The subject matter or figure imaged by, or seen through, an optical system.

Optical Density

Abbreviated OD.  The logarithm to the base ten of the reciprocal of the transmittance at a particular wavelength, where: Dλ = log10 (1/τ λ) where τ λ is the transmittance at the wavelength of interest. Symbol: D(λ), D λ or OD. Optical Density is commonly used to describe the protection factor provided by a filter (such as used in eyewear, viewing windows, etc.) at a specific wavelength. Each incremental unit of OD (1, 2, 3….etc.) represents a 10x increase in protection.

Filament of quartz or other optical material capable of transmitting light along its conformation and emitting it at the end.

Exciting the lasing medium by the application of light rather than electrical discharge from anode and cathode.