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What is NIR
The near-infrared (NIR also called SWIR for short-wave infrared) extends from just beyond the visible 3000 nm where conventional optical glasses cease to transmit. The visible boundary is not well-defined because the eye has some sensitivity out to 780 nm. Typically, visible-rejection filters have a cutoff in the 800-900 nm range.

Optec supplies lenses tailored to two sections of the NIR band. The standard NIR lenses are coated to optimize transmission and are color-corrected for best resolution in the 900-1700 nm band. Versions are also available coated and corrected for 1700-2300 nm. These bands match two common forms of InGaAs detectors. Lenses with extended range to 3000 nm can also be supplied but the have relatively low transmission because of strong absorption by the glasses above 2500 nm. Lenses for imaging beyond 3000 nm must be made from more exotic materials.
 
NIR Lens Characteristics
Three factors are important in the design of NIR lenses:
  • The glasses used must have high transmission in the NIR
  • The lens surfaces must be anti-reflectance coated for the NIR
  • The lens formula must include color correction for the NIR

Although many lenses designed for visible imaging will pass enough NIR energy to form images, these lenses are much more prone to ghost images from internal reflections and to fuzzy images from poor chromatic aberration correction. As InGaAs sensors improve, these problems become more and more apparent.

Minimum transmission for 6 surfaces AR-coated for the 900-1700 nm NIR band



Equally-weighted sagittal and tangential MTF for the 900-1700 nm NIR band

 


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