In brief
A reflection reaches the observation point if the direction to a point on the PV area lies within the reflection lobe. The lobe surrounds the reflection direction, which follows from the sun position and the module orientation; its size results from the module type and the diameter of the solar disc (0.53°).
From the solar position (Ch. V.1) and the surface normal (Ch. V.2), the reflection direction, the centre of the reflection lobe, follows for every time step. The lobe spans its angular range around this direction. A cell of the PV area counts as hit if the direction from the observation point to it lies within the lobe; in this direction the observer sees a reflection.
V.3.1 Reflection direction
The sun must illuminate the front side of the modules. The ideal specular direction follows from the law of reflection; the reflection offset δ turns it within the plane of incidence towards flatter angles.
V.3.2 Lobe boundary
The deviation between the reflection direction and the viewing direction to the cell is split into two components: one in the plane of incidence (major axis) and one perpendicular to it (minor axis). A hit occurs if the point lies within the ellipse whose semi-axes are half the lobe width plus half the solar diameter. The solar diameter is taken as 0.53°, because the sun is not a point source: every point of the solar disc produces its own lobe.
V.3.3 Several zones
For module types with zones, the ellipse is checked per zone, from the inside out. The innermost zone that still contains the point is decisive; its reflection profile determines the luminance. If the point lies outside all zones, there is no hit. All zones share the same centre and the same offset δ.
V.3.4 Edge cases
Solar positions for which the reflection lies far beside the PV area are discarded in advance without detailed checking. The margin of this pre-filter is 10° plus the largest lobe width of the module type beyond the extent of the area; it does not change the result.
For the perfect mirror, the hit region is the solar disc (radius 0.265°) without offset; the check is carried out point by point (Ch. V.2, section V.2.3, deep dive "Accuracy of the hit check").
Deep dive: formulas
Notation: d direction of propagation of sunlight (from the sun to the area), n surface normal (unit vectors in the east, north, up system).
- Front side. A time step qualifies if d·n < 0. Angle of incidence θ = arccos(−d·n).
- Ideal specular direction. r = d − 2(d·n)n. The direction in which the observer must look to see the reflection is −r.
- Offset δ. Rotation of −r by δ(θ) about the axis n × d, i.e. within the plane of incidence. For δ = 0 the rotation is skipped; at normal incidence the plane of incidence is undefined and the rotation has no effect.
- Lobe axes. Minor axis eᵩ = normal of the plane of incidence (n × d, normalised); major axis eₗ perpendicular to it and to the reflection direction.
- Hit. With v = unit vector from the observation point to the cell centre (or calculation point), u = v·eₗ, w = v·eᵩ:
(u / a)² + (w / b)² ≤ 1
a = (major axis(θ) + 0.53°) / 2 + capture margin
b = (minor axis(θ) + 0.53°) / 2 + capture margin
Capture margin: half the cell diagonal (0.354°) on the cell grid, half the point-grid diagonal for the point-by-point check. For a circular lobe, a = b = (spread(θ) + 0.53°) / 2 + capture margin. a and b are used in radians; for small angles u and w are the angular deviations in radians. - Zones. The test runs per zone with its a, b. Whether a cell is hit is decided by the cell centre with the capture margin; any zone suffices, and this determines the glare minutes. Which zone supplies the luminance is decided at zone boundaries by the calculation points of the cell: the innermost zone containing one of them (with half the point-grid diagonal as margin), i.e. the part of the cell that lies on the area; never a zone further in than the cell centre with margin gives. If only the margin hits, the outermost zone containing the cell with margin applies. In the point-by-point check, a cell receives the luminance of the innermost zone containing one of its hit points; each point contributes to the solid angle with the ratio of its zone luminance to that value, so the illuminance remains the sum over the zones (up to calculation state 2026.10.11-6 the mean of the zone luminances, too low at zone boundaries). Up to calculation state 2026.10.11-4 the zone was also chosen by the cell centre with margin; at the edge of the area, luminance and illuminance were therefore too high by up to the ratio of the zones, the glare minutes were not affected. - Curves. Values between the 10° supporting points are interpolated linearly. The calculation reads them from a table in 0.1° steps and also interpolates between these steps; this applies to the reflection profile, lobe axes, zones and offset. Up to calculation state 2026.10.12-8 the value at the angle of incidence rounded down to 0.1° applied; with steep curves and grazing incidence the luminance was therefore up to about 6 % too low.