Infrared lenses are optical lenses that use anti-reflection coatings to maximize performance for applications operating above 700 nm including thermal imaging, or spectroscopy. Infrared lenses are also used to collect, focus, or collimate light in the near-infrared, short-wave infrared, mid-wave infrared, or long-wave infrared spectra. Near infrared refers to 700 – 900 nm. The Infrared blocking filters are often used in digital video & still cameras that use CCD or CMOS sensors to prevent unwanted Infrared light from reaching the sensor, which is sensitive to near infrared.

There is no specific chemical element that is highlighted in the NIR. What appears bright and what dark depends on the spectral reflectance of the material in the range from ~720 nm (depending of course on the IR filter you use) to approximately 1200 nm, which is the end of the sensors sensitivity. Vegetation, for example, appears bright in NIR images because chlorophyll has high reflectance above ~750 nm ( see Article Assessment of Vegetation Stress Using Reflectance or Fluores...

If you decide not to use an IR filter with the IR blocking filter being removed from your camera you will get color distortions, because the RGB channels of the sensor get additional, invisible for our eyes (NIR) information. So you either should mount an IR blocking filter to the lens of your IR camera if you want to use it normal way, or mount an IR filter for IR images or videos.

A good starting point for some information on scientific use of NIR might be a search for "NDVI". Apart from scientific use, you may search google images for "IR photography" which gives you tons of images.

Thank you for the answers!

Is this lenses made by yourself? If so, I am interesting to it.

Dear Hongfu Yuan ,

Actually, no, there were provided with the camera system and tracking software we bought from Noldus https://www.noldus.com/office/fr/ethovision-xt.

I bought some additional filters from MidOpt company https://midopt.com/