The Role of Infrared Filters in Temperature Sensors

In today’s smart world, accurate temperature measurement is everywhere. From handheld thermometers to factory robots, infrared temperature sensors keep machines and people safe.
Yet, few people notice the small part that makes these sensors reliable — the infrared optical filter. It looks simple, but this tiny filter decides which light the sensor “sees” and which light it blocks. That choice makes all the difference.

At first glance, it’s only a coated piece of glass. But inside that layer are complex optical films that control how light travels. Let’s take a closer look at why filters matter, how they function, and how Bodian Optical designs them to meet strict real-world needs.

Why Temperature Sensors Need Infrared Filters

Every object that has heat gives off infrared radiation. Even things that seem cold, like ice, still emit some energy. The hotter an object becomes, the stronger its infrared radiation is, and the shorter its main wavelength grows.

Infrared temperature sensors detect this invisible energy and turn it into electrical signals. Then, a small chip calculates the real surface temperature.

But there’s a problem — the world is full of light and heat. Lamps, sunlight, and reflections from nearby surfaces all give off electromagnetic waves. Without control, the detector would collect all kinds of mixed signals. The result? Confused readings and wrong data.

This is where the infrared filter comes in. It acts like a spectral gatekeeper. The filter only allows certain infrared wavelengths to pass through while stopping everything else, including visible and ultraviolet light.

Without this filter, even the best infrared sensor couldn’t deliver consistent results.

How Infrared Filters Work

An infrared filter sits in front of the sensor or inside its optical system. It’s built from many thin film layers, each one measured in nanometers. These layers have different refractive indexes. When light enters, some wavelengths pass through and others bounce off.

This is called optical interference — a process that uses light’s own wave pattern to separate colors and wavelengths. Engineers control the film design so that only the target infrared range gets transmitted. Everything else is blocked.

Let’s see what that means for real sensors.

1. Blocking Visible Light

Visible light, from around 380 nm to 780 nm, can be much stronger than infrared radiation. If the sensor takes it in, the small thermal signal could disappear in the noise. An infrared filter is almost opaque to visible light but transparent to infrared. That’s why thermometers work fine even under bright light or direct sunlight.

2. Choosing the Right Wavelength Band

Different jobs require different wavelength ranges.

• Long-wave infrared (8–14 µm): This range is ideal for body or room temperature detection. It lies in the “atmospheric window,” where air absorbs almost no radiation.
• Mid-wave infrared (3–5 µm): Used for very hot objects like engines, furnaces, and molten glass.
• By adjusting the film’s layer thickness, the designer can tune which wavelengths get through. That’s how sensors measure the right target without background interference.

3. Improving Signal Quality

Because only the needed wavelengths reach the detector, noise drops sharply. The useful signal stands out, making readings smoother and more accurate. Even weak infrared emissions can be captured clearly.

4. Protecting the Sensor

Sometimes too much light or heat hits the detector. It can overload or “saturate” the sensor. The filter helps keep light levels within range, extending the sensor’s life and maintaining steady performance.

Common Applications of Infrared Filters

Forehead and Ear Thermometers

This is where most people encounter infrared filters daily. Human skin emits heat mostly around 9–10 µm, so medical thermometers use an 8–14 µm band-pass filter. The filter focuses only on body radiation and ignores visible or near-infrared light. This makes readings fast and accurate, even in bright rooms.

Industrial Temperature Monitoring

Factories use infrared sensors to check materials like steel, plastic, or glass without touching them. These environments are noisy, with glowing furnaces and strong reflections. Filters tuned to the 3–5 µm range isolate the true signal from all that background light.

For such work, filters must survive heat, dust, and humidity. Bodian Optical designs industrial filters that stay stable from –50 °C up to 200 °C. They keep measuring correctly where many others fail.

Thermal Imaging and Security Systems

In thermal cameras, every pixel is like a mini infrared sensor. Filters help ensure that all pixels detect the same range of wavelengths. This gives clear and uniform images. Some advanced cameras even have rotating filter wheels that switch between bands for various conditions.

Smart Home and Automotive Devices

Infrared filters also appear in air conditioners, smart thermostats, and vehicle night-vision systems. These products need filters that are thin, light, and affordable, yet still strong. Using magnetron sputtering and vacuum evaporation, Bodian Optical creates precise coatings that meet both performance and cost goals.

Main Types of Infrared Filters

Infrared filters come in several styles, depending on how they manage light:

• Band-Pass Filters: Allow only a fixed range of wavelengths, like 8–14 µm, to pass.
• Long-Pass Filters: Stop short wavelengths and transmit longer infrared ones.
• Short-Pass Filters: Do the opposite, letting short infrared waves through.
• Anti-Reflection (AR) Coatings: Not true filters, but used on lenses to reduce glare and reflection, improving transmission.

Each design can be fine-tuned for its center wavelength (CWL), bandwidth (FWHM), or optical density (OD). That’s why customization is so important — and it’s something Bodian Optical does exceptionally well.

How Bodian Optical Produces Precision Filters

Bodian Optical (Beijing Jingyi Bodian Optical Technology Co., Ltd.) has over forty years of experience in optical coating. The company manages every step — from substrate polishing to multilayer coating and spectral testing.

Its production line includes Leybold Syrus, Helios magnetron sputtering systems, and Optorun OTFC coating equipment, along with Agilent Cary and PerkinElmer spectrometers for inspection and quality control.

Some standout features of Bodian’s filters include:

• High transmission (above 90%) and deep blocking up to OD 4.
• Excellent stability under temperature and humidity swings.
• Custom wavelength and size options using materials like Germanium, Silicon, or Zinc Selenide.
• Automated production for even coating layers and consistent optical results.

Because Bodian keeps the entire process in-house, it can respond quickly to custom orders and deliver filters that balance performance with price. These qualities make Bodian’s filters ideal for thermal cameras, laboratory sensors, and industrial inspection systems.

The Future of Infrared Sensing

Infrared sensing is entering a new stage. Devices are becoming smaller and more connected, while AI helps interpret their data faster. Yet no matter how advanced the software gets, every measurement still depends on clean optical input — and that comes from the filter.

New multi-layer and multi-material designs are now being developed. They can handle several infrared ranges in one filter or switch between them automatically. Bodian Optical is already exploring such innovations to help engineers build smarter, more compact temperature sensors for tomorrow’s products.

Conclusion

The infrared filter is a small part, but it carries a big role. It decides what the sensor truly “sees.” By cutting out visible and stray light, it keeps temperature readings accurate and stable.

Whether it’s a medical thermometer, an industrial furnace monitor, or a car night-vision camera, the same principle applies — precision starts with clean infrared filtering.

With long experience and advanced coating technology, Bodian Optical produces filters that combine accuracy, durability, and flexibility. They continue to support industries that rely on reliable infrared measurement day after day.

FAQs

Q1: What materials are used for infrared filters in temperature sensors?
A: Filters often use Germanium, Silicon, Zinc Selenide, Zinc Sulfide, or Calcium Fluoride. These materials transmit infrared light effectively and can handle heat changes without cracking or warping.

Q2: Why is the 8–14 µm range important in thermometers?
A: This range matches the heat radiation from human skin and also sits in an atmospheric window, where air absorbs very little energy. It gives the most steady and natural readings for people and the environment.

Q3: How does Bodian Optical create filters for different applications?
A: Bodian adjusts film thickness, layer count, and material mix to change how light passes through. This lets them design filters that fit each device — from small home sensors to large industrial cameras.