Abstract: The invention incorporates an opto-electronic section in the tip of the sheath cover for a diagnostic probe that makes both electric and optical measurements of tissue contacted by the sheath. In addition the invention provides a piece of turbid material such as a polymer of controlled composition as a built-in calibrator attached to the outside of the tip of the sheath. In one embodiment the probe sheath in fabricated from a flexible material which can be rolled up prior to fitting and when fitted to the tip of the probe can be rolled down the shaft of the probe for use and then rolled back off the shaft for disposal.