A truly noninvasive medical measurement is any measurement system that does not physically breach the skin or enter the body deeply through an external orifice. Thus, the measurement of body temperature with a thermometer in the mouth, rectum, or ear canal is considered noninvasive, as is the use of an otoscope to examine the outer surface of the eardrum. Similarly, the opthalmoscope and the slit lamp, which shine light in the eyes to examine the retina and the cornea and lens, respectively, are considered noninvasive procedures
