The BACKGROUND command estimates the background in the image. The method used divides the image into equal boxes to estimate the local background intensity. The event distribution in each box is checked for spatial uniformity and consistency with the expected statistical distribution (Poisson or Gaussian, depending on image type). If inconsistency between the observed and expected distribution is found the box is rejected. The distribution of the average background values in all boxes is then compared to a Gaussian distribution with mean equal to the mean of all the background values found. All boxes where the background level is more than three standard deviations away from the mean are rejected. If more than 70 % of the boxes are rejected the background routine stops and an error message is generated. The value of the background intensity returned is the average of the values measured in all accepted boxes. A P(D) can be obtained using the background/npd=n/box=m/plot. The output is written to the file pd.qdp.