def exptimewarning(bin_ix, events, verbose=0, ratio=0.5):
    """
    Passes a warning if the effective exposure time within a bin is
        significantly less than the raw exposure time, which might produce
        anomalous values due to counting statistics or be a symptom of a problem
        in the exposure time correction for this bin.

    :param bin_ix: Array indices designating which events are in the time bin
        of interest.

    :type bin_ix: numpy.ndarray

    :param events: Set of photon events to check if there is an effective
        exposure time warning.

    :type events: dict

    :param verbose: Verbosity level, a value of 0 is minimum verbosity.

    :type verbose: int

    :param ratio: The ratio of effective to raw exposure time in a bin below
        which the bin will be flagged.

    :type ratio: float

    :returns: bool -- Returns True/False whether a bin has a low exposure.
    """

    return (events['exptime'][bin_ix]/
            (events['t1'][bin_ix]-events['t0'][bin_ix]) < ratio)