Modern heart rate monitors usually comprise two elements: a chest strap transmitter and a wrist receiver (which usually doubles as a watch) or mobile phone. In early plastic straps, water or liquid was required to get good performance. Later units have used conductive smart fabric with built-in microprocessors that analyze the EKG signal to determine heart rate. More recent devices utilise optics to measure heart rate using Infrared light.[2] This is achieved by production of infrared light by an internal bulb, as Infrared light is absorbed by the blood, a sensor measures the amount that the infrared light is darkened by. If it is significantly darker, due the pulse causing a temporary increase in the amount of blood that is travelling through the measured area, that is counted as a pulse.[3]