1 Introduction

ISOGAL is an ISOCAM survey at 7 $\mu$m and 15 $\mu$m, at a spatial resolution in pixel-field-of-view of 6 $^{\prime\prime}$ and sensitivity about 10 mJy, of about 16 deg², toward the Galactic plane mostly interior to $\vert l\vert = 30\hbox{$^\circ$ }$ (Omont et al. 2003). About 200 fields observed are well spread in the inner bulge and in the Galactic disk. In combination with the DENIS data (Epchtein et al. 1997), the colors between 15 $\mu$m, 7 $\mu$m, $K_{\rm S}$, J, I in the ISOGAL-DENIS catalogue (Schuller et al. 2003) allows a detailed study of cold stellar populations. For example, this survey shall result in a practically complete census of mass-losing AGB stars in the fields of the inner bulge and in some parts of the Galactic disk. The stars at the RGB tip may also be well characterized in the ISOGAL catalogue, as well as nearby or massive young stellar objects (YSOs). In addition to the study of cold stellar populations, another goal of ISOGAL is to study the Galactic structures in regions highly obscured through the inner Galaxy with a sensitivity and spatial resolution about two orders of magnitude better than IRAS.

FC-01863+00035 is one of the disk fields within the ISOGAL survey. In order to avoid strong sources saturating the ISOCAM detectors, an ISOGAL field is usually limited to a small $l \times b$ raster where no bright IRAS objects exist. The field FC-01863+00035 covers an area of about 0.1 deg² in the Galactic plane. Unlike the fields studied in the Galactic bulge by Glass et al. (1999) and Omont et al. (1999), this disk field suffers serious interstellar extinction in the Galactic plane. We picked it to make a case study of ISOGAL data in the Galactic disk, taking advantage of the recent availability of the ISOGAL-DENIS PSC. This line of sight is interesting because it crosses four spiral arms, with large values of visible extinction up to 30 mag and beyond, corresponding to strong CO emission (Bronfman et al. 1989). Being much closer to the Galactic Center than the field of the early study by Pérault et al. (1996), at $l = -45\hbox{$^\circ$ }$, it is more typical of the majority of ISOGAL fields. Being outside of the tangential direction of the molecular ring, it avoids too strong perturbations of the quality of ISOGAL data by star forming regions, while keeping nevertheless a non negligible number of detected YSOs. On the other hand, it is far enough from the Galactic Center so that disk sources well prevail against bulge ones, although this direction is neither very far from the far end of the bar structure (e.g. Lopez-Corredoira et al. 2001). In addition to the discussion of stellar populations detected by ISOGAL, which is more difficult than in the bulge because of the larger uncertainty on the distance of the sources, the main goal of this paper is to show how the combination of ISOGAL and DENIS data allows to study the properties and the structure of interstellar extinction in the inner Galactic disk.