The instruments of Level A
VFL (visual fault locator) — a red laser that lights up breaks, macrobends and connector faults within a few kilometers; also confirms fiber identity. Optical power meter — reads absolute power in dBm at calibrated wavelengths. OLTS (light source + meter) — the pair that certifies a link's insertion loss, the Tier 1 test every installation must pass.
Referencing — where accuracy is won or lost
Before measuring, the source and meter are joined by a reference cord and the meter is zeroed: the one-cord reference is the standards-preferred method. Reference cords must be inspected, cleaned and treated like instruments; a damaged reference cord poisons every measurement of the day.
Building the loss budget
Expected loss = fiber length × dB/km (0.35 at 1310, 0.25 at 1550 for the field) + 0.75 dB × mated connector pairs + 0.1 dB × splices. Compare the measured value at each required wavelength, in both directions, against this number. Passing means measured ≤ budget; a pass with suspiciously low margin is a prompt to inspect, not to celebrate.
Recording results
Every tested link gets an identifier, the measured values per wavelength and direction, the reference method, instrument serials and the date. This record is the baseline all future troubleshooting will compare against — links without records are links you will one day re-test at your own expense.