The invention incorporates electrodes in the tip of the sheath cover for a diagnostic probe that makes both electric and optical measurements of tissue contacted by the sheath. The electrodes are positioned relative to the optical window in such a way as to ensure that sealing is not compromised by the presence of the electrodes. The electrodes are close by to ensure that both the optical and electrical measurements are made on the same area of tissue. The invention uses the hoop tension in the sheath to apply the needed spring force to the electrode contact. 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.