In fiber optics, the radiation angle is half the vertex angle

Vertex angle

In geometry, a vertex angle is the angle associated with a vertex of a polygon.The vertex angle in a polygon is measured by the interior side of the vertex. For any simple n-gon, the sum of the interior angles is π radians or 180 degrees....

 of the cone of light

Light

Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 emitted at the exit face of an optical fiber

Optical fiber

An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...

.

The cone boundary is usually defined (a) by the angle at which the far-field irradiance

Irradiance

Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...

 has decreased to a specified fraction of its maximum value or (b) as the cone within which there is a specified fraction of the total radiated power

Power (physics)

In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

 at any point in the far field.