Etendue

The etendue (French: spread ) measures the length of a beam in the geometrical optics. The etendue is made up of the cross section and the solid angle. For an infinitesimal ray bundle the infinitesimal etendue is defined as:

In which

• The solid angle
• The surface vector of the cross section
• Normalized to the refractive index of the radiation are Richtungsvekor.

The etendue is conserved geometrical optics, ie does not change during the passage of a beam through an optical system. She is a Lagrange invariant.

For an imaging system, the etendue is also referred to as the etendue of light conductance and measures the maximum of the transmitted radiation.

Examples

For non-imaging systems, such as those found in lighting technology, describes the etendue of a light source, how strong the light is already fanned:

• A light source such as a fluorescent lamp, a correspondingly large etendue, it can - if there is no light output should be lost - narrowly focused light are produced only with an extremely large reflector.
• A point light source such as a metal halide lamp has a much smaller etendue and thus allows the directed emission of large parts of the light having a relatively small reflector.