ECE 495 — Electromagnetic Warfare and Survivability

By the end of this lesson, you will be able to:

• Explain why every warm object radiates, and how temperature sets both how much it emits and where its energy peaks
• Identify the MWIR and LWIR atmospheric windows and explain why the IR fight lives in them
• Distinguish reflective-band sensing (needs illumination) from emissive-band sensing (sees the target's own heat)
• Contrast photon and thermal detectors, and explain why high-performance IR sensors are cooled

Radar sees the energy you reflect. An IR sensor sees the energy you emit.

• You can shape and coat an airframe to shrink its RCS — but you cannot stop it from being warm
• IR/EO sensing is passive: no warning to the target, hard to jam, very precise in angle
• The price: line-of-sight only, and the atmosphere and weather can close the door
• Naturally warm backgrounds are clutter — the IR problem is as much background as target

Every object above absolute zero radiates across a spread of wavelengths. A perfect radiator is a blackbody; real surfaces emit some fraction of it, set by their emissivity.

Those two facts — how much and where — are the whole physics of an IR signature.

Where a source peaks tells you which band sees it best:

Hot, fast-moving parts live in the MWIR; cool skin and the world behind it live in the LWIR. That split drives band choice for the rest of the block.

The target's temperature says where it wants to be seen; the atmosphere says where you get to look. Water vapor and CO₂ close broad stretches of the IR — a water-vapor wall at ~5–8 µm, and a CO₂ notch near 4.3 µm inside the MWIR.

Humidity and weather shrink IR range: the IR advantage is real, but not all-weather.

Put the two rules on one axis. Wien sets the peak; the atmosphere decides whether anyone can use it.

The peak sits outside the 3–5 µm block. So why is the MWIR still the band you hunt a plume in?

Same law, a cooler source: 325 K skin peaks at 8.92 µm, and the marker crosses the closed band to get there.

A peak that lands between 5 and 8 µm is worst of all: the water-vapor wall is shut, and the sensor has to work the shoulders on either side.

An IR detector is a transducer: photons in, electrical signal out. Two families dominate:

The threat that turns this physics into a weapon is the IR-guided missile — passive, no radar warning, fire-and-forget.

• Reticle / spin-scan seekers (early): a spinning pattern chops the incoming IR into a signal that encodes direction to a bright hot spot. Cheap and effective — and fooled by a brighter spot, i.e. a flare
• Imaging IR seekers (modern): a focal-plane array forms an actual thermal picture and tracks shape. It tells a flare from an aircraft and is far harder to defeat

• IR sensing (L21–L23): blackbody and bands today; IR emitters and the five signature components; then seekers, IRST, and FLIR
• IR countermeasures (L24): flares, DIRCM, plume suppression, exhaust mixing
• RF signature (L25–L27): RCS physics, faceting and curvature, RAM, EMCON
• Multi-spectral (L28): acoustic, visual, and EO signatures, and how they fuse
• Project 3 (L29–L30): a B-21 IR/RCS signature trade study — HW3 and Quiz 3 land at L30

Type this with me — the five sources from the table, in one pass.

T = [5800 1000 800 325 290];        % sun, plume, tailpipe, skin, bkgd
lam = 2898 ./ T;                    % Wien peak [um]
M = 5.670e-8 * T.^4;                % Stefan-Boltzmann total [W/m^2]
win = strings(size(T));
win(lam < 0.75) = "visible";
win(lam >= 0.75 & lam < 3) = "MWIR edge";
win(lam >= 3 & lam <= 5) = "MWIR";
win(lam > 5 & lam < 8) = "H2O wall";
win(lam >= 8) = "LWIR";
for k = 1:numel(T)
    fprintf('%5.0f K  %6.2f um  %s\n', T(k), lam(k), win(k));
end
fprintf('plume/skin power ratio = %.1f x\n', M(2)/M(4));

The plume peaks at 2.9 µm, outside the window. Why hunt it in the MWIR?

The plume is 3× hotter than the skin. Why is it 90× brighter?

Two sources on one notional aircraft at cruise altitude, where the thin, cold air holds the skin far below the reading's 325 K.