A capsule for determining the blood content of living tissue in vivo in a patient to detect tumors. The capsule includes a light source and a light detector for directing light onto the tissue and for receiving interacted light therefrom. By analyzing the interacted lights a determination can be made of the blood content of that tissue. There are various ways of positioning the capsule so that it contacts the tissue in the desired orientation of the light source and light detector. There is also a system for determining the actual contact between the capsule and the tissue. A calibration system is also used that allows a self calibration that can be carried out easily and accurately just prior to the use of the capsule.