Difference between Thermal Imaging and Night Vision

2019-11-14

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Let’s start with a little background. Our eyes see reflected light. Daylight cameras, night vision devices, and the human eye all work on the same basic principle: visible light energy hits something and bounces off it, a detector then receives it and turns it into an image.

Whether an eyeball, or in a camera, these detectors must receive enough light or they can’t make an image. Obviously, there isn’t any sunlight to bounce off anything at night, so they’re limited to the light provided by starlight, moonlight and artificial lights. If there isn’t enough, they won’t do much to help you see.

Thermal Imaging Cameras

Thermal imagers are altogether different. In fact, we call them “cameras” but they are really sensors. To understand how they work, the first thing you have to do is forget everything you thought you knew about how cameras make pictures.

Thermal cameras make pictures from heat, not visible light. Heat (also called infrared, or thermal, energy) and light are both parts of the electromagnetic spectrum, but a camera that can detect visible light won’t see thermal energy, and vice versa.

Thermal cameras detect more than just heat though; they detect tiny differences in heat – as small as 0.01°C – and display them as shades of grey or with different colors. This can be a tricky idea to get across, and many people just don’t understand the concept, so we’ll spend a little time explaining it.

Night Vision Devices

Those greenish pictures we see in the movies and on TV come from night vision goggles (NVGs) or other devices that use the same core technologies. NVGs take in small amounts of visible light, magnify it greatly, and project that on a display.

Cameras made from NVG technology have the same limitations as the naked eye: if there isn’t enough visible light available, they can’t see well. The imaging performance of anything that relies on reflected light is limited by the amount and strength of the light being reflected.

NVG and other lowlight cameras are not very useful during twilight hours, when there is too much light for them to work effectively, but not enough light for you to see with the naked eye. Thermal cameras aren’t affected by visible light, so they can give you clear pictures even when you are looking into the setting sun. In fact, you can aim a spotlight at a FLIR and still get a perfect picture.

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