Infrared theory and thermography fundamentals
Domain 1 - Infrared theory and thermography fundamentals
This domain tests the physics behind a radiometric reading: Planck's law, Wien's displacement, Stefan-Boltzmann, emissivity/reflectivity/transmissivity, spectral band selection, atmospheric windows, spatial resolution, and thermal sensitivity.
Core rules
- A blackbody emits the maximum spectral radiance at a given temperature; a real surface emits ε times that, with ε + ρ + τ = 1. For opaque targets τ = 0, so ε = 1 − ρ.
- Planck's law gives the spectral distribution; Wien's law λmax ≈ 2898 µm·K / T locates the peak; Stefan-Boltzmann gives M = εσT⁴, σ = 5.67 × 10⁻⁸ W·m⁻²·K⁻⁴.
- Ambient targets near 300 K peak near 10 µm, so 8–14 µm longwave cameras suit building and electrical work; hot targets shift toward shorter wavelengths.
- The camera infers temperature from received radiance; the result depends on entered ε, reflected temperature, distance/atmosphere, and focus.
- IFOV = FOV ÷ horizontal detector pixels. A measurement spot should cover at least about 3 × 3 pixels to avoid background averaging. NETD is thermal sensitivity, not absolute accuracy.
Worked example
An 80 °C (353 K) busbar is measured with ε set to 0.95, but true ε is 0.85. The camera solves ε_set T_app⁴ = ε_true T_true⁴. T_app = 353 × (0.85/0.95)^0.25 ≈ 343 K = 70 °C. The 10 °C low reading is an emissivity error, not a real temperature drop. Correct ε or use a contact reference.
Traps
Using °C in T⁴ equations; ignoring reflected temperature from nearby hot surfaces; assuming shiny metal has high ε; using a longwave camera on a 1000 °C target without checking range; and reporting a single-pixel hot spot as a measured temperature.