EXFO OTDR 2 - Manual (Page 95)

Analyzing the Results Manually
OTDR 87
Getting Event Loss and Maximum Reflectance
Getting Event Loss and Maximum Reflectance
Event loss (expressed in dB) is calculated by measuring the signal level
reduction in Rayleigh backscatter (RBS) caused by this event. Event loss
can result from both reflective and non-reflective events.
The loss calculation provided is called the “four-point event loss”. The
four-point event loss calculation uses the least-square approximation (LSA)
method to determine the event loss. The four-point event loss corresponds
to the loss displayed in the events table.

For the four-point event loss: the LSA method is used to fit a straight
line to the backscatter data within the two regions defined by
markers a, A and b, B, that is over the regions to the left and to the right
of the event bordered by markers A and B, respectively.
The two fitted lines are then extrapolated toward the center of the
event and the loss event is directly read from the drop in power
between the two lines.
Four-point
event loss
a
b