def int_intensity(F2, L, P, I0, wavelength, cell_vol, cryst_vol):
    """
    Calculate the reflection intensities scaling factor
    
    INPUT:
    F2        : the structure factor squared
    L         : Lorentz factor
    P         : Polarisation factor
    I0        : Incoming beam flux
    wavelength: in Angstroem
    cell_vol  : Volume of unit cell in AA^3
    cryst_vol : Volume of crystal in mm^3

    OUTPUT:
    int_intensity: integrated intensity

    """
    emass = 9.1093826e-31
    echarge = 1.60217653e-19
    pi4eps0 = 1.11265e-10
    c = 299792458.0
    k1 = (echarge ** 2 / (pi4eps0 * emass * c ** 2) * 1000) ** 2
    k2 = wavelength ** 3 * cryst_vol * 1e+21 / cell_vol ** 2
    return k1 * k2 * I0 * L * P * F2
