  Hardness ratios are commonly used in X-ray photometry to indicate spectral
properties roughly. It is usually defined as the ratio of counts in two
different wavebands. This definition, however, is problematic when the counts
are very limited. Here we instead define hardness ratio using the $\lambda$
parameter of Poisson processes, and develop an estimation method via Bayesian
statistics. Our Monte Carlo simulations show the validity of our method. Based
on this new definition, we can estimate the hydrogen column density for the
photoelectric absorption of X-ray spectra in the case of low counting
statistics.
