Hot parts (nozzle/turbine), exhaust plume, airframe skin emission, reflected sunlight, and reflected sky/earthshine — three emitters and two reflectors.
Hot parts and the plume dominate in the MWIR (3–5 µm); the airframe skin dominates in the LWIR (8–12 µm).
\(I_{\text{band}} \propto \varepsilon \cdot A \cdot f_{\text{band}}(T) \cdot \sigma T^{4}\): \(\varepsilon\) emissivity (radiating efficiency), \(A\) projected area seen (where aspect enters), \(f_{\text{band}}(T)\) band fraction (set by Wien), and \(\sigma T^{4}\) total output that explodes with temperature.
The \(\sigma T^{4}\) term — radiated power scales with the fourth power of absolute temperature, so temperature dominates over area.
About 700–900 K, peaking in the MWIR near 3–4 µm. Small area, but the \(T^{4}\) term makes them brilliant.
Near 4.3 µm in the MWIR — the CO₂ emission band of the exhaust gas (shifted by Doppler and pressure). The ~800 K continuum peak sits around 3.6 µm.
It radiates at near-ambient temperature (up to ~330 K subsonic) in the LWIR over a large area and is visible from every aspect, so it carries the beam and head-on signature. You cannot point it away.
During the day in the MWIR, where it can rival a ~300 K graybody with \(\varepsilon \approx 0.5\); in the LWIR it is negligible against the surface's own thermal emission.
From astern the sensor looks up the tailpipe at the exposed hot nozzle and plume; from the beam those are masked and only the skin (LWIR) remains. Projected area \(A\) toward the sensor changes with aspect.
Look-up places the target on cold sky for high contrast and easy detection; look-down places it on warm cluttered terrain for low contrast and hard detection (the penetrator's friend).
The moments — about an hour after sunrise and after sunset — when skin and background temperatures become equal, so the LWIR contrast \(\Delta T\) goes to zero and the airframe blends into its background.
Midday the sun-baked terrain is hotter than the skin, so the target reads cold (negative \(\Delta T\)); at night the terrain radiates to the cold sky and drops below the skin, so the target reads warm (positive \(\Delta T\)).
No — crossover only zeroes the LWIR skin contrast. The plume and hot parts still shout in the MWIR. You cannot disappear in every band at once.
About 7.5 h and 19.5 h — roughly one hour after sunrise and one hour after sunset.
Hot parts: shield the nozzle / avoid afterburner. Plume: mix with bypass air to cool it / route over the airframe. Skin: low-emissivity coatings. Reflections: manage geometry and finish to kill sunglint. For an LO bomber like the B-21 these are baked into the airframe (Lesson 24 / IRCM).