The ratio of reflected to incident power at one discrete point, expressed as a negative dB value. A glass-to-air break or open connector reflects about 4 percent, roughly minus 14 dB, from Fresnel reflection; a good UPC connector is near minus 55 dB and APC beats minus 60 dB. OTDRs report reflectance for every reflective event on the trace, and high reflectance also drives ghosts and wide dead zones.
Reflectance at the second panel is minus 35, scope that connector, it's either dirty or cracked.
Also heard as
- back reflection
Do not mix it up with
- Return loss: Return loss is a positive dB value usually for a whole link; reflectance is a negative value for one event.
Related terms
Return loss
How much weaker reflected light is than the light sent in, in dB, where bigger numbers mean less reflection.
Event
Any point on an OTDR trace where the signal departs from the smooth fiber slope, such as a splice, connector or break.
APC
Angled Physical Contact: a singlemode connector polish with an 8 degree angle that sends reflections into the cladding.
Ghost
A false event on an OTDR trace caused by a strong reflection bouncing back and forth in the fiber.
Event dead zone
The distance after a reflective event during which an OTDR cannot detect a second, separate event.