Radiant intensity

Radiant intensity is radiant power emitted per unit solid angle in a specified direction. Its SI unit is the watt per steradian.

Radiant flux: Radiant flux is the rate at which radiant energy is emitted, transferred, or received, measured in watts. It is the total power whose directional distribution radiant intensity describes.
Inverse-square law: The inverse-square law states that a point source's intensity decreases with the square of distance in an unobstructed field. It converts source intensity into irradiance at a distance.
Laser: A laser is a source of coherent, highly directional electromagnetic radiation. Its narrow beam can concentrate radiant power into a small solid angle.
Irradiance: Irradiance is radiant power incident on a surface per unit area, measured in watts per square metre. Unlike radiant intensity, it is defined at a receiving surface rather than per direction at a source.
Solid angle: Solid angle measures the extent of a region on a sphere as seen from its center, in steradians. It supplies the directional area over which radiant power is counted.
Goniophotometer: A goniophotometer measures how a source's emitted radiation varies with direction. Its angular measurements can determine radiant intensity distributions.
Light-emitting diode: A light-emitting diode is a semiconductor device that emits light when electrically driven. Its angular radiant-intensity profile helps characterize optical output.
Radiant exposure: Radiant exposure is radiant energy received per unit area over a specified time, measured in joules per square metre. It accumulates incident energy over time instead of specifying directional power.
Steradian: The steradian is the SI unit of solid angle, equal to the area subtended on a unit sphere. Radiant intensity is expressed per steradian.
Radiation pattern: A radiation pattern describes how a source's emitted power varies with direction. Radiant intensity is the directional quantity commonly plotted in such patterns.

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