The Laser Doppler Flowmeter provides a relative measure of tissue perfusion in the measurement area immediately surrounding the probe. Signal strength can be affected by pressure (between the probe and tissue), temperature and other factors. Thus, the measurement provided is usually acute, with relative changes being the dominant parameter. If repeated measurements over longer period are to be compared, the best approach is to use a challenge to the tissue to prompt a dynamic response in perfusion (temperature or occlusion challenges being the most frequently utilized).