Non-contact infrared temperature measurement is a foundational technology across industries, from industrial process control and aerospace systems to medical diagnostics and consumer devices. By detecting the infrared (IR) radiation naturally emitted by objects, these sensors can accurately determine temperature without physical contact.
At the heart of this technique lies spectral sensitivity; the specific range of wavelengths that a sensor responds to. The choice of this wavelength band has a major influence on measurement accuracy, emissivity correction, and resistance to environmental interference. At Dexter Research, our thermopile detectors feature a broad, flat spectral response, offering the flexibility to tailor sensitivity precisely to each application through custom optical filters and window materials.
Basics of Spectral Selection in IR Sensors
All objects above absolute zero emit electromagnetic radiation according to blackbody radiation principles. Wien’s law tells us that as temperature increases, the peak emission wavelength shifts to shorter values. For example, a surface at room temperature (~300 K) emits most strongly around 10 μm, while molten metal at 1200 °C peaks near 2–3 μm.
This relationship forms the foundation for spectral selection. A broadband detector captures a wide range of wavelengths, generating a stronger overall signal. However, it may also collect unwanted background radiation or emissions from the environment. In contrast, a narrowband detector — typically achieved through an optical filter — limits detection to a specific wavelength range, improving selectivity but reducing total signal strength.
Designers must balance these trade-offs between signal strength and spectral purity to ensure reliable temperature readings under real-world conditions.
Thermopile Detectors and Our Approach
Our thermopile detectors stand out for their flat spectral absorption extending from the ultraviolet through the far infrared. This means the detector itself is largely neutral to wavelength; instead, the filter or window defines the effective spectral band.
This characteristic gives system designers exceptional freedom. By selecting an appropriate optical bandpass filter or window material, the sensor’s spectral sensitivity can be finely tuned to the target and environment. For instance:
- 8–14 μm band – ideal for industrial surface temperature measurement, as it matches the emission peak of most materials near ambient or moderate temperatures.
- 3–5 μm band – optimal for measuring hot metals or ceramics at elevated temperatures, where shorter wavelengths carry more radiative energy.
- Shorter wavelengths (1–3 μm) – useful in specialized cases, such as when glass or atmospheric gases interfere in the longer IR bands.
Key Criteria When Selecting Spectral Sensitivity
Target Emissivity and Material
Emissivity — how efficiently a surface emits IR radiation — varies by wavelength. Some coatings, metals, and transparent materials behave differently across the spectrum. Selecting a spectral band that aligns with the material’s emissivity curve ensures accurate readings.
Ambient and Background Interference
Atmospheric gases, furnace windows, and background sources can distort readings if they emit or absorb in the same spectral band. Choosing a wavelength range outside these absorption peaks (for instance, avoiding water vapor bands) minimizes interference.
Window and Optics Transmission
Every optical element in the measurement path affects spectral transmission. The detector window, protective optics, and filters must all pass the chosen band efficiently. (We’ll explore window materials in more detail in an upcoming article.)
Signal-to-Noise and Response Time
A narrow spectral band limits total energy reaching the detector, which may lower signal strength and slow response. Our high-responsivity thermopile designs help maintain excellent signal-to-noise performance even with narrower filters.
Example: Measuring Molten Steel
Consider non-contact temperature measurement of molten steel at approximately 1200 °C. The peak emission occurs around 2–3 μm. Engineers might select a 1.6–3 μm or 3–5 μm band to capture strong emitted energy while avoiding ambient reflections. A sapphire or chalcogenide window provides the needed transmission, and a thermopile detector equipped with the appropriate filter delivers stable, accurate readings.
The process includes selecting the filter, calibrating emissivity, and verifying linearity — steps that we support through our engineering and applications expertise.
Because our thermopile detectors have a broad native spectral response, the customization of bandpass filters, windows, and packaging becomes a powerful design tool. Our in-house control of these parameters allows us to deliver detectors tailored to the exact needs of each infrared temperature measurement system.
Whether you require a specific optical filter, window material, or aperture size, we provide a range of standard and custom options to achieve optimal performance. This flexibility helps design engineers make the best trade-offs between sensitivity, selectivity, and environmental resilience.
Looking for Thermal Measurement Solutions?
Selecting the right spectral sensitivity is critical to achieving accurate, stable, and interference-free infrared temperature measurements. Starting with a detector that has a broad, uniform response and then tailoring the optical filter and window allows engineers to fine-tune performance for their specific application.
If you’re developing a non-contact infrared thermometer or thermal measurement system, our applications team can help you identify the ideal wavelength band, filter, and packaging configuration.
Contact Dexter Research Center today to discuss your infrared temperature sensing requirements and discover how spectral customization can elevate your system performance.