IR Corrected CCTV Lenses
Noise, blur and loss of focus can impact on static and moving elements viewed by a day/night camera.
Should day/night cameras only be installed with IR corrected lenses? What is the impact on performance if they are not?
A: When you’re using day/night CCTV cameras, IR corrected lenses are vital for outdoor applications and any indoor applications with artificial lighting. Some techs might think you’d only use IR corrected lenses when installing cameras in support of IR illumination but that’s not the case, because natural light and some light sources also contain elements of the IR spectrum.
Day/night camera manufacturers push IR sensitivity in their cameras as hard as they can to increase minimum scene illuminations and that means that when an ordinary lens is used, the scene may be out of focus in night mode. Essentially this happens because the wavelength of IR light is different from that of visible light and this creates a displaced focal point in a viewed scene.
When a tech sets a fixed camera focus correctly for daytime with an ordinary lens, as soon as IR illumination becomes a dominant light source with the cut filter removed, focus is lost. You can go around a network fitted with PTZs or remotely adjustable lens cameras and manually refocus every night but it’s a high touch solution. An IR-corrected lens rectifies the issue by focusing both the visible and the infrared light in the same vertical plane to remove IR chromatic aberrations. This means a sharper picture with the cut filter removed, because all light available to the camera is being focused in the same plane.
It’s not just sunlight that contains visible and IR light. Artificial light sources with a significant IR component include halogen, high pressure sodium, fluorescent, incandescent and high-pressure mercury. If you’re using colour cameras, IR-corrected lenses will also make a difference to colour correctness and image quality. Something to consider is that the latest cameras will execute an automatic refocus when they go from day to night mode. But not all do so – you’ll soon notice which don’t. Finally, locking a camera in day mode will avoid the issue, so long as colour performance after dark is supported by ambient lighting.