An optical filter comprising: an upstream laterally variable bandpass optical filter; and a downstream laterally variable bandpass optical filter, wherein the downstream laterally variable bandpass optical filter is sequentially disposed downstream of the upstream laterally variable bandpass optical filter and separated by a distance L along an optical path of an optical beam, wherein the upstream and downstream laterally variable bandpass optical filters (21A, 21B) each have a bandpass center wavelength that gradually varies in a mutually coordinated fashion along a first direction represented by an x-axis, transverse to the optical path, thereby providing for each bandpass center wavelength having a same dependence on an x-coordinate along the first direction, wherein a dependence of spectral selectivity of the optical filter on a degree of collimation of the optical beam is less than a corresponding dependence of spectral selectivity of the downstream laterally variable bandpass optical filter on the degree of collimation of the optical beam, wherein a transparent medium having a refractive index n is disposed between the upstream and downstream filters (21A, 21B), and wherein the refractive index n varies along the first direction, and wherein the bandpass center wavelengths of the upstream and downstream laterally variable bandpass optical filters (21A, 21B) are monotonically increasing in the first direction. an upstream laterally variable bandpass optical filter; and a downstream laterally variable bandpass optical filter, wherein the downstream laterally variable bandpass optical filter is sequentially disposed downstream of the upstream laterally variable bandpass optical filter and separated by a distance L along an optical path of an optical beam, wherein the upstream and downstream laterally variable bandpass optical filters (21A, 21B) each have a bandpass center wavelength that gradually varies in a mutually coordinated fashion along a first direction represented by an x-axis, transverse to the optical path, thereby providing for each bandpass center wavelength having a same dependence on an x-coordinate along the first direction, wherein a dependence of spectral selectivity of the optical filter on a degree of collimation of the optical beam is less than a corresponding dependence of spectral selectivity of the downstream laterally variable bandpass optical filter on the degree of collimation of the optical beam, wherein a transparent medium having a refractive index n is disposed between the upstream and downstream filters (21A, 21B), and wherein the refractive index n varies along the first direction, and wherein the bandpass center wavelengths of the upstream and downstream laterally variable bandpass optical filters (21A, 21B) are monotonically increasing in the first direction. wherein the downstream laterally variable bandpass optical filter is sequentially disposed downstream of the upstream laterally variable bandpass optical filter and separated by a distance L along an optical path of an optical beam, wherein the upstream and downstream laterally variable bandpass optical filters (21A, 21B) each have a bandpass center wavelength that gradually varies in a mutually coordinated fashion along a first direction represented by an x-axis, transverse to the optical path, thereby providing for each bandpass center wavelength having a same dependence on an x-coordinate along the first direction, wherein a dependence of spectral selectivity of the optical filter on a degree of collimation of the optical beam is less than a corresponding dependence of spectral selectivity of the downstream laterally variable bandpass optical filter on the degree of collimation of the optical beam, wherein a transparent medium having a refractive index n is disposed between the upstream and downstream filters (21A, 21B), and wherein the refractive index n varies along the first direction, and wherein the bandpass center wavelengths of the upstream and downstream laterally variable bandpass optical filters (21A, 21B) are monotonically increasing in the first direction.