Photonics Dictionary: L

A complete unit containing a light source, globe, reflector, housing, socket and other necessary components for lighting.

Luminous flux emitted from a surface per unit solid angle per unit of area, projected onto a plane normal to the direction of propagation. Also known as brightness and luminous sterance.

Ratio of the luminance of a specimen to that of a perfect reflecting or transmitting diffuser identically illuminated.

A type of photometer calibrated in luminance units (candles per square unit, or lamberts). In photography an exposure meter contains a luminance meter to record the average luminance of a scene.

An objective measure of an object's brightness that is derived from the ratio of the luminance of its lightest section to that of its darkest section.

Luminescence is the emission of light that occurs without the involved substance undergoing a significant increase in temperature. In other words, it is the production and emission of light by a...

Fiber that emits luminescent radiation excited by ultraviolet, x-ray or high-energy particles.

A collector for photovoltaic power generation that consists of a thin plate that strongly absorbs sunlight striking its faces and luminesces efficiently. A large fraction of the luminescence is...

Quality or state of being luminous.

Plot of spectral luminous efficiency vs. wavelength.

Pertaining to electromagnetic radiation as perceived by the eye; that is, with the contributions as wavelengths in the visible region weighted according to V(λ), which symbolizes the spectral...

Quotient of total luminous flux divided by total radiant flux; lumens per watt. (For a source, quotient of total luminous flux divided by lamp power input.)

Ratio of radiant flux weighted according to V(l), the spectral luminous efficiency, to the corresponding radiant flux.

Luminous flux emitted per unit area of a source, expressed as lumens per area.

A measure of the time-integrated amount of flux. It has units of lumen-seconds and might be used to describe such things as the radiant energy that the eye would receive from a photographic flash.

The total luminous flux given out per unit area; i.e., the sum of the luminous emittance and any radiation that is reflected from and/or transmitted through the surface: expressed as lumens per area.

Descriptive of the radiant power of visible light modified by the eye response. It is the measure of the flow of visible light energy past any given point in space in a given time period, and is...

Luminous flux emitted by a source in a given range of directions; the unit of measure is the lumen/steradian, now known as the candela.

A mixture of phosphor with a trace of a radioactive element that emits faint light continually. It is used on watch dials and as a marker inside dark rooms to show the location of switches.

A photometric instrument having two prisms clamped in optical contact to produce a photometric field with an acute dividing line.

A high-resolution spectroscopic device commonly used in the early 20th century as a component of double-beam interferometers. Light introduced at one end of the plane-parallel glass plate will be...

A dielectric sphere with an index of refraction that varies with distance from the sphere center. A parallel beam of rays is focused on the lens surface at a point diametrically opposite the...

SI unit of luminous incidence or illuminance, equal to 1 lumen per square meter.

SI unit of light exposure.

An illuminometer designed to measure illumination in terms of luxes.

A spectrum that is continuous in the UV region with borders ranging from the visible to 300 A and lower.

The hydrogen-derived, ultraviolet radiation running from 1216 to 512 A, discovered by Theodore Lyman in 1914.

A type of filter consisting of a series of birefringent crystals and polarizers invented by French astronomer Bernard Lyot for isolating and observing significant wavelengths of solar light.

A physical stop that is conjugate to the entrance pupil and is used primarily to reduce diffraction effects at longer wavelengths.