Thermal Physics and Heat Transfer Fundamentals
Three modes, one camera
Conduction moves heat through solids and between touching surfaces; convection exchanges heat between a surface and a moving fluid; radiation is the only mode that needs no medium, and it is the mode a thermal imager detects. A thermographer reasons about all three, because what appears on screen is a surface radiosity pattern shaped by the other two.
Laws the camera implicitly solves
Planck law describes spectral radiance as a function of wavelength and temperature; Wien law locates the peak (about 9.9 um at 20 C, about 7.5 um at 100 C); Stefan-Boltzmann integrates over all wavelengths, giving q = epsilon x sigma x A x (T1^4 - T2^4). Kirchhoff law states that spectral emissivity equals spectral absorptivity, and the radiosity balance epsilon + rho + tau = 1 links emission, reflection and transmission.
Traps that appear in questions
- Confusing heat with temperature: heat is energy in transit, temperature is a state.
- Using Celsius in a T^4 relationship; Kelvin is required.
- Treating emissivity as a single universal number: it varies with wavelength, viewing angle, temperature and surface condition.
- Ignoring transient behaviour: thermal capacity and thermal effusivity explain why heavy assemblies lag and why timing of an inspection matters.
- Forgetting that convection coefficients depend on orientation, surface finish and air movement, so similar surfaces can cool very differently.
Worked example
A 60 C (333 K) painted panel with epsilon 0.95 loses heat by radiation to a 20 C (293 K) room. q/A = 0.95 x 5.67e-8 x (333^4 - 293^4) = about 265 W/m2. Doubling the emissivity error to 0.50 instead of 0.95 would understate the flux by roughly half, which is why surface preparation matters more than camera resolution in many surveys.