Multi-spectral-band systems are steadily emerging as a desired feature in a camera system. Each spectral band offers different image characteristics. Shorter-wavelength spectral bands provide the potential for better resolution due to the smaller impact of diffraction on the size of the optics blur. This means that a smaller-aperture optical system with a shorter wavelength can be used to see a further distance than that of a longer wavelength.
A dual-field-of-view, all-refractive, infrared optical system was developed that images the midwave infrared light in one field of view, and the shortwave infrared light in the second field of view onto the same detector. The two fields of view vary in focal length by a factor of six.
The narrow field of view images the SWIR radiation at a slow f/number of 10.0, while the wide field of view images the MWIR radiation at f/1.9. The f/number in the MWIR is defined by the aperture stop located inside the dewar, which is cooled to cryogenic temperatures and referred to as a cold stop. This cold stop is attached to a radiation shield and controls emissive energy in the MWIR from outside the field of view from reaching the image plane. The field of view is changed via a single lens that changes its axial position within the lens, resulting in an axial zoom.
The change in focal length and f/number at the same time enables an increased focal length without having to increase the aperture size by the ratio of the focal length change, but rather by the ratio of the focal length change divided by the ratio of the f/number change.