Short-Wavelength vs Long-Wavelength Infrared Sensors

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.

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.