Narrow bandwidth linear variable filters (NB-LVF) bring hyperspectral imaging to a wide range of applications in a compact, low weight, rigid structure. The center wavelengths of the narrow bandpass of a linear variable filter changes smoothly in one dimension and are constant in the orthogonal dimension along the surface of the filter. The filter, which is the size of the camera’s detector, is placed directly ahead of the detector and successive frames are acquired as the camera skews or as the camera platform moves across a scene. The full width, half maximum bandwidth of the filter used is 0.8% of the center wavelength and the spectral range is 400 to 900 nm with a wavelength gradient of 50 nm/mm. Examples using the LVF camera for emission spectroscopy, absorption spectroscopy, machine vision, and industrial process control and hyperspectral imaging are presented.
Laser based applications including optical communications, LIDAR and Raman spectroscopy benefit from ultra-narrow (< 1.0 nm) bandpass and high edge slope dichroic optical filters by rejecting off-band ambient and scattered light. However, applications for these filters are limited by shifts in wavelength due to temperature and angle of incidence, system f-number, doppler shift and pointing error of the gimbal as well as the stability of the source. Passive design techniques such as athermalization, use of high refractive index materials and widening the passband are compared with active tuning options. Adding thermal or tilt tuning can expand the operational range of the filter and mitigate the compromise to signal to noise which follows from widening the passband.