Incremental photoelectric scanning method employed by Specto renders it stable over long periods of operation, eliminating need for calibration. Offered in ranges of 12 and 30 mm, gauge attains ±1 µm accuracy, tolerates plunger velocities up 1.2 m/sec, and provides reference mark. Design features ball-bush plunger and extensive contamination protection. Versions supporting pneumatic plunger extension are available.