- optical resonator
- optical retardance calibration
- optical return loss tester
- optical rotary dispersion
- optical rotation
- optical rotation; optical activity
- optical rotatory dispersion
- optical rotatory dispersion (ORD)
- [optical] rotatory power
- optical rotatory power
- optical router
- optical scalar equation
- optical scanner
- optical scanning
- optical seeker
- optical sensibilization
- optical sensor
- optical separation
- optical shutter
- optical sight
- optical signal-to-noise ratio
- optical soliton
- optical soliton communication system
- optical sound recording process workflow
- optical space astronomy