The `Hipparcos magnitude', designated as , is defined
by the pass-band of the Hipparcos main detection
chain, which ranges from 340 to 850 nm. This broad-band system yields
magnitudes close to the visual 
magnitude , but
shows residuals with respect to especially for red stars. For the
prediction of the magnitude with the specified accuracy (necessary for
the optimum allocation of satellite observing time, which varies strongly
with magnitude especially towards the fainter stars), it was necessary to
obtain both a magnitude and a colour index for each programme star.
Transformation formulae were established to obtain
from the magnitudes and colours available or newly observed in
various photoelectric systems (Grenon 1988b).
magnitudes given in the Input Catalogue have been derived using
on-ground calibration of the detection chain and preliminary
transformation formulae (which will be refined for the derivation of
the magnitudes from the satellite data). An
approximate relation valid for O-F stars and G-K giants in terms of the
Johnson magnitude and 
colour index is:
For variable stars, further information is given in field 26, and in fields 39-44.