  Continuum intensity observations obtained with the Michelson Doppler Imager
(MDI) on-board the SoHO mission provide long time series of filtergrams that
are ideal for studying the evolution of large-scale phenomena in the solar
atmosphere and their dependence on solar activity. These filtergrams, however,
are not taken in a pure continuum spectral band, but are constructed from a
proxy, namely a combination of filtergrams sampling the Ni I 676.8 nm line. We
studied the sensitivity of this continuum proxy to the shape of the nickel line
and to the degradation in the instrumental transmission profiles. We compared
continuum intensity measurements in the nearby of nickel line with MDI proxy
values in three sets of high resolution spectro-polarimetric data obtained with
the Interferometric Bidimensional Spectrometer (IBIS), and in synthetic data,
obtained from multi-dimensional simulations of magneto-convection and
one-dimensional atmosphere models. We found that MDI continuum measurements
require brightness corrections which depend on magnetic field strength,
temperature and, to a smaller extent, plasma velocity. The correction ranges
from 2% to 25% in sunspots, and is, on average, less than 2% for other
features. The brightness correction also varies with position on the disk, with
larger variations obtained for sunspots, and smaller variations obtained for
quiet sun, faculae and micropores. Correction factors derived from observations
agree with those deduced from the numerical simulations when observational
effects are taken into account. Finally, we found that the investigated
potential uncertainties in the transmission characteristics of MDI filters only
slightly affect the brightness correction to proxy measurements.
