The Horizontal Sight Angle is the measurement at a point of the viewing angle towards the outside. The openings are the windows looking towards the exterior of the building. The horizontal sight angle of a room completely closed is 0°, the horizontal sight angle of a flat surface without any wall around is 360°.
The Horizontal Sight Angle indicator is used in particular by the European standard EN 17037 (in UK: BS EN 17037:2018 Daylight in buildings).
The EN 17037 standard uses 3 thresholds for the sight angle: 14°, 28° and 54°. This extension makes it possible to analyze a surface according to a customizable scale that can give more information during the design phase than these 3 thresholds.