HR2 and HR3 hardness ratio distribution of the optically bright sources (large dots) and optically faint/blank-field sources (small dots) from the hard-spectrum sample. The underlying grey-scale and contours show the pn-camera hardness ratio distribution of the entire high Galactic latitude (∣b∣ > 20°) 1XMM catalogue, and have a logarithmic scaling (Watson et al. 2003).