(1) Plethysmographic determination in which the intensity of light reflected from the skin surface and the red cells below is measured to determine the blood volume of the respective area. There are two types, transmission and reflectance.
Photoplethysmography is based on the determination of the optical properties of a selected skin area. For this purpose non-visible infrared light is emitted into the skin. More or less light is absorbed, depending on the blood volume in the skin. Consequently, the backscattered light corresponds with the variation of the blood volume. Blood volume changes can then be determined by measuring the reflected light and using the optical properties of tissue and blood.
Modern measuring methods, using sophisticated equipment, allow you to greatly improve the accuracy and reliability of photometric methods. Nevertheless, keep in mind that only relative measurements are possible so that only blood volume changes, not absolute levels, can be recorded.
The measured signal records venous blood volume changes as well as the arterial blood pulsation in the arterioles.
Light-reflection-rheography (LRR) also refers to the venous test. The name is derived from a special kind of sensor that has emitting and sensing parts side by side on the same side of the sensor. The sensor measures the reflected part of the emitted light. (Note that the other sensors described below, transmission sensors, can only measure pulse waves. Only the reflective sensors will work for the venous test.)
(2) A photoplethysmograph (PPG) is an optically obtained plethysmograph, a volumetric measurement of an organ. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption (Shelley and Shelley, 2001). A conventional pulse oximeter monitors the perfusion of blood to the dermis and subcutaneous tissue of the skin.