# What does an IR sensor ‘see’?

By
Bram Stelt

This is an important question to answer when selecting and using an IR sensor.

A good analogy is …………..shining a flashlight. When you hold a flashlight close to a wall, the illuminated spot will be small and bright. Moving the flashlight away, the light spot becomes bigger, but also more blurry.

In the case of an IR sensor, the sensor isn’t shining light towards a surface, it is receiving infrared radiation from a surface. Kind of the reverse of the beam of the flashlight. But the analogy holds: when a sensor is close to a surface, it measures a small spot, when you place it further away the measured spot will be bigger.

How much bigger? That depends on the measurement angle of the sensor (just like the angle of the lightbeam of the flashlight). Hence, the combination of angle and distance determine the field of view of the sensor.

But before we go into that, what is your objective? How big is the object or target you want to measure?

Just like the flashlight that illuminates any object that is in the lightbeam, an IR sensor will measure the temperature of any object that is in the field of view of the sensor (any object emitting IR radiation towards the sensor).

When using and installing an IR sensor, you always need to consider what the field of view of the sensor is, and you need to make sure that the object you want to measure is completely filling the field of view of the sensor.

When the object you want to measure is not completely filling the field of view of the sensor, other heat sources in the vicinity or background, that fill the field of view, will affect the temperature measurement and introduce errors: the temperature you measure is not a good measurement of the object of interest.

To avoid measurement errors you need to consider where we started with:

• Measurement angle
• Distance
• Minimum spotsize

Most IRt/c sensors have diverging measurement angles. This means the minimum spotsize as sensor can measure is the size of the detector: when the sensor is almost touching the object, the spot that is measured is equal to the size of the detector. Moving the sensor further away increases the spotsize.

Ask yourself the question: given a certain measurement angle of the sensor, how far can the sensor be moved away from the object without making the field of view bigger than the object?

Please note that due to reflections and possible interference from other heat sources, we always recommend to position the sensor as close to the target as possible. This reduces the risk of introducing errors and thus allows the most accurate and reliable readings.

Also, if the object is smaller than the field of view of the sensor (or even smaller than the detector size) it is still possible to measure it’s temperature and correct for background errors. See technote 57 for more information