Short vs. Long-Wavelength

According to Planck’s law, a hotter object emits significantly more infrared energy, and the peak of its emission spectrum shifts to shorter wavelengths as temperature increases. In practical terms, high-temperature targets emit a larger fraction of their radiation in the short-wavelength infrared range, resulting in a much stronger signal for short-wavelength sensors. At any given temperature, the radiated energy grows exponentially as the measurement wavelength becomes shorter, leading to a steeper sensor response curve for short-wavelength detectors. By contrast, long-wavelength sensors exhibit a more gradual increase in signal with temperature because they operate in a part of the spectrum where the temperature dependence is less dramatic. This means short-wavelength pyrometers are extremely responsive at high heat levels, while at lower object temperatures most of the radiation shifts to longer wavelengths – leaving short-wave detectors with very little signal.

These principles explain why short-wavelength sensors excel at measuring high temperatures, whereas long-wavelength sensors are often necessary for accurately measuring colder objects.

Figure 1 shows the measurement deviation caused by a 10% error in emissivity. The temperature deviation, plotted on a logarithmic Y-axis, is substantial. At shorter wavelengths, this deviation decreases exponentially.

Advantages of Short-Wavelength Pyrometers and Thermal Cameras

Short-wavelength infrared sensors offer multiple advantages, particularly in high-temperature or challenging measurement conditions:

• Reduced Sensitivity to Emissivity Errors: Short-wavelength measurements are dominated by the target’s temperature, so an incorrect emissivity setting has a smaller effect on accuracy. In the short-wave IR range, the exponential temperature dependence outweighs emissivity effects, meaning measurement error due to emissivity uncertainty is much lower.
• Greater Robustness to Emissivity Variations: Thanks to the reduced influence of emissivity at short wavelengths, these sensors maintain accuracy even if a target’s emissivity is unknown or changing. Short-wavelength sensors are therefore more trustworthy in applications where emissivity cannot be precisely defined or may vary during operation.
• Better Measurement on Metallic Surfaces: Many metals and shiny materials have higher emissivity at short IR wavelengths. Short-wavelength pyrometers can thus read metallic target temperatures more reliably, whereas long-wave sensors might give unstable or under‑estimated readings on polished metal surfaces.
• High Signal Strength at High Temperatures: At very high object temperatures, short-wavelength sensors receive a much stronger infrared signal. Most of the thermal radiation from a hot object falls into shorter wavelengths, yielding a higher detector output in the short-wave band.

Advantages of Short-Wavelength Pyrometers and Thermal Cameras

Long-wavelength infrared sensors have their own strengths in certain scenarios:

• Measures Low Temperatures: Long-wavelength detectors can measure colder targets than short-wave sensors. Low-temperature objects emit most of their thermal radiation at longer IR wavelengths, so a long-wavelength sensor receives a stronger signal in those conditions.
• Suitable When Emissivity is Well-Characterized: If the target’s emissivity is high, stable, and accurately known, a long-wavelength sensor can perform well with minimal error. In controlled situations where low-temperature measurement is required and the emissivity of the material is well-characterized, using a long-wavelength instrument is an appropriate choice.

Selection Strategy for Pyrometer and Thermal Camera Wavelength

When choosing between a short- or long-wavelength pyrometer or thermal camera, it is critical to account for the trade-off between measurement wavelength and the sensor’s start-up temperature. The start up temperature is here referred to as the lowest temperature at which it can produce a reliable reading. Short-wavelength thermal cameras or pyrometers inherently have higher start-up temperature limits – they simply cannot detect very low temperatures because cold objects emit too little energy at short IR wavelengths. This means the infrared camera or pyrometer’s measurement range must start below the lowest temperature expected in the process. If the process or target can be quite cold at any stage, a long-wavelength sensor may be necessary to capture those low-temperature readings.

In general, the best approach is to select the shortest infrared camera or pyrometer’s wavelength that is just below the application’s minimum temperature or even sacrificing the lower end of the temperature range for better repeatability and accuracy in general. Avoid choosing an unnecessarily long-wavelength model solely to reach slightly lower temperatures – using a longer wavelength than needed can decrease measurement accuracy and increase sensitivity to emissivity errors.

Choose the shortest possible wavelength to maximize accuracy and robustness while still covering the full temperature range!