  We present ISOPHOT observations between 60 and 200 microns and a
near-infrared extinction map of LDN 1780. We have used the ISOPHOT data
together with the 25, 60 and 100 microns IRIS maps to disentangle the warm and
cold components of large dust grains that are observed in translucent and dense
clouds. The warm and cold components in LDN 1780 have different properties and
spatial distributions, with the warm component surrounding the cold component.
The cold component is associated with molecular gas at densities of ~1000
cm^-3. The warm component has a uniform colour temperature of 25 K, and the
colour temperature of the cold component slightly varies between 15.8 and 17.3
K (beta=2, Delta(T)=0.5 K). The ratio between the 200 microns emission of the
cold component and Av is Ic(200)/Av=12.1 MJy/sr/mag and the average ratio
tau(200)/Av=2.0x10^-4 mag^-1. The far-infrared emissivity of the warm component
is significantly lower than that of the cold component. The Halpha emission and
Av correlate very well; a ratio I(Halpha)/Av=2.2 Rayleigh/mag is observed. This
correlation indicates the presence of a source of ionisation that can penetrate
deeply into the cloud. Using the ratio I(Halpha)/Av we have estimated a
ionisation rate for LDN 1780 that results to be ~10^-16 photons/s. We interpret
this relatively high value as due to an enhanced cosmic ray flux of ~10 times
the standard value. The origin of the cosmic rays could be from supernovae in
the Scorpio-Centaurus OB association and/or the runaway zeta Ophiuchus.
