01. The Discovery of Infrared Light

Radiometry deals with the measurement of electromagnetic radiation and forms the scientific foundation of infrared temperature measurement. Its development spans from Herschel’s discovery of infrared radiation to Planck’s blackbody theory, which established the physical laws still used in modern IR sensing.

01. The Discovery of Infrared Light

02. Infrared Radiation

Infrared radiation extends beyond visible light and occupies a key part of the electromagnetic spectrum used for non-contact temperature measurement. It is divided into wavelength bands such as NIR, SWIR, MWIR, and LWIR, each suited to different sensing technologies and applications.

02. Infrared Radiation

03. Planck’s Law

Planck’s radiation law forms the physical basis of non-contact temperature measurement by linking emitted infrared radiation to object temperature and wavelength. It explains why hotter objects emit exponentially more radiation and shift toward shorter wavelengths, enabling accurate infrared temperature sensing.

03. Planck’s Law

04. Rayleigh-Jeans Approximation

The Rayleigh–Jeans approximation describes blackbody radiation at long wavelengths and high temperatures, providing a simplified model for parts of the infrared spectrum. Its limitations at shorter wavelengths highlight why accurate temperature measurement requires Planck’s law instead.

04. Rayleigh-Jeans Approximation

05. Lambert’s Cosine Law

Lambert’s cosine law describes how thermal radiation from a diffuse surface appears weaker at oblique viewing angles due to geometric projection. It explains why infrared sensors measure the highest intensity when viewing a surface perpendicular to its normal and reduced intensity as the angle increases.

05. Lambert’s Cosine Law

06. Stefan–Boltzmann Law

The Stefan–Boltzmann law describes how the total thermal radiation emitted by an object increases with the fourth power of its absolute temperature. It shows how emissivity and surface area determine the total radiated power, forming a fundamental basis for infrared temperature measurement.

06. Stefan–Boltzmann Law

07. Wien’s Law

Wien’s displacement law describes how the peak wavelength of thermal radiation shifts toward shorter wavelengths as temperature increases. It explains why hot objects emit more energy at shorter infrared wavelengths, forming the basis for wavelength selection in temperature measurement.

07. Wien’s Law

08. Reflection, Absorption & Transmission

Reflection, transmission, and absorption describe how infrared radiation interacts with a material, with their contributions always summing to one. These effects depend on wavelength, material properties, angle of incidence, and polarization, and are governed by fundamental optical laws such as Snell’s law and Fresnel equations.

08. Reflection, Absorption & Transmission

09. Kirchhoff’s Law

Kirchhoff’s law states that, at thermal equilibrium, a material’s emissivity equals its absorptivity at a given wavelength. This explains why good absorbers are also good emitters and why emissivity is a critical parameter for accurate infrared temperature measurement.

09. Kirchhoff’s Law

10. Emissivity

Emissivity describes how efficiently a material emits infrared radiation compared to a blackbody and is a key parameter in accurate IR temperature measurement. It depends on wavelength, material, surface properties, and viewing angle, making directional emissivity especially important for infrared cameras and pyrometers.

10. Emissivity

11. Infrared Windows

Infrared windows enable temperature measurement through closed systems but must match the sensor’s wavelength range to ensure accuracy. Window material, transmissivity, and coatings directly affect signal strength and must be considered and compensated for during calibration.

11. Infrared Windows

12. Measurement Spot Size

Infrared measurements are only accurate when the target fully fills the sensor’s measuring spot defined by the optics. Standard, far-field, and close-focus optics control how spot size changes with distance, enabling optimized measurements for long-range or high-resolution close-up applications.

12. Measurement Spot Size

13. Optical Resolution & MFOV

Optical resolution defines how precisely an infrared sensor can resolve small measurement spots and accurately measure temperature. It is limited by the combined effects of optics quality, detector or pixel size, and diffraction, which ultimately determine the smallest resolvable spot.

13. Optical Resolution & MFOV

14. Infrared Signal Processing

Infrared sensors determine object temperature by measuring emitted radiation and converting it into temperature using Planck’s law and the target’s emissivity. In real measurements, additional radiation sources and broadband detection require numerical evaluation and compensation for the sensor’s own temperature to obtain accurate results.

14. Infrared Signal Processing

15. Infrared Sensor Design

Pyrometers are non-contact infrared thermometers that measure surface temperature at a single spot by detecting emitted infrared radiation. An optical system focuses the radiation onto a wavelength-specific detector, which converts it into an electrical signal that is processed and output as a temperature value.

15. Infrared Sensor Design

16. Short- vs. Long-Wavelength

Temperature measurement sensitivity depends strongly on wavelength, as described by Planck’s law and Wien’s displacement law. Short-wavelength infrared sensors provide a much stronger, more temperature-dependent signal for hot objects than long-wavelength sensors, improving sensitivity at high temperatures.

16. Short- vs. Long-Wavelength

17. Ratio / Dual / Two-Colour Pyrometer

Two-color pyrometry uses two wavelengths to estimate object temperature while reducing sensitivity to unknown emissivity and common optical losses. The method relies on assumptions about the emissivity ratio and wavelength spacing, revealing a trade-off between emissivity robustness and temperature sensitivity.

17. Ratio / Dual / Two-Colour Pyrometer

18. Pixel Pitch

Advances in uncooled microbolometer manufacturing have reduced costs and increased pixel density, driving the widespread use of thermal cameras in industrial applications. At the same time, pixel size, NETD, and optical diffraction jointly limit sensitivity and resolution, defining how reliably temperatures can be measured.

18. Pixel Pitch

19. Thermal Sensitivity

NETD describes the thermal sensitivity of an infrared camera or pyrometer, indicating the smallest temperature difference the system can distinguish above its noise level. It represents short-term random noise, not absolute accuracy or drift.

19. Thermal Sensitivity