def set_illuminant_datable(illuminant, CCT):
    """
    Set the *Illuminant* `DataTable` content for given illuminant name.

    Parameters
    ----------
    illuminant : str
        Existing illuminant name or *Custom*, *Daylight* or *Blackbody*.
    CCT : numeric
        Custom correlated colour temperature (CCT) used for the *Daylight* and
        *Blackbody* illuminant types.

    Returns
    -------
    tuple
        Tuple of data and columns.
    """

    logging.info(
        'Setting illuminant datatable for "%s" illuminant and "%s" CCT...',
        illuminant,
        CCT,
    )

    labels = ["Wavelength", "Irradiance"]
    ids = ["wavelength", "irradiance"]
    precision = [
        None,
        Format(precision=DATATABLE_DECIMALS, scheme=Scheme.fixed),
    ]
    columns = [
        {
            "id": ids[i],
            "name": label,
            "type": "numeric",
            "format": precision[i],
        }
        for i, label in enumerate(labels)
    ]

    if illuminant == "Custom":
        data = [
            dict(wavelength=wavelength, irradiance=None)
            for wavelength in CUSTOM_WAVELENGTHS
        ]

    else:
        if illuminant == "Daylight":
            xy = CCT_to_xy_CIE_D(CCT * 1.4388 / 1.4380)
            illuminant = sd_CIE_illuminant_D_series(xy)
        elif illuminant == "Blackbody":
            illuminant = sd_blackbody(CCT)
        else:
            illuminant = SDS_ILLUMINANTS[illuminant]

        data = [
            dict(
                wavelength=wavelength,
                irradiance=illuminant[wavelength],
            )
            for wavelength in illuminant.wavelengths
        ]

    return data, columns