  Eta Car is one of the most luminous and massive stars in our Galaxy and is
the brightest mid-infrared (mid-IR) source in the sky, outside our solar
system. Since the late 1990s the central source has dramatically brightened at
ultraviolet and optical wavelengths. This might be explained by a decrease in
circumstellar dust extinction. We aim to establish the mid-IR flux evolution
and further our understanding of the star's ultraviolet and optical
brightening. Mid-IR images from $8-20~\mu$m were obtained in 2018 with VISIR at
the Very Large Telescope. Archival data from 2003 and 2005 are retrieved from
the ESO Science Archive Facility and historical records are collected from
publications. We present the highest angular resolution mid-IR images of $\eta$
Car to date at the corresponding wavelengths ($\geq 0.22''$). We reconstruct
the mid-IR evolution of the spectral energy distribution of the spatially
integrated Homunculus nebula from 1968 to 2018 and find no long-term changes.
Eta Car's bolometric luminosity has been stable over the past five decades. We
do not observe a long-term decrease in the mid-IR flux densities that could be
associated with the brightening at ultraviolet and optical wavelengths, but
circumstellar dust must be declining in our line-of-sight only. Short-term flux
variations within about 25% of the mean levels could be present.
