def path_emission(iterable, emission=0):
    """Construct a common emission from an iterable of :class:`Material` objects

    Parameters
    ----------
    iterable: list_like
        List of :class:`Material` objects

    emission: scalar or :class:`~lentil.radiometry.Spectrum`, optional
        Emission collected upstream of the iterable path. This could be used to
        represent things like zodiacal background or warm cavities emitting
        into the first element of a path.

    Returns
    -------
    emission: :class:`~lentil.radiometry.Spectrum`

    """
    for item in iterable:
        emission = (emission * item.transmission) + item.emission
    return emission