Definition
The RUF is the ratio of the "luminous flux into the sky caused by the current lighting design" to the "luminous flux into the sky that cannot be avoided when the lighting task is fulfilled".
Calculation
Luminous flux into the sky caused by the current lighting design
Starting from the luminaires of the obtrusive light scene, photons are emitted taking the light distribution curves into account. The photons that "escape" directly into the sky are stored.
The photons that hit scene surfaces are redirected considering the material properties of the surfaces hit. If a photon no longer hits a surface on its way through the scene, this photon is also stored.
Once a sufficient number of photons have been sent from all luminaires to be considered, the "sky photon storage" is evaluated, and the luminous flux is calculated from it.
In the RUF calculation, the RUF surfaces are treated as gray scene surfaces with their set reflectance. The set reflection factor should ideally correspond to the average reflectance of the surfaces they occlude.
Luminous flux into the sky that cannot be avoided
For a RUF surface, the user sets a reflectance ๐ and a target illuminance E-Target. To calculate the "unavoidable luminous flux," it is assumed that the RUF surface receives a uniform illuminance E-Target. The theoretically unavoidable luminous flux into the sky is calculated as follows:
A (the area of the RUF surface) is determined by the construction of the surface. Cutouts are taken into account. Other geometries are ignored.
Multiple RUF surfaces in one scene
The RUF is a metric calculated for the entire scene and not for each RUF surface separately.
In the following example, two RUF surfaces are placed. The resulting RUF in this example is calculated as follows:
Evaluation of the RUF
The larger the RUF, the greater the avoidable portion of obtrusive light.
A RUF of 1 is theoretically ideal.
If the RUF is less than 1, it indicates that not all target illuminances of the RUF surfaces have been achieved. However, since obstruction is ignored when calculating the "unavoidable obtrusive light," it is possible to obtain results < 1 even with ideal illumination of all RUF surfaces.