Gains in sensitivity with ISOCAM, compared to IRAS at 12 µm, have made it possible to extend the range of MIR counts by three orders of magnitude. Similarly ISOPHOT allowed to perform, for the first time, surveys at 175 µm, a range of considerable cosmological interest. The ISO counts give a radically new view of star formation in the universe between now and z ~ 2.
6.1. MIR templates and K correction effects
ISOCAM surveys have essentially been performed in two filters,
around 6.75 and 15 µm (but see also the 12 µm
survey of
[Clements
et al. 1999]).
Throughout this review, we have
presented, discussed or alluded to template spectra of the various
galaxy types in the MIR. For nearby objects, the 6.75 µm filter
probes infrared bands, while the 15 µm filter probes
preferentially warm dust and neon lines. As the redshift increases,
the infrared bands are more and more shifted to the 15 µm
filter.
For galaxies intrinsically bright in these bands, the K correction at
15 µm is positive (sources appear to be fainter with increasing
distances), but flat between
z 
0.4 and
1.3. For objects with
z > 0.4, the 6.75 µm filter has access only to the
starlight or to
an eventual AGN contributions; the same is true at 15 µm if
z > 1.5 (see
[Aussel et
al. 1999a];
[Elbaz et
al. 1999]).
The situation is even more favorable for surveys with ISOPHOT at 175 µm; there, since most galaxy SEDs peak well below this wavelength, as z increases, the K correction is negative, favoring detection of distant galaxies.
The fields surveyed have been selected for their low zodiacal and cirrus emission; the second point is particularly important for the FIR studies. They can be found, in the Northern hemisphere, in the Lockman Hole, and in the Southern hemisphere, in the Marano field. Both fields have been the subject of multiwavelength studies, but often not to the depth required to interpret the results of ISOCAM surveys. These surveys are therefore well complemented by studies on well-known fields, the HDF North and a CFRS field at 6.75 and 15 µm, and the SSA13 field at 6.75 µm. Few results are available on the other fields at 6.75 µm, where observations at other wavebands, not yet available for many of the surveys, are crucial to avoid contamination by galactic stars (but see [Taniguchi et al. 1997]; [Flores et al. 1999a]; [Sato et al. 1999]), and for lack of space we concentrate here, for ISOCAM, on 15 µm results.