A probe, comprising: a hermetically sealed outer housing that makes the probe robust for immersion; a light source for transmitting light onto a material; a window in the outer housing, wherein the window is optically transparent at one or more of transmitted, refracted or reflected wavelengths of light from the material; an optical filter for capturing light transmitted, refracted or reflected from the material, and for separating the captured light into a spectrum of constituent wavelength signals; and a detector array for providing power readings for a plurality of the constituent wavelength signals, wherein the probe further comprises a controller for storing the power readings, each power reading being for a constituent wavelength signal of the plurality of the constituent wavelength signals; characterized in that the optical filter comprises a spatially variable optical filter comprising: an upstream spatially variable bandpass optical filter; and a downstream spatially variable bandpass optical filter that is disposed downstream of the upstream variable bandpass optical filter and separated from the upstream spatially variable bandpass optical filter by a fixed predetermined distance along an optical path of the captured light; and the upstream and downstream spatially variable bandpass optical filters (35A, 35B) each have a bandpass center wavelength that varies in a mutually coordinated fashion along at least a common first direction transversal to the optical path, and wherein light at the bandpass center wavelength along the upstream spatially variable bandpass optical filter outside a predetermined acceptance angle is attenuated by more than light inside the predetermined acceptance angle; and wherein the outer housing comprises a shape that is spherical. a hermetically sealed outer housing that makes the probe robust for immersion; a light source for transmitting light onto a material; a window in the outer housing, wherein the window is optically transparent at one or more of transmitted, refracted or reflected wavelengths of light from the material; an optical filter for capturing light transmitted, refracted or reflected from the material, and for separating the captured light into a spectrum of constituent wavelength signals; and a detector array for providing power readings for a plurality of the constituent wavelength signals, wherein the probe further comprises a controller for storing the power readings, each power reading being for a constituent wavelength signal of the plurality of the constituent wavelength signals; characterized in that the optical filter comprises a spatially variable optical filter comprising: an upstream spatially variable bandpass optical filter; and a downstream spatially variable bandpass optical filter that is disposed downstream of the upstream variable bandpass optical filter and separated from the upstream spatially variable bandpass optical filter by a fixed predetermined distance along an optical path of the captured light; and the upstream and downstream spatially variable bandpass optical filters (35A, 35B) each have a bandpass center wavelength that varies in a mutually coordinated fashion along at least a common first direction transversal to the optical path, and wherein light at the bandpass center wavelength along the upstream spatially variable bandpass optical filter outside a predetermined acceptance angle is attenuated by more than light inside the predetermined acceptance angle; and wherein the outer housing comprises a shape that is spherical. an upstream spatially variable bandpass optical filter; and a downstream spatially variable bandpass optical filter that is disposed downstream of the upstream variable bandpass optical filter and separated from the upstream spatially variable bandpass optical filter by a fixed predetermined distance along an optical path of the captured light; and the upstream and downstream spatially variable bandpass optical filters (35A, 35B) each have a bandpass center wavelength that varies in a mutually coordinated fashion along at least a common first direction transversal to the optical path, and wherein light at the bandpass center wavelength along the upstream spatially variable bandpass optical filter outside a predetermined acceptance angle is attenuated by more than light inside the predetermined acceptance angle; and wherein the outer housing comprises a shape that is spherical.