Determine the DISC to DIRINT multiplier `dirint_coeffs`.

    dni = disc_out['dni'] * dirint_coeffs

    Parameters
    ----------
    times : pd.DatetimeIndex
    kt_prime : Zenith-independent clearness index
    solar_zenith : Solar zenith angle
    w : precipitable water estimated from surface dew-point temperature
    delta_kt_prime : stability index

    Returns
    -------
    dirint_coeffs : array-like

def _dirint_coeffs(times, kt_prime, solar_zenith, w, delta_kt_prime):
    """
    Determine the DISC to DIRINT multiplier `dirint_coeffs`.

    dni = disc_out['dni'] * dirint_coeffs

    Parameters
    ----------
    times : pd.DatetimeIndex
    kt_prime : Zenith-independent clearness index
    solar_zenith : Solar zenith angle
    w : precipitable water estimated from surface dew-point temperature
    delta_kt_prime : stability index

    Returns
    -------
    dirint_coeffs : array-like
    """
    kt_prime_bin, zenith_bin, w_bin, delta_kt_prime_bin = \
        _dirint_bins(times, kt_prime, solar_zenith, w, delta_kt_prime)

    # get the coefficients
    coeffs = _get_dirint_coeffs()

    # subtract 1 to account for difference between MATLAB-style bin
    # assignment and Python-style array lookup.
    dirint_coeffs = coeffs[kt_prime_bin-1, zenith_bin-1,
                           delta_kt_prime_bin-1, w_bin-1]

    # convert unassigned bins to nan
    dirint_coeffs = np.where((kt_prime_bin == 0) | (zenith_bin == 0) |
                             (w_bin == 0) | (delta_kt_prime_bin == 0),
                             np.nan, dirint_coeffs)
    return dirint_coeffs