The 13VDIR models have infrared-corrected optics for day/night
operation. These models are designed specifically for use with true
day/night cameras, which are designed with a moving filter
arrangement, or with fixed focal length black-white cameras. The
13VDIR models work especially well with CCC1390H, CC3770H, and
CC3770UH cameras.

Why use an IR Corrected Lens with a Day/Night Camera?
Lens correction is very common and can be related to our own vision problems and why we wear glasses or have our vision corrected. Most vision problems occur when the eye cannot focus the image onto the retina. Nearsightedness occurs when a distant object looks blurred because the image comes into focus before it reaches the retina. Farsightedness occurs when a close object looks blurred because the image doesn't come into focus before it gets to the retina. Corrective lenses, then, are prescribed to correct for these problems, to adjust the focal point onto the retina.

In the same relation, IR light is displaced behind the focus point of visible light. This causes a blurred picture when using monochrome and day/night cameras. Monochrome and day/night cameras are sensitive to IR light although it cannot be seen with the human eye.  This problem can be resolved by using and IR corrected lens which provides a sharper picture because it adjusted for IR light and visible light to focus on the same plane.