Apparatus (2) is provided, including a heartbeat measurement device (3), which senses a cardiac parameter of a patient and generates a cardiac parameter signal responsively thereto. An optical measurement device acquires data by emitting towards tissue of the patient 400-1000 niti light, and receiving light reflected from the tissue. An integrator unit (10) receives the cardiac parameter signal and, in response thereto, actuates the optical measurement device to acquire the data.