A finger receptor is used with a non-invasive monitoring device to determine non-invasively the concentration of known constituents in blood or tissue. The receptor has a channel for receiving a finger of a user. The channel has a light entrance and a light exit so that light can be passed from a light source through a finger located in the channel in a direction generally normal to the finger. Extraneous light is excluded and the finger is held in position by a spring-mounted roller. The receptor has sensing means to determine when a finger has been properly positioned in the channel. Previous devices are not capable of achieving repeatable results to a sufficient degree to replace invasive methods of testing.
