A patient interface and method of locating the patient interface for use particularly in spectroscopy applications. The patient interface includes a concave region and first and second convex regions. A wing extends from the concave region to help locate the patient interface properly. The convex regions provide additional adhesion support, particularly when used on the thenar eminence. The patient interface may be placed in a number of locations on a patient to determine an optimum location for measurement prior to affixing the interface to the patient.