The two point method compares the power levels at the input and the output points.This method is much more accurate than the OTDR method because its main limitation is source stability.Therefore the two point loss measurement is the most commonly used method.
At the input point ,a unit acting as a transmitter launches light into the fiber.
The light signal is derived from a continuous source ,stabilized in power and operating at a known and fixed wavelength.
At the output point ,a unit,acting as a receiver and calibrated for the signal central wavelength and bandwidth,measures the light throughput.
The transmitter-receiver pair is available as individual components or as integrated instrument,also called an optical loss test set(OLTS).