except that instead of reducing the convection thermal exchange mechanism, it reduces the radiative thermal exchange mechanism.  Thus, assuming that the object covered by the broad-band coating continues to generate heat at a constant rate, covering the object with a broad-band low emissivity coating will result in an increase in its temperature.
A narrow-band low emissivity coating will also reduce the radiative thermal exchange mechanism, but only over a restricted spectral range.  This is analogous, for example, to putting a blanket only over one's feet.  The body  under the blanket will increase in temperature, but the effect will be much less significant than if one covers one's entire body with a blanket.
Hence we expect generally that applying a narrow-band low emissivity coating over an object that is heated at a constant rate will result in a smaller increase in the temperature of the body than if we apply a broad-band low emissivity coating.
To determine the magnitude of the effect, we performed the following thought experiment:  We consider a hypothetical black body cube of dimensions of 1 cm per side and insert into it a heating mechanism that heats the cube at a constant rate, so that the black body cube reaches an equilibrium temperature of 700 Kelvin in an environment at 20 C.  We can then calculate the rise in temperature our black body cube with different low emissivity coatings applied over its surface.  In particular, we consider the effect of an LRPL narrow-band low emissivity coating and a broad-band low emissivity coating (e = 0.2).   The entire calculation may be seen here, and the result of the calculation is shown in Table 1 below:
Table 1: Equilibrium temperature in Kelvin for a blackbody cube heated at a constant rate in an environment at 20 C, and the same cube coated with indicated low emissivity coatings.

Source

Equilibrium Temperature [Kelvin]

Blackbody (BB)

700 

Narrow-band coated BB

771 

Broad-band coated BB

981 

Table 2: Summary of results.

Source

Power Radiated in Band II

(arbitrary units)

Maximum Detection Distance

(arbitrary units)

Blackbody

1

1

Broad-band coated BB

0.93

0.96

Narrow-band coated BB

0.73

0.84

 

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