Infrared or IR Corrected Lenses use Low dispersion glass (LD glass) which is a type of glass that greatly reduces Chromatic Aberration.
Chromatic Aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point or image plane as shown in the left image of (Fig. 1)
Different Types of LD Glass Listed Below:
Special low dispersion glass (SLD glass) and extraordinary low dispersion glass (ELD glass) are glasses with yet lower dispersion (and yet higher price). Other glasses in this class are extra-low dispersion glass (ED glass), and ultra-low dispersion glass (UL glass).
Standard lenses do not focus all frequencies, wavelengths or colors of light on to an image plane at the same location (Fig. 2).
Standard lenses do focus most of the visible light on to the image plane but because CCD and CMOS image sensors are sensitive in the Infrared range your image will look softer or out of focus when Near Infrared and Infrared light is also prevalent in the captured scene (Fig. 3). The image on the color camera is not as blurry because the inherent design of color imaging cameras require an IR Cut Filter which is in place in these color images. This all holds true when using a Day/Night “Color/Monochrome” cameras. The IR Cut filter is removed when Day/Night cameras switches to Night or Monochrome mode.
Fig. 3 (Bright Sun / +100°F)
This focus shift is mostly apparent in the Near Infrared and Infrared frequency range. When using a standard lens in a normal lighting condition and then switch to a dark condition with Infrared Illumination your image will become out of focus and refocusing the lens will be needed.
Infrared or IR Corrected Lenses should be used on both color and monochrome cameras in all lighting conditions to achieve a crisp sharp image at all times.