OBR Reflection Amplitude Scaling: dB vs. dB/mm

The OBR can report reflection amplitude in a couple of different scales, and it's easy to grab the wrong one without realizing it, since, at quick glance, both are just "amplitude in dB".

Amplitude (dB) is tied directly to Return Loss. If you integrate the underlying linear data over a reflection event and convert to dB, you get the Return Loss for that event — this is the scaling you want when you're measuring a discrete reflection like a connector or splice.

Amplitude (dB/mm) is the same thing, but normalized per unit length to account for the sample spacing. It's the better choice for distributed reflectors like Rayleigh scatter, because the scatter level stays put even if you change the laser scan range (and therefore the sample spacing). The tradeoff is that dB/mm readings for a discrete event will shift depending on scan width, so it's not the scaling you want for a discrete, connector-style, RL measurement.

If you need the full math behind how these are derived from the raw S/P reflection coefficients, and want to reconstruct an RL reading from extracted data, see the attached engineering note for the complete derivation and worked examples.