Time evolution of the mid-IR flux of the Homunculus nebula around η Car from 1968 to 2018 in four wavelength regions, chosen for best temporal coverage. Vertical dashed lines indicate periastron passages. The 1σ region of the average flux for each wavelength region is shown (averages exclude the lower ISO flux values). There is no evidence for a long-term change, but variations with the orbital period cannot be ruled out.