Infrared Temperature Sensors - The Importance of Field of View and Spot Size

When designing or integrating an infrared (IR) temperature measurement system, one of the most critical considerations is Field of View (FOV) and its direct impact on spot size. The spot size determines the physical area from which your sensor is collecting infrared radiation to compute a temperature reading.

• Too large a spot size, and you risk averaging unwanted background signals, reducing measurement accuracy.
• Too small a spot size relative to your target can miss critical thermal details or even the target altogether.

In many industrial and research applications—where measurement precision is paramount—understanding the geometry behind FOV and spot size is crucial.

Defining Field of View and Spot Size

Field of View (FOV)

• The Field of View is usually specified as a full cone or angle ($\theta$) for which the sensor can detect radiant energy.
• In data sheets, you may also see FOV expressed indirectly as a Distance-to-Spot ratio (D:S). For example, a “12:1” sensor means that at 12 units of distance, the spot diameter is 1 unit.

Spot Size

• The spot size (also called the measurement target size) is effectively the diameter of the circular area on the target surface that the IR sensor is measuring.
• Formally, if an IR sensor has a half-angle $\theta/2$ and is placed a distance $D$ from the target, the approximate spot diameter $S$ can be calculated by:

Alternatively, if the sensor uses a D:S ratio $R$, you can approximate the spot size $S$ at distance $D$ using:

​​Why Spot Size Matters

1. Accuracy: If your target is smaller than the spot size, you’re effectively measuring part of the background (e.g., the conveyor belt under a small product) rather than your target’s true temperature.
2. Resolution: A sensor with a better D:S ratio (e.g., 50:1 compared to 10:1) allows you to stand farther away from a small target and still measure it accurately.
3. Application Constraints: In some cases (like a high-temperature furnace), you can’t get the sensor too close. Knowing the spot size becomes essential so you do not inadvertently measure surrounding walls or other heat sources.

Working Through an Example

Suppose your IR sensor has a D:S ratio of 10:1. You need to measure a steel bolt that is roughly 2 cm in diameter. How far away can the sensor be and still measure only the bolt?

Using:

​you want $S \leq 2 \,\text{cm}$. So:

Hence, you should place your sensor no farther than 20 cm from the bolt to ensure the bolt’s 2 cm diameter fills the measurement spot.

Visualizing Spot Size vs. Distance

 

Understanding the Chart

1. Horizontal Axis (Distance from Target): As you increase the sensor’s distance from the target, the measurement spot grows linearly for a given D:S ratio.
2. Vertical Axis (Spot Diameter): The spot size for a 10:1 ratio grows more rapidly with distance than it does for a 50:1 ratio.

This simple chart helps you see the large differences in potential measurement accuracy at various distances.

Additional Considerations

1. Optical Geometry: In reality, IR sensor lenses and detectors are not perfectly ideal, and the measured spot can have a transition region. Manufacturers often specify a 90% energy spot size or something similar. Always confirm how the spot is defined in your sensor’s data sheet.
2. Ambient Environment: Dust, smoke, or steam can widen the effective measurement area by scattering IR radiation.
3. Aligning the Sensor: Be sure to physically align the sensor’s optical axis so that the target you’re measuring is centered in the field of view.
4. Emissivity and Reflectivity: Even with a perfectly sized spot, the emissivity of your target drastically influences readings. For reflective metals, you may need additional corrections.

Conclusion

Whether you’re measuring the temperature of a tiny electronic component or the surface of a large industrial furnace, properly understanding Field of View and Spot Size is essential to obtaining reliable IR temperature measurements. By matching the D:S ratio or FOV angle to your target size and distance constraints, you can significantly improve measurement accuracy and process control.

Check out IOThrifty's line of Infrared Temperature Sensors. If you need additional assistance in selecting the right infrared temperature sensor for your application, please reach out to our team of experts at service@iothrifty.com.