Infrared light is a component of daylight. Unlike the eye, CCTV cameras have sensitivities extending into the infrared. During camera operation in the daylight a filter is required to eliminate the infrared wavelengths. With a sufficient amount of natural or artificial light the image recorded by the camera is clear and the colours are rendered correctly. A complete range of visible and infrared wavelengths reaching the camera will affect the colour rendering and overall image quality. Thus, the infrared wavelengths should be limited from reaching the image sensor by a mechanical infrared cut-off filter.
If the illumination is insufficient, the IR illuminator will automatically activate and synchronise the operation of the ICR filter.
The IR illuminators arranged around the camera lens emit infrared radiation and illuminate the surrounding monitored by the camera. With the filter open, the camera is sensitive to the IR radiation illuminating the object. As a result, the camera can operate in complete darkness. The camera switches to monochrome mode providing a clear and sharp image.
Depending on the camera model, the filter mechanism operates in different modes. However, the function remains the same. The animation shows the operation of the mechanical infrared cut-off filter in APTI cameras (Fig. 1). A click can be heard when the camera switches between the day mode and the night mode.
Fig. 1. Operation of the mechanical infrared cut-off filter (ICR) in APTI cameras.
Almost all new Day/Night cameras for 24h monitoring with CMOS image sensor recording high resolution images include an ICR filter. The interferences due to infrared wavelengths should be eliminated to the greatest extent possible. The cameras without a mechanical infrared cut-off filter has this component built-in. A spectral-response characteristic of the camera is practically constant and switching between colour and monochrome images is carried out by an electronic module which also processes the images from the image sensor. Those are mostly analogue cameras with CCD image sensors with an adequate efficiency of image recording at night.