An optical spectrometer (102;202) comprising: an adjustable sampling space (104;11) having two generally opposing, relatively movable, side-walls (106,108;18) between which in use a sample for analysis is charged and in at least one of which is formed an optical interface (106,108; 12,14) translucent to optical energy emitted by an optical energy source (120;13); an actuator (116;1) mechanically coupled to one or both of the opposing side-walls (108;18) and operable in response to a command signal applied thereto to effect their relative movement; and an optical position sensor (110,112,114;210,212,214) adapted to detect interference fringes generated by optical energy traversing the distance between the side-walls (106,108;18) a plurality of times having passed through the at least one optical interface (108; 14) and to generate the command signal in dependence thereof, wherein the position sensor comprises a spectral instrument adapted to generate an output indexing intensity of input optical energy against an indication of its wavelength; characterised in that the position sensor (110,112,114;210,212,214) is adapted to process the detected interference fringes to determine therefrom a reference position of the actuator (116,1) at which the two side-walls (106,108; 18) will be parallel and to generate the command signal thereafter to effect movement of the actuator (116,1) relative to the reference position so as to bring the side-walls (106,108;18) into a predetermined relative angular orientation at which the interference fringe formation is mitigated thus achieving a predetermined degree of non-parallelism of the side-walls (106,108;18) and thereby bringing the adjustable sampling (104;11) space into a analysis position at which the side walls (106,108;18) are relatively inclined to form a wedge shape at which position sample analysis is performed. an adjustable sampling space (104;11) having two generally opposing, relatively movable, side-walls (106,108;18) between which in use a sample for analysis is charged and in at least one of which is formed an optical interface (106,108; 12,14) translucent to optical energy emitted by an optical energy source (120;13); an actuator (116;1) mechanically coupled to one or both of the opposing side-walls (108;18) and operable in response to a command signal applied thereto to effect their relative movement; and an optical position sensor (110,112,114;210,212,214) adapted to detect interference fringes generated by optical energy traversing the distance between the side-walls (106,108;18) a plurality of times having passed through the at least one optical interface (108; 14) and to generate the command signal in dependence thereof, wherein the position sensor comprises a spectral instrument adapted to generate an output indexing intensity of input optical energy against an indication of its wavelength; characterised in that the position sensor (110,112,114;210,212,214) is adapted to process the detected interference fringes to determine therefrom a reference position of the actuator (116,1) at which the two side-walls (106,108; 18) will be parallel and to generate the command signal thereafter to effect movement of the actuator (116,1) relative to the reference position so as to bring the side-walls (106,108;18) into a predetermined relative angular orientation at which the interference fringe formation is mitigated thus achieving a predetermined degree of non-parallelism of the side-walls (106,108;18) and thereby bringing the adjustable sampling (104;11) space into a analysis position at which the side walls (106,108;18) are relatively inclined to form a wedge shape at which position sample analysis is performed.