Patent ID: 8760640

Claim:
A field head of an optical instrument for measuring a refractive index of a medium, or a quantity derivable therefrom, comprising: a measurement prism having a boundary surface in contact with the medium; a first wave guide for providing broad-band light from a broad band light source; a dispersive element for dispersing the broad-band light into at least one component light beam belonging to a plurality of component light beams, so that each incident component light beam in the plurality has a differently directed propagation path and at least one different wave length distinctive relative to others of the plurality of component light beams; and a condenser for collecting into a second wave guide at least one component light beam as reflected at the measurement prism boundary surface; wherein the dispersive element is arranged to direct at least one component light beam into a critical angle of total reflection from the boundary surface, and at least another component light beam into an angle leading into the condenser arranged to collect at least one of the another component light beams to be passed to a spectrometer.