Knowledge Library

Accuracy and Resolution in Infrared Imaging

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.

Accuracy and Resolution in Infrared Imaging

Emissivity in Infrared Measurement

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.

Emissivity in Infrared Measurement

From Planck’s Law to Stefan–Boltzmann

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.

From Planck’s Law to Stefan–Boltzmann

Infrared Radiation and Temperature

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.

Infrared Radiation and Temperature

Infrared Radiation and the Electromagnetic Spectrum

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.

Infrared Radiation and the Electromagnetic Spectrum

Infrared Thermometry and Imaging

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.

Infrared Thermometry and Imaging

IR Windows for Non-Contact Temperature Measurement

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.

IR Windows for Non-Contact Temperature Measurement

Kirchhoff’s Law of Thermal Radiation

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.

Kirchhoff’s Law of Thermal Radiation

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.

Lambert’s Cosine Law

NETD – 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.

NETD – Thermal Sensitivity

Ratio Method in Two-Colour Pyrometry

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.

Ratio Method in Two-Colour Pyrometry

Rayleigh-Jeans and Planck’s Law for Blackbody Radiation

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.

Rayleigh-Jeans and Planck’s Law for Blackbody Radiation

Short-Wavelength vs Long-Wavelength Infrared Sensors

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.

Short-Wavelength vs Long-Wavelength Infrared Sensors

Temperature Calculation

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.

Temperature Calculation

The Discovery of Infrared Radiation

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.

The Discovery of Infrared Radiation

The Interaction of Light with Materials

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.

The Interaction of Light with Materials

The Shift in Black-Body Radiation

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.

The Shift in Black-Body Radiation

Why Pixel Size and Diffraction Define Thermal Camera Accuracy

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.

Why Pixel Size and Diffraction Define Thermal Camera Accuracy

Why Spot Size Matters

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.

Why Spot Size Matters