linearly variable filter

A linearly variable filter (or linear variable filter, abbreviated LVF) is a filter such that across one spatial direction, the passband varies (as to the wavelengths transmitted). They can be used in optical systems for a kind of spectrography: for sufficiently bright sources, a "moving" scan across the survey field can gather data, which can be post-processed into spectral energy distributions for each point, i.e., a data cube, serving as a kind of imaging spectrograph. This technique is utilized by the SPHEREx plan.