  Coronal active regions are observed to get fuzzier and fuzzier (i.e. more and
more confused and uniform) in harder and harder energy bands or lines. We
explain this evidence as due to the fine multi-temperature structure of coronal
loops. To this end, we model bundles of loops made of thin strands, each heated
by short and intense heat pulses. For simplicity, we assume that the heat
pulses are all equal and triggered only once in each strand at a random time.
The pulse intensity and cadence are selected so as to have steady active region
loops ($\sim 3$ MK), on the average. We compute the evolution of the confined
heated plasma with a hydrodynamic loop model. We then compute the emission
along each strand in several spectral lines, from cool ($\leq 1$ MK), to warm
($2-3$ MK) lines, detectable with Hinode/EIS, to hot X-ray lines. The strands
are then put side-by-side to construct an active region loop bundle. We find
that in the warm lines ($2-3$ MK) the loop emission fills all the available
image surface. Therefore the emission appears quite uniform and it is difficult
to resolve the single loops, while in the cool lines the loops are considerably
more contrasted and the region is less fuzzy. The main reasons for this effect
are that, during their evolution, i.e. pulse heating and slow cooling, each
strand spends a relatively long time at temperatures around $2-3$ MK, and that
it has a high emission measure during that phase, so the whole region appears
more uniform or smudged. We make the prediction that the fuzziness should be
reduced in the hot UV and X-ray lines.
