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The balloon borne experiment PRONAOS/SPM has measured the submillimeter emission from 200 to 600 pm with an angular resolution of 2-3.5' of a quiescent translucent filament, (A(v) similar to4) in the Taurus molecular complex. In order to analyse these data, we have developed a model for the emission of the filament using an independent tracer of the total column density (star count method on 2MASS stars) and a radiative transfer code. We first use in the model the optical properties of the dust from the standard model of Desert et al. (1990). The computed brightness profile fails to reproduce the data inside the filament. The agreement between data and model can only be obtained by changing the dust properties inside the filament. We have removed all particles not in thermal equilibrium from the densest part of the filament (typically n(H) similar to 10(4) cm(-3)), and multiplied the submillimeter emissivity by a significant factor (similar to3). This suggests that grain-grain coagulation into fluffy aggregates occur inside the filament.