Horizontal direction

Method

Each direction of the ground in front of the plane keeps its own weight, the cosine of its incidence angle times its solid angle. Its angular response is read at the incidence angle of the horizontal direction with the same azimuth (1), not at its own incidence angle. On a plane tilted 35° with the Fresnel curve for anti-reflective glass the ground factor is 0.944, against 0.830 with the true angles. A plane tilted less than 2° sees no ground, and its factor is 1.

The math

1. Ground-reflected light

$$F_{IAM,gnd} = \frac{\int_{gnd} F(\theta_0)\,\cos\theta\,d\Omega}{\int_{gnd} \cos\theta\,d\Omega}, \qquad \theta_0 = \arccos(\cos\gamma_r\,\sin\beta) \tag{1}$$

where

Symbol	Meaning	Unit
$F_{IAM,gnd}$	the angular-response factor for the ground-reflected light	—
$F(\theta_0)$	the angular-response curve read at $\theta_0$	—
$\theta_0$	the incidence angle of the horizontal direction with the same azimuth	degrees
$\gamma_r$	the azimuth of the direction measured from the plane's facing	degrees
$\beta$	the tilt of the module plane	degrees
$\theta$	the incidence angle of the ground direction itself	degrees
$d\Omega$	an element of solid angle below the horizon, taken where $\cos\theta > 0$	sr

References

1. Analysis of PVsyst 8.1.5 outputs shows this convention in its results.
2. PVsyst 6.8.8, the program's computation of the incidence-angle distribution of the isotropic diffuse and of the albedo, shown in its Detailed Study of IAM dialog (Effect on Diffuse and Albedo tab): the source of this method's bins, grid and ground rule, read from the program

API

iam.ground_iam_angle = horizontal_direction

Symbols follow the symbol registry.