How does Infrared Cameras Work?

How does Infrared Cameras Work?

Summary

Infrared detectors can "see" in the dark by converting the heat naturally emitted by any object above absolute zero into electrical signals, which are then used to generate images. Here's how an infrared camera works.

Like visible light, infrared (IR) radiation, sometimes called infrared light, is a type of electromagnetic radiation. Infrared has longer wavelengths than visible light - invisible to the human eye, it responds to only a small part of the electromagnetic spectrum. Infrared cameras can "see" in the dark by converting the heat naturally emitted by any object above absolute zero into electrical signals, which are then used to generate images.
 
Principle

All everyday objects emit thermal energy - even ice cubes! The hotter the object, the more thermal energy it emits. The energy emitted by an object is called the thermal or thermal signature of the object. Two objects side by side can have different thermal characteristics.

For example, animals, motors, or machines generate their own heat biologically or mechanically. Objects such as soil, rocks, and plants absorb heat from the sun during the day and release it at night.

Considering that different materials absorb and release thermal energy at different rates, an area that appears to be uniform in temperature is actually made up of a mosaic of different temperatures.

Spectrum

The infrared spectrum can be divided into three main regions. The exact boundaries between these spectral regions may vary slightly by application. The spectral region used in infrared thermography is typically 0.9µm to 16µm, more specifically, in the range of 2µm to 5µm and 7µm to 15µm.

NIR = Near Infrared

SWIR = Short Wave Infrared

MWIR = Medium Wave Infrared

(V)LWIR = (Very) Long Wave Infrared
 
Working principle

Thermal or infrared detection systems utilize sensors to capture radiation in the infrared portion of the electromagnetic spectrum. Infrared cameras detect thermal energy, or heat, emitted by the scene being viewed and convert it into electrical signals. This signal is then processed to produce an image. 

The heat captured by thermal imaging cameras can be measured with high precision. This means that thermal imaging cameras can be used to examine thermal performance and determine the relative severity of thermal-related problems. The warmer an object or object is, the more radiation it emits.

Contrary to popular belief, infrared cameras cannot penetrate walls or other solid objects. They can only measure the heat emitted by the scene being observed. For example, if there is a heat source behind a wall, a thermal image of the wall will show the heat flow through the wall, but it cannot "see" the heat source itself.

However, in the 0.7µm to 4µm portion of the electromagnetic spectrum, infrared radiation is measured in terms of light reflected from the material or scene being observed. This capability is very useful in the semiconductor, glass, and steel industries.

Thermal imager

Thermal imaging cameras are made with cooled or uncooled infrared detectors. Cooled detectors offer better image quality and accuracy, while uncooled detectors are less accurate, but also less expensive.

Cooled infrared detectors must be combined with cryocoolers to reduce the detector temperature to cryogenic temperatures and reduce thermally induced noise to levels below the signal emitted by the scene.

Uncooled image detectors do not require cryogenic cooling. They are designed using a device called a microbolometer--a special type of bolometer that is sensitive to infrared radiation.
 
When the camera's sensors receive infrared radiation, the data is converted into a colored representation of the scene. The camera's settings can be adjusted to show different temperature gradients before taking the image. And, depending on the desired accuracy, resolution can also be an important factor. 

For example, in industrial maintenance, the parts to be inspected may be large and have high thermal contrast, so a thermal imager with a low spatial resolution (from 60x60 pixels) is sufficient. A higher spatial resolution (from 640x480 pixels) is a must for more detailed inspection or to observe small details with equally small temperature differences.
 
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