The reciprocal of transmittance, 1/T.

Open Installation

Any location where lasers are used which will be open to operating personnel during laser operation and may or may not specifically restrict entry to observers.


The performance of the laser or laser system over the full range of its intended functions (normal operation). It does not include maintenance or services as defined in this glossary.

Optic Axis

A birefringent crystal, such as calcite, will divide an entering beam of monochromatic light into two beams having opposite polarization. The beams usually propagate in different directions and have different speeds. There will be only one or two directions within the crystal in which the beam will remain collinear and both polarizations will propagate at the same speed. These directions are referred to as optic axes (not to be confused with optical axes). A uniaxial birefringent crystal has a single optic axis; a biaxial birefringent crystal has two optic axes.

Optical Axis

The optical axis is the line passing through the centers of curvature of a lens. For a train of lenses or optical components, it is the optical centerline for all the centers of the optical components. Note: This is not the same as the optic axis.

Optical Cavity (Resonator)

Space between the laser mirrors where lasing action occurs.

Optical Density

A logarithmic expression for the attenuation produced by an attenuating medium, such as an eye protection filter.

Optical Fiber

A filament of quartz or other optical material capable of transmitting light along its length by multiple internal reflection and emitting it at the end.

Optical Flats

Precision ground and polished, clear disks ranging from about 1 inch in diameter to over 14 inches in diameter. They are used for measuring shape and the flatness optical parts.

To determining the flatness of a part, the optical flat is placed on the part to be measured. The two parts are examined, through the optical flat, under a monochromatic light. Interference fringes are generated at the surfaces. The pattern of the fringes indicated the shape of the part. The wavelength of the light and the number, or straightness of light bands, determine the flatness for the part.

Optical Pumping

The excitation of the lasing medium by the application of light rather than electrical discharge.

Optical Radiation

Ultraviolet, visible and infrared radiation (0.35-1.4 nm) that falls in the region of transmittance of the human eye.

Optically Pumped Lasers

A type of laser that derives energy from another light source such as a xenon or krypton flashlamp or other laser source.

Output Coupler

Partially reflective mirror in laser cavity which allows emission of laser light.

Output Power

The energy per second measured in watts emitted from the laser in the form of coherent light.