Etendue
The étendue (French: expansion ) measures the expansion of a beam in geometrical optics . The étendue is made up of the cross section and the solid angle . For an infinitesimal bundle of rays, the infinitesimal étendue is defined as:
in which
- the solid angle ,
- is the area vector of the cross-section and
- is the directional vector of the radiation normalized to the refractive index .
The étendue is a conservation quantity of geometrical optics, so it does not change when a beam passes through an ideal optical system . It is a Lagrange invariant .
For an imaging system, the étendue is also known as the light guide value and measures the étendue of the maximum transmitted radiation.
Examples
For non- imaging systems , such as those found in lighting technology , the étendue of a light source describes how strongly the light is already fanned out:
- Has a light source such as If, for example, a fluorescent lamp has a correspondingly large étendue, tightly bundled light can only be generated with an extremely large reflector if no light output is to be lost .
- A point light source such as a metal halide lamp has a much smaller étendue and thus allows the directional emission of large parts of the light with a relatively small reflector.