import math


def effective_emissivity_two_dimension(emissivity_1: float, emissivity_2: float, l_d: float, l_s: float) -> float:
    """
    有効放射率の計算（二次元空間の場合）

    :param emissivity_1:    面1の放射率, -
    :param emissivity_2:    面2の放射率, -
    :param l_d:             通気層の厚さ, m
    :param l_s:             通気胴縁または垂木の間隔, m
    :return:                有効放射率, -
    """
    effective_emissivity = 1.0 / (1.0 / emissivity_1 + 1.0 / emissivity_2 - 2.0 + 1.0 /
                                  (1.0 / 2.0 * (1.0 + math.sqrt(1.0 + l_d ** 2.0 / l_s ** 2.0) - l_d / l_s)))
    return effective_emissivity