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Near Infrared Technology |
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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. |
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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.
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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 |
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Equally-weighted sagittal and tangential MTF
for the 900-1700 nm NIR band
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