  At a redshift of 0.18, Abell 1689 is so far the most distant cluster of
galaxies for which substantial mid-infrared (MIR) data have been published. We
present here the follow-up optical photometric and spectroscopic observations
which were used to study the individual properties of the galaxies members of
A1689. Sources with a low [15 micron] / [6.75 micron] flux ratio typically
consist of luminous passive early-type galaxies while those with a high MIR
color index are mainly luminous, blue, emission-line, morphologically disturbed
spirals, i.e. the star-forming galaxies usually associated with the
'Butcher-Oemler' effect. On the other hand, at least 30% of the 15 micron
sources have optical counterparts showing no evidence of current star-formation
activity, while their 15 micron emission is most likely due to obscured star
formation. We argue that the LW3 luminosity measured in the cluster members is
a reliable tracer of the total infrared luminosity, L(IR). We derive from L(IR)
a star-formation rate free of dust extinction, SFR(IR), which we compare with
that determined in the optical from the flux of the [OII] emission line,
SFR(opt). The highest total star formation rates (11 Msun/yr) and dust
extinction are measured in those galaxies exhibiting in their optical spectrum
a signature of a dusty starburst. In contrast, none of the galaxies with
post-starburst optical spectra has been detected by ISOCAM. We find a median
SFR(IR) of the LW3-detected galaxies of 2 Msun/yr, that is ten times higher
than the median SFR(opt) of the [OII] detected galaxies. The ratio
SFR(IR)/SFR(opt) is in fact very high, ranging between 10 and 100 for
LW3-detected galaxies with [OII] emission. We conclude that a major part, at
least 90%, of the star formation activity taking place in Abell 1689 is hidden.
