def intensity_perpendicular_from_monomer_n(M, A_1, A_2, b):
    """
    Calculate perpendicular intesity nominator from monomer fraction,
    monomer and dimer anisotropy, brightness relation and monomer brightness.
    """
    return (((1-A_1) - (1-A_2)*b) *M + (1-A_2)*b)