def intensity_parallel_from_monomer(M, A_1, A_2, b, c_b_1):
    """
    Calculate parallel intesity from monomer fraction, monomer and dimer
    anisotropy, brightness relation and monomer brightness.
    """
    return (2/3)*(((A_1+0.5)-(A_2+0.5)*b) *M + (A_2+0.5)*b)*c_b_1