Infrared Testing (IR) Method Fundamentals and Physics
Core Physics
Infrared testing relies on detecting thermal radiation emitted by objects according to their temperature. The fundamental relationship is given by Planck's law, which describes the spectral radiance of a blackbody as a function of wavelength and temperature. Wien's displacement law states that the peak wavelength of emission is inversely proportional to absolute temperature (λ_max ≈ 2898 μm·K / T). Stefan-Boltzmann law gives total emitted power proportional to T^4. Emissivity (ε) is the ratio of an object's emitted radiation to that of a blackbody at the same temperature; it varies with material, wavelength, temperature, and surface condition. Real objects are often non-blackbodies, so knowledge of emissivity and corrections are essential. Reflectivity (ρ), absorptivity (α), and transmissivity (τ) are related by ε + ρ + τ = 1 (for opaque materials, τ=0, so ε + ρ =1). Atmospheric transmission windows (e.g., 3-5 μm and 8-14 μm) are important because water vapor and CO2 absorb infrared radiation outside these bands.
Traps in Questions
- Confusing emissivity with reflectivity; remember that good reflectors (low ε) are poor emitters.
- Forgetting that emissivity is wavelength-dependent; a material may have high ε in one band and low in another.
- Misapplying Wien's law: peak wavelength decreases as temperature increases.
- Assuming all infrared cameras measure temperature directly; they measure radiance and convert using assumed emissivity.
- Overlooking the effect of distance and atmospheric attenuation on temperature readings.
Worked Example / Rule of Thumb
A common rule: For a blackbody at 300 K, peak wavelength is about 10 μm (Wien's law: 2898/300 ≈ 9.66 μm). If a material has emissivity of 0.8, the emitted radiance is 80% of a blackbody at the same temperature. To estimate temperature error when emissivity is misjudged: if actual ε is 0.8 but set to 1.0, the indicated temperature will be lower than actual; the error can be approximated using the ratio of emissivities raised to the fourth power (T_indicated ≈ T_actual * (ε_actual/ε_set)^0.25). This is a useful rule of thumb for small errors.