A characterization of three sky scanners available on the market is done and a side by side comparison of two similar instruments is conducted over a six months 15 minutes measurements period. On the basis of a 4 years-instruments of 15 minutes measurements acquisition period in Geneva, Switzerland, a definition of a data bank validation criterion is attempted. The conclusion is that it is very difficult to define a common procedure. In fact, the instruments are too different in their geometry, calibration, spectral sensitivity, scan duration, aperture angle, dynamic, etc.