  Total solar irradiance (TSI) has been monitored from space since 1978. The
measurements show a prominent variability in phase with the solar cycle, as
well as fluctuations on timescales shorter than a few days. However, the
measurements were done by multiple and usually relatively short-lived missions
making the possible long-term trend in the TSI highly uncertain. While the
variability in the UV irradiance is clearly in-phase with the solar cycle, the
phase of the variability in the visible range has been debated. In this paper,
we aim at getting an insight into the long-term trend of TSI since 1996 and the
phase of the solar irradiance variations in the visible part of the spectrum.
We use independent ground-based full-disc photometric observations in Ca~II~K
and continuum from the Rome and San Fernando observatories to compute the TSI
since 1996. We follow the empirical San Fernando approach based on the
photometric sum index. We find a weak declining trend in the TSI of
-7.8$^{+4.9}_{-0.8}\times10^{-3}$ Wm$^{-2}$y$^{-1}$ between the 1996 and 2008
activity minima, while between 2008 and 2019 the reconstructed TSI shows no
trend to a marginally decreasing (but statistically insignificant) trend of
-0.1$^{+0.25}_{-0.02}\times10^{-3}$ Wm$^{-2}$y$^{-1}$. The reference TSI series
used for the reconstruction does not significantly affect the determined trend.
The variation in the blue continuum (409.2 nm) is rather flat, while the
variation in the red continuum (607.1 nm) is marginally in anti-phase, although
this result is extremely sensitive to the accurate assessment of the quiet Sun
level in the images. These results provide further insights into the long-term
variation of the total solar irradiance. The amplitude of the variations in the
visible is below the uncertainties of the processing, which prevents an
assessment of the phase of the variations.
