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Glossary: L

Lagrange Invariant

It is a version of the optical invariant that uses the chief ray and the marginal ray as the two rays of interest for ray tracing calculations.

  See also Optical Invariant


A perfectly diffuse reflecting surface, where light is scattered equally in all directions.

  See also Specular


Acronym that stands for light amplification by stimulated emission of radiation.  The basic structure is based on an active medium (either a gas or semiconductor) contained between two reflectors. A laser's reflectors contain light by oscillating it through a medium repeatedly allowing the energy to build up with each pass. Laser radiation escapes due to a partially reflecting mirror in the assembly.

Laser Damage Threshold

The maximum amount of laser power per area that a surface can withstand before it is damaged. For pulsed lasers, typically measured in  mJ/cm2; for continuous wave lasers, typically in  W/cm2.  This measurement is very important for mirrors or any other optical component used in conjunction with laser products.

  See also Power Density , Laser

Lateral Color

The difference in image height as a function of wavelength.

LBO Crystal

Lithium triborate(LiB3O5) is a nonlinear optical crystal with a number of unique and useful optical properties.

Light Pipe Homogenizing Rod

An optical component that utilizes total internal reflection to turn a non-uniform light source into uniform illumination.

Line Pair (lp)

A pair of alternating black and white bars at a specific frequency.

Line Width

The width of the line created by a line generating laser is equivalent to the beam spot diameter at the target without the line generating optic. The length of the line is defined by the fan angle.

  See also Fan Angle

Linear Polarizer

A type of filter used to limit the polarization of light to a specific plane by absorbing components oscillating in planes other than the specified polarization transmission axis.

Littrow Dispersion Prism

Also known as a 30-60-90 dispersion prism, it is available in two versions: uncoated and coated. An uncoated Littrow dispersion prism disperses white light into its spectral components, similar to an equilateral prism; a coated Littrow dispersion prism deviates the ray path by 60° without inverting or reverting the image. The hypotenuse of the coated version is at Brewster's angle for 514.5nm, resulting in retroreflection at that wavelength. Rotating the prism changes the retroreflected wavelength and makes this prism suitable for line selection in a laser. 

  See also Dispersion

Longitudinal Chromatic Aberration (LCA)

A type of chromatic aberration where different wavelengths focus at different points along the horizontal optical axis as a result of dispersion properties of the glass. The refractive index of a glass is wavelength dependent, so it has a slightly different effect on where each wavelength of light focuses, resulting in separate focal points for red, yellow, and blue light along a horizontal plane.

Longitudinal Mode Spacing

The spacing between multiple frequencies of oscillation within a laser that results from a combination of the gain curve for a laser and the resonance characteristics of its cavity. Since the mode spacing is defined as the speed of light divided by twice the cavity length, short cavities can be used in applications that require a minimal frequency spread. A short cavity can extend the mode spacing such that only one frequency experiences gain during oscillation. 

  See also Spatial Mode , Laser

Longpass Filter

A type of filter in which the transmission band is a higher wavelength region as compared to the region blocked

  See also Shortpass Filter

Louver Angle

For light control film, the angle orientation of the closely spaced micro-louvers. The louvers simulate a tiny venetian blind that blocks out unwanted ambient light and controls the direction of display light. Unlike blinds, the film is a solid material.

  See also Viewing Angle

Luminous Flux

The perceived power of visible light or the flow of this visible light past a fixed point per unit of time, measured in lumen.

  See also Minimum Sensitivity


Unit of measure of luminous flux incident per unit area. One lux (lx) equals one lumen per square meter.

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