Principles of LPRL low-observable technology.

spectral radiant emittance

LPRL IR stealth coating technology alters the thermal emissivity of a host. Emissivity is defined as the ratio of radiant energy emitted by a body to the radiant energy emitted by a black body at the same temperature. Formally,

where e is the emissivity, and Wgray body (Wblack body) is the total radiant energy emitted by a gray body (black body) at a given temperature. Thus, an emissivity of unity corresponds to a perfect black body, while an emissivity of zero corresponds to a completely nonemissive material.

The radiation emitted by a black body is described by Planck’s Law and is shown below in the figure at left for several temperatures. As the temperature increases, the peak emission wavelength shifts towards the blue. This is known a Wien’s law and is described by the formula
Wein's law
LPRL IR stealth coatings exploit LPRL’s patented selective emissivity technology, which enable our coatings to shift the energy of the emitted radiation outside the detectable band. For example, we show below the spectral radiant emittance from a body at 700 K (427 °C) that is coated with a coating of LPRL low observable paint, and the same thing coated with a broad-band metallic-based low observable paint. Note that the emittance from the LPRL coated object is reduced in band II (3 to 5 mm), and is increased outside of this band (red curve is above black dashed curve outside of band II). The same object coated with a metallic-based low emissivity coating also emits less in band II (although not to the extent of the LPRL coating), but does not shift the emittance into the atmospheric absorption band (5 to 10 mm) as does the LPRL coating. In addition, since the broad-band coating acts as a thermal insulator, the temperature of the underlying object is increased to a much greater extent than for the LPRL coating.
The LPRL coating not only renders IR detection more difficult by reducing the emission in band II, but, by perturbing the spectral distribution of the emitted radiation, also deceives thermal images that rely on reference spectra to arrive at the brightness temperature of an object. In addition, being nonmetallic in nature, LPRL low emissivity coatings are also compatible with radar absorbing materials (RAM), and may therefore be used in multipurpose coatings (IR/RAM), which we describe in more detail below Finally, LPRL coatings minimize the thermal penalty of low emissivity coatings.

Combined Infrared and Radar Stealth

To this point, we have discussed IR stealth technology as separate from radar stealth technology. However, in the near future coatings will have to perform both functions simultaneously in order to counter the threat of dual mode missile guidance systems such as the MICA air-to-air missile that equips, among others, the Mirage 2000-5.  In general, a unit’s coating will need to satisfy the requirements of the missions for which it was designed. For example, ground-support aircraft face very different threats than tanks or ships or aircraft designed with air superiority assumed, and the coatings of each will be different. Certain zones need only an IR stealth or RAM coating, while other zones require a combined IR/RAM stealth coating.

As discussed above in the brief review of stealth technologies, it is difficult to combine IR and radar stealth because current IR stealth materials are based on electrically conductive elements, which are highly reflective in the radar domain. Hence the dielectric nature of the LPRL IR stealth coatings proves advantageous for creating combined IR/RAM coatings because the active elements that provide IR stealth do not perturb the RAM characteristics. On the contrary, LPRL IR stealth coatings actually provide a modest amount of radar furtivity.

To demonstrate this effect, we show in Table 2 below the mean attenuation coefficient for a 150 mm thick LPRL IR stealth coating.  While the magnitudes of these attenuation coefficients obviously are not sufficient to provide the primary radar stealth properties of a vehicle, they do demonstrate the compatibility of LPRL  IR coatings with radar cross-section (RCS) reduction schemes.
Polarization Incident Angle Mean Attenuation Coefficient [dB] at given frequency
    7.5 - 12.5 GHz 28.5 - 40.5 GHz 75 - 100 GHz
Horizontal Normal 0.03 0.3 0.026
Horizontal   1 1.7 2.28
Horizontal 10º   4.75 6.5 6.25
Vertical Normal 0.035 0.025 0.05
Vertical   2.0 1.9 2.0
Vertical 10º   7.5 7.0 6.5

 

To provide true multipurpose coatings (combined IR and radar stealth coatings) LPRL IR low observable coatings must be combined with radar absorbing materials, using either a multilayer or homogeneous approach.  The schematic diagram below gives  the essence of these two approaches.

The multilayer architecture consists of applying sequential layers of different coatings. In this approach, the outer layer typically provides the IR stealth properties, without perturbing the RCS of the host. The inner layer is designed to reduce the RCS.
In the homogenous material the active dopants that provide IR or radar stealth are blended together in precise stoichiometric ratios that are varied depending on the application (aircraft, ship, tank, …) and/or mission. These coatings are typically much thicker than IR low observable coatings, since the wavelength in the GHz (radar) regime is longer than in the IR regime.

We note that although LPRL has developed RAM coatings, our core competency is IR stealth coatings, and tuning these coatings for RAM compatibility or to meet other mission-specific requirements. Therefore, our primary thrust in this field is to exploit our IR stealth technology in partnership with current manufacturers of RAM coatings to develop multifunction coatings.

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