def low_luminance_tritanopia_factor(L_A):
    """
    Returns the low luminance tritanopia factor :math:`F_t` from given adapting
    field *luminance* :math:`L_A` in :math:`cd/m^2`.

    Parameters
    ----------
    L_A : numeric
        Adapting field *luminance* :math:`L_A` in :math:`cd/m^2`.

    Returns
    -------
    numeric
        Low luminance tritanopia factor :math:`F_t`.

    Examples
    --------
    >>> low_luminance_tritanopia_factor(318.31)  # doctest: +ELLIPSIS
    0.9996859...
    """

    F_t = L_A / (L_A + 0.1)
    return F_t