  The so called Li&Ma formula is still the most frequently used method for
estimating the significance of observations carried out by Imaging Atmospheric
Cherenkov Telescopes. In this work a straightforward extension of the method
for point sources that profits from the good imaging capabilities of current
instruments is proposed. It is based on a likelihood ratio under the assumption
of a well-known PSF and a smooth background. Its performance is tested with
Monte Carlo simulations based on real observations and its sensitivity is
compared to standard methods which do not incorporate PSF information. The gain
of significance that can be attributed to the inclusion of the PSF is around of
10% and can be boosted if a background model is assumed or a finer binning is
used.
