– see Section \[isoimages\])).

ISOCAM observations
-------------------

### Observational parameters

The dense cores were mapped between April 1997 and March 1998 using the LW2 (5–8.5 $\mu$m) and LW6 (7–8.5 $\mu$m) filters of ISOCAM (Cesarsky et al. 1996), the spectro-imager aboard ISO (Kessler et al. 1996), centered at 6.75 $\mu$m and 7.75 $\mu$m respectively (except for Oph D, see below). These wavelengths approximately correspond to a minimum in the dust extinction curve (see the model of Draine & Lee 1984 for diffuse clouds), providing a good opportunity to peer deep inside dark clouds. We used a pixel field of view of 6$\arcsec$, which also roughly corresponds to the FWHM size of the ISOCAM point spread function (PSF) at $\sim$ 7 $\mu$m. For a “standard” observation, an area of $10.7\arcmin \times 11\arcmin$ was covered for each core with a raster map of $6 \times 4$ images and an overlap of 17 pixels in right ascension and of 6 pixels in declination. At each raster position, about 18 single read-outs were taken with an integration time per read-out of 2.1s. About 50 stabilisation read-outs had to be added at the beginning of each observation to allow the detectors to lose memory of the 