def calc_finesse(FSR, FWHM):
    """
    Returns the FP Finesse.

    Parameters
    ----------
    FSR (float) : free-spectral-range in BCV or A
    FWHM (float) : full-width-at-half-maximum in BCV or A

    Returns
    -------
    F (float) : the finesse

    Observations
    ------------
    Both FSR and FWHM have to have same units.
    """
    return float(FSR) / float(FWHM)