An apparatus which permits an analytical probe to be calibrated by pressing the tip against a block of material of controlled characteristics. A transparent elastomer is used into which has been incorporated a light-scattering material. Elastomer calibrators may also be made part of a preassembled probe component such as a disposable sheath. In that case the turbid elastomer is assembled on the tip with the other components of the sheath and is removed after calibrating the system and before the probe put into use. An additional film of flexible polymer having optical characteristics analogous to the material to be detected by the probe may be placed on top to simulate the structure of the material that is to be measured.