IR Corrected vs Standard Lenses

Digital cameras, using CCD or CMOS chips, are highly sensitive in the near infrared.

This is a wavelength domain which is very interesting for planetary imaging but the human eye cannot see it.
However, there is one situation where we absolutely don’t want it: when we use a color camera.

Color is not a subjective notion: it is due to the physical properties of the observed objects, to their capacity to absorb or reflect some wavelengths more than others. Its perception depends on the human eye, this why it can make sense only in relation to the wavelengths that this eye can see.

For this reason alone, we must use an IR-cut filter with a color camera, otherwise the wavelengths the camera will record, will differ from that of our eye. For a correct reproduction of the original colors of any objects, both must meet.

For the most part, Color Image Sensors had the IR Cut Filter permanently mounted to them.
There are some situations and applications where you would not want to use the IR Cut Filter.

  • When operating a color camera in Color Mode, which allows the Infrared wave length through to the color sensor.
  • With a monochrome or color sensor in monochrome mode, allowing the IR energy will improve the image in low light or when using an IR (Infrared) Illuminator on your subject.

Figure 1

Figure 1.

Chromatic Aberration

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

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).

Figure 2

Figure 2.

Figure 3

Figure 3.

Figure 4

Figure 4. (Bright Sun / +100°F)

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 (See 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 (See Fig. 4).

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.