A METHOD OF DETERMINING A POSITION AND/OR ORIENTATION OF A HANDHELD DEVICE WITH RESPECT TO A SUBJECT, A CORRESPONDING APPARATUS AND A COMPUTER PROGRAM PRODUCT

According to an aspect, there is provided a method of determining a position and/or orientation of a hand-held device with respect to a subject. The hand-held device is for use on a body of the subject. The method comprises receiving (101) images from an imaging unit arranged in or on the hand-held device; receiving (103) a displacement signal from a displacement sensor that is arranged in or on the hand-held device to measure displacement of the hand-held device along the body when the hand-held device is in contact with the body; processing (105) the received images to determine whether a body part of the subject can be identified in the received images; determining (107) whether the hand-held device is in contact with the body; determining (109) a mode of operation to use to determine a position and/or orientation of the hand-held device based on whether a body part can be identified and whether the hand-held device is in contact with the body; wherein the mode of operation to use is determined as (i) a first mode when a body part can be identified, (ii) a second mode when a body part cannot be identified and the hand-held device is not in contact with the body, and (iii) a third mode when the hand-held device is in contact with the body; and determining (111) the position and/or orientation of the hand-held device with respect to the body of the subject using the received images and/or received displacement signal according to the determined mode of operation.