A probe having a probe head in which is formed an opening for receiving a sample to be analyzed, the head comprising a pair of optical interfaces disposed at an opposing inner surface of the opening to delimit a path for optical radiation through the opening, wherein one of the pair of optical interfaces comprises a transparent element adapted to permit optical radiation in one or more wavelength regions of interest to travel between the probe head and the opening, the optical probe comprising a movable diaphragm in which one of the pair of optical interfaces is located for movement therewith and an actuator operably connected to the diaphragm to control its movement so as to vary the path length wherein the probe head comprises a hinge system cooperable with the actuator to move the optical interface in the movable diaphragm in an arc to vary the path length.
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