Definitions: L

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 lymphocyte is a type of white blood cell (leukocyte) that plays a central role in the immune system. Lymphocytes are produced in the bone marrow and mature in the lymphoid organs, such as the...

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.