Lesson 21 Flashcards#
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1. Why does every object above absolute zero radiate?
Any object above absolute zero emits thermal (blackbody) radiation across a spread of wavelengths. A perfect radiator is a blackbody; real surfaces emit some fraction of that ideal, set by their emissivity.
2. How does temperature set how much an object radiates?
Total emitted power climbs with the fourth power of absolute temperature (Stefan-Boltzmann, \(M_{\text{total}} \propto T^4\)). Doubling the temperature radiates roughly 16× as much power.
3. How does temperature set where an object's emission peaks?
By Wien's displacement law, \(\lambda_{\text{peak}}\,[\mu\text{m}] \approx 2898 / T\,[\text{K}]\). As temperature rises, the peak wavelength shifts shorter (bluer).
4. Where does the Sun (~5800 K) peak, and in what band?
About 0.5 µm, in the visible band (\(2898/5800 \approx 0.5\) µm).
5. Where does a ~325 K airframe skin peak, and in what band?
About 8.9 µm, in the LWIR (8–12 µm) band (\(2898/325 \approx 8.9\) µm).
6. Which two IR atmospheric windows do military thermal systems fight in?
MWIR 3–5 µm (hot targets: plumes, engines) and LWIR 8–12 µm (cool targets: skin, terrain, FLIR).
7. Why is the 5–8 µm region unusable, and what closes it?
It is largely opaque, blocked mainly by atmospheric water vapor, with CO₂ also cutting a notch near 4.3 µm. Water vapor and CO₂ close broad stretches of the IR, and humidity/weather shrink IR range — the IR advantage is real but not all-weather.
8. What is the difference between reflective-band and emissive-band sensing?
Reflective band (~0.4–3 µm) sees reflected light (sun, moon, laser) and goes dark without a source. Emissive/thermal band (~3–14 µm) sees the target's own radiated heat and works in total darkness.
9. Why can an LWIR thermal imager form a picture on a moonless night?
It senses the heat objects radiate on their own (emissive band), so no external illumination is needed.
10. What does a thermal sensor fundamentally live on, and what is thermal crossover?
It lives on contrast — the temperature difference between target and background. Thermal crossover is when that difference vanishes (e.g., cool skin against sun-warmed desert) and the target can disappear.
11. Contrast photon and thermal IR detectors.
Photon (quantum) detectors free a charge carrier per photon: fast, sensitive, band-selective (InSb MWIR, HgCdTe/MCT LWIR), but must be cooled. Thermal detectors (microbolometer) sense a temperature rise: broadband and uncooled, cheaper and smaller, but slower and less sensitive.
12. Which detector material goes with which band?
Silicon (Si) for VIS/NIR, indium antimonide (InSb) for MWIR, and mercury cadmium telluride (HgCdTe, MCT) for LWIR.
13. Why are high-performance photon IR detectors cooled, often to ~77 K?
Warm, a photon detector drowns in its own thermally generated noise. Cooling (liquid nitrogen or a Stirling cooler) suppresses that noise and raises SNR so faint targets are detectable, letting the limit become the background itself (BLIP).
14. In one line, how does radar sensing differ from IR sensing?
Radar sees the energy you reflect (active, can warn/jam); IR sees the energy you emit (passive, no warning, hard to jam, but line-of-sight and weather-limited).
15. Why do hot engine parts live in MWIR while cool skin lives in LWIR?
Wien's law: hotter parts peak at shorter wavelengths (MWIR ~3–5 µm for plumes and tailpipes), while cool skin and background peak longer (LWIR ~8–12 µm).