How to measure light attenuation with a red optical power meter

The insertion loss method uses a calibrated source and power meter to measure loss across the fiber non-destructively. Divide loss by length to get attenuation. You measure optical power in dBm or insertion loss in dB. Consistent procedures ensure accuracy. [pdf]

What is the use of a strong light source fiber optic red light pen

It emits a stable red light driven by a constant current source, which is coupled into the optical fiber through an interface to perform fiber fault detection functions. These include checking fiber connectivity and locating faults such as fiber breaks and bends. [pdf]