A small sensor in a production logging tool that distinguishes the type of fluid in its vicinity as it is moved up and down a production well. The optical probe is sensitive to the optical index of the fluid in front of the probe. It can distinguish easily between gas and liquid, and less easily between oil and water. Light is sent from a source down a fiber-optic cable to the tip of the probe, and reflected back to a sensor, which converts it into an electrical signal. Gas, with a low optical index, reflects a significant amount of light and gives a large signal, while liquids give low signals.

As with other local probes, the output is set to be nearly digital, so that the percentage of the time that the probe sees gas is a direct measure of gas holdup in front of the probe. By using several probes, it is possible to obtain the average holdup and an image of the flow structure across the well.