Infrared light is a type of light that cannot be seen by a person’s eyes. This is because Infrared light uses wavelengths longer than light of the visible spectrum. Infrared illumination allows a camera to “see” even in total darkness. It does this through the use of lamps or diodes that emit infrared light at a specific wavelength. The two wavelengths that are common to infrared illuminators are 715 nanometers and 830 nanometers. The human eye can see up to 780 nanometers and therefore can faintly see illuminators that use the 715 nm wavelength. For true covert nighttime surveillance, illuminators that operate at 830 nm should be used.
The light produced by the bulbs is filtered so at a minimum the only wavelengths emitted are either 715 nm or 830 nm respectively. These numbers are starting points with regards to the frequency emitted; they are the absolute low end of the spectrum utilized by the camera. Wavelengths lower than the spectrum used by the camera are reflected and collected inside the IR illuminator. This collection generates heat in addition to the heat of the wavelengths used by the camera. If close enough, a person can thus feel that an IR camera is on even though they cannot see the wavelengths used.
Lux and IR cameras
The ability of a camera to capture a picture based on the level of light is measured in lux. The lower the lux rating, the better the camera can see in low light. All IR cameras have a lux rating of 0 – meaning that they can see in no light at all. Color IR cameras switch to B&W at night for the purposes of monitoring because of the greater sensitivity that can be achieved this way. A photo cell inside the camera monitors daylight and determines when to switch. Some cameras even use a lens to disperse the IR waves. A distinction should be made between IR cameras and day/night cameras. Day/night cameras can operate effectively in low light but they do not come bundled with LEDs, making it impossible for them to work in no light at all, unlike an IR camera.
How to choose an IR camera
When using an IR camera for outdoor applications, it is best to use an outdoor IR camera. Combining an indoor IR camera with an outdoor housing will not work properly as the IR light may reflect off the glass of the housing. Some success has been reported when the camera is placed flush up against the glass, though. Also, one should always look at the distance rating or range when purchasing and IR camera or illuminator. Purchasing an IR camera for indoor use that has a greater range than the interior dimensions of the building will overwhelm the camera and result in a washed out bright picture. It should be noted that IR cameras cannot see through smoke. In order to achieve this, a thermal camera must be used.