Based on IRAS images, we have selected a sample of dense clouds with regular IR morphology and well defined cold cores. The cores have been identified by their strong 100 micron emission and lack of any 60 micron emission. We want to map these clouds in the far-IR with full ISO resolution at the longest ISOPHOT wavelengths. These observations provide unique data to address the following aims: (1) to study the structure of clouds in the very high density regions which are normally only observable in molecular transitions at radio and mm wavelengths. (2) to refine the estimate of the far IR mass absorption coefficient to constrain proposed dust models. The mass absoption coefficient is still highly uncertain in dense regions, improvement will also serve the interpretation of other ISO observations. (3) to study the variation of the IR colors on the edges of the cloud cores. The IRAS observations have shown that there exists a small transition region surrounding the core where the FIR colors dramatically change.