Short- vs. Long-Wavelength Infrared Sensor

Based on the Planck law, temperature measurement behaves differently at longer and shorter wavelengths. According to Planck’s law, the spectral radiance of a blackbody increases sharply with temperature and shifts toward shorter wavelengths—a phenomenon described by Wien’s displacement law. As a result, for hot objects, a greater proportion of the emitted infrared radiation falls into the short-wavelength region. The Planck law results in the fact that, at a given temperature, the emitted radiance increases exponentially as the wavelength decreases. This provides a stronger signal to the detector. When comparing short-wavelength sensors to long-wavelength sensors. The characteristic signal response of infrared sensors as a function of object temperature for different spectral bands, each approximated by a power-law relationship [math]~T^n[/math], where n depends on the sensor’s wavelength sensitivity. Short-wavelength sensors exhibit a steeper increase in signal with temperature due to the stronger temperature dependence of Planck’s law at shorter wavelengths. In contrast, long-wavelength sensors show a more gradual response, corresponding to lower exponents. Figure 1 illustrates that behavior on a logarithmic scale.

Short-wavelength pyrometers are preferred for high-temperature measurements and fast dynamic changes, while long-wavelength sensors are better suited for lower temperatures and materials with low or varying emissivity.

Nevertheless, short-wavelength infrared temperature sensors generally have higher minimum measurable temperatures (“start-up temperatures”) due to the physics of Planck’s radiation law and signal-to-noise constraints. At low temperatures, most of the thermal emission shifts to longer wavelengths (as described by Wien’s displacement law. At short wavelengths, the spectral radiance becomes very small for low temperatures, often below the detection threshold of the detector. This means that at lower temperatures, there simply isn’t enough infrared energy emitted at short wavelengths to generate a usable signal.

Additionally, the emissivity characteristics of many shiny objects play an important role and favour short-wavelength sensors over long-wavelength sensors. Many metals exhibit increasing emissivity as wavelength decreases. In the mid- and long-wave IR, emissivity may drop significantly, especially on polished surfaces. At shorter wavelengths, emissivity tends to be higher and more stable, improving temperature accuracy on metallic targets. Therefore, materials that may not be measurable with conventional infrared sensors may become measurable with shorter wavelengths due to their higher emissivity in those spectral ranges.
Additionally, due to the nature of the Planck law, short-wavelength sensors are less prone to errors in case the emissivity is set incorrectly. Due to the exponential power-law relationship [math]~ε \cdot T^n[/math] a linear emissivity factor does not affect the infrared flux coming from the target as dominantly as the temperature itself for short wavelengths, when the wavelength-dependent exponent n is high. With long-wavelength measurements, the influence of emissivity is on the same scale as that of the temperature itself.

Figure 2 displays the measurement deviation that occurs when the emissivity is set incorrectly by 10%. The temperature deviation is much more significant. Note that the temperature deviation is plotted logarithmically on the Y Axis. Notably, with a shorter wavelength, the possible measurement deviation is reducing exponentially.

Sensors with longer measurement wavelengths show markedly higher sensitivity to emissivity errors, whereas short-wavelength sensors maintain lower deviation across the full range. Figure 3 illustrates the maximal measurement deviation over wavelength. It uses the same calculation as before but references the measurement deviation to the measurement range. Shorter wavelengths show lower sensitivity to emissivity variations, whereas longer wavelengths on the right exhibit greater susceptibility.

Figure 4 illustrates the improved measurement stability of short-wavelength sensors in scenarios with uncertain or varying emissivity. Sensors that operate at longer wavelengths are considerably more affected by uncertainties in emissivity, whereas short-wavelength sensors are less sensitive to such variations. This makes short-wavelength sensors more suitable for applications where emissivity cannot be accurately defined.

In summary, the accuracy, signal strength, and robustness to emissivity variations of infrared temperature measurement are highly dependent on the choice of sensor wavelength. Based on the Planck and Wien laws, the following key points justify the use of short-wavelength sensors:

• Higher Signal Intensity at High Temperatures: According to Planck’s law, spectral radiance increases exponentially as wavelength decreases. For hot objects, most of the emitted energy shifts into the short-wavelength range, resulting in a much stronger detector signal.
• Stronger Temperature Sensitivity: Short-wavelength sensors exhibit a steeper signal–temperature response. This means that small changes in temperature produce more pronounced signal changes, enabling better resolution and dynamic response.
• Reduced Sensitivity to Emissivity Errors: At shorter wavelengths, the influence of temperature dominates over emissivity due to the exponential nature of the signal–temperature relationship. As a result, errors from incorrect emissivity settings have less impact on measurement accuracy.
• Higher Emissivity of Metals at Short Wavelengths: Many reflective or polished metals have significantly higher and more stable emissivity in the short-wave IR range, improving measurement reliability compared to mid- or long-wave sensors, which may yield unstable or underestimated readings.
• Improved Measurement Accuracy under Uncertain Emissivity: Because of the reduced influence of emissivity at short wavelengths, these sensors offer better accuracy in applications where emissivity is unknown, variable, or difficult to control (e.g., changing surface conditions).

Nevertheless, one key consideration has to be made, when choosing a sensor.

• Limitations at Low Temperatures: Due to Wien’s displacement law, low-temperature objects emit very little radiation in the short-wave range. This leads to higher “start-up temperatures” for short-wavelength sensors, making them less suitable for cold targets.

This trade-off of wavelength and startup temperature should be considered early in the sensor selection process to optimize performance and measurement reliability for the specific application.

As a rule, engineers should choose the shortest wavelength that still allows reliable measurement across the intended temperature range. This approach ensures maximum measurement accuracy, especially in high-temperature applications or on materials with uncertain or variable emissivity. Selecting unnecessarily long wavelengths just to cover lower temperatures may result in degraded accuracy and higher sensitivity to emissivity variations.