Dust emission, extinction, and scattering in LDN 1642
  We study the near-infrared (NIR) scattering in LDN 1642, its correlation with
the cloud structure, and the ability of dust models to simultaneously explain
sub-millimetre emission, NIR extinction, and NIR scattering.
  We use observations from the HAWK-I instrument to measure the NIR surface
brightness and extinction. These are compared with Herschel data on dust
emission and, with radiative transfer modelling, with predictions calculated
for different dust models.
  We find an optical depth ratio $\tau(250\,\mu{\rm m})/\tau(J)\approx
10^{-3}$, confirming earlier findings of high sub-millimetre emissivity. The
relationships between the column density derived from dust emission and the NIR
colour excesses is linear and consistent with the standard NIR extinction
curve. The extinction peaks at $A_J=2.6\,$mag, the NIR surface brightness
remaining correlated with $N({\rm H}_2)$ without saturation.
  Radiative transfer models can fit the sub-millimetre data with any of the
tested dust models. However, these predict a NIR extinction that is higher and
a NIR surface brightness that is lower than in observations. If the dust
sub-millimetre emissivity is rescaled to the observed value of
$\tau(250\,\mu{\rm m})/\tau(J)$, dust models with high NIR albedo can reach the
observed level of NIR surface brightness. The NIR extinction of the models
tends to be higher than directly measured, which is reflected in the shape of
the NIR surface brightness spectra.
  The combination of emission, extinction, and scattering measurements provides
strong constraints on dust models. The observations of LDN 1642 indicate clear
dust evolution, including a strong increase in the sub-millimetre emissivity,
not yet fully explained by the current dust models.
