Human body tissues are relatively translucent to penetrating light what makes possible to perform the effective transillumination for noninvasive measurements os some biophysical quantities by utilization of tissue optics. However, light is strongly attenuated by different tissue components and only proper processing of the transmitted fraction of light at the wavelengths included in the transillumination window, allows us to acquire measurable electrical signals. Volumes of living body parts are cyclically modulated according to the individual arterial pulsations. As a result, making transillumination of a sufficiently thin layer enables to observe rhythmical changes in light intensity transmitted by such an object. Optical density of blood and other tissues is very high, and so, values of light intensity components which are very low, cause many problems with reliability, sensitivity, and range of the detected selective changes. In order to choose a particular device and serve it properly, end-users of medical monitors should always know their advantages and limitations which can be of biophysical as well as technical nature. A lot of artifacts, noises, and disturbances affect the processing procedures and cause the decrease in useful output signals to be converted. On the other hand, there are many problems with empirical verification on human subjects. It is important to solve such complex questions because all the erroneous responses are potentially dangerous. One of the significant diagnostic problems is to detect global as well as local changes in tissue oxygenation which can appear for a lot of reasons. Basing on some novel light on use of the pulse oximetry idea, a model of the living object exposed to noninvasive transillumination can be considered in order to do technical recommendations useful in practice. Such an approach has been presented in the paper from the metrological point of view.