Understanding Infrared

How Thermal Cameras Work

• Thermal cameras detect infrared radiation (heat) emitted by objects and convert that into a temperature reading.
• They don't "see" temperature directly but infer it from emitted infrared energy.

Why the Shell Casing Reads as -1°C

• Emissivity Difference:

Metals, especially polished ones like brass, have very low emissivity. This means they don't emit much infrared radiation and instead reflect ambient infrared radiation from their surroundings (like the sky).

• Sky Reflection Effect ("Cold Sky Effect")

On a clear day, the sky acts as a cold source because space is extremely cold (often measured as -60°C). The brass casing reflects the sky’s infrared radiation rather than emitting its own.The thermal camera, seeing this reflection, interprets it as if the brass is much colder than the actual ambient temperature.

• Actual Temperature vs. Apparent Temperature

The brass may physically be close to ambient temperature (20°C), but since the camera detects reflected infrared, it falsely appears as -1°C.

Conclusion

The thermal camera is being "fooled" by the brass casing’s reflective properties, making it look much colder than it actually is. This is a common issue with metals and is why thermal imaging often struggles with polished surfaces. In this case it aids our search of the site and makes light of finding spent shell cases at the scene over a large area that could take lots of personnel in a long drawn out search.

This is why Regions of Interest are captured and revisited by air assets or ground assets. It is now so easy to send an operative or asset to the area detected.

You don't need and often can't see what you are looking for so a good understanding of the principles allows you to make sense of the non visual!