structure of starless cores. The outer density structure of cores and the possible presence of edges are difficult to constrain from (sub)millimeter observations. Fortunately, starless cores can also be studied in [*absorption*]{} at mid-IR wavelengths (e.g. Abergel et al. 1996, Egan et al. 1998), and this technique turns out to be more sensitive to the outer parts of cores. Another advantage of the mid-IR absorption technique over (sub)millimeter emission mapping is that the column density structure of the absorbing material may be derived without any assumption about the dust temperature distribution.

In order to gain further, independent insight into cloud core structure prior to protostar formation, we undertook dedicated mid-IR imaging observations toward a broad sample of starless dense cores with the ISOCAM mid-IR camera aboard the [*Infrared Space Observatory*]{} (ISO). The present paper discusses the results of this mid-IR imaging program, whose immediate goal was to detect the sample cores in absorption against the diffuse mid-IR background produced by the mean galactic interstellar radiation field (e.g. Mathis, Mezger & Panagia 1983) and/or arising from the envelopes of the parent molecular clouds (e.g. Bernard et al. 1993).

The layout of the paper is as follows. In Sect. 2, we give 