  We consider the effect of mass segregation on the observable integrated
properties of star clusters. The measurable properties depend on a combination
of the dynamical age of the cluster and the physical age of the stars in the
cluster. To investigate all possible combinations of these two quantities we
propose an analytical model for the mass function of segregated star clusters
that agrees with the results of N-body simulations, in which any combination
can be specified. For a realistic degree of mass segregation and a fixed
density profile we find with increasing age an increase in the measured core
radii and a central surface brightness that decreases in all filters more
rapidly than what is expected from stellar evolution alone. Within a Gyr the
measured core radius increases by a factor of two and the central surface
density in all filters of a segregated cluster will be overestimated by a
similar factor when not taking into account mass segregation in the conversion
from light to mass. We find that the $V-I$ colour of mass segregated clusters
decreases with radius by about 0.1-0.2 mag, which could be observable. From
recent observations of partially resolved extra-galactic clusters a decreasing
half-light radius with increasing wavelength was observed, which was attributed
to mass segregation. These observations can not be reproduced by our models. We
find that the differences between measured radii in different filters are
always smaller than 5%.
