Using spectroscopy to probe stellar global parameters, such as effective temperature, is much exploited in the literature. In the solar case it can be used as an indicator of magnetic cycle variations. In this work we study the sensitivity to bright active regions of three photospheric lines (Fe I 537.9 nm, C I 538.0 nm and Ti II 538.1 nm), which have been monitored on the sun for more than twenty years. In our analysis we compare our experimental results, from observations with the THEMIS telescope, with theoretical results, obtained by spectral synthesis with FAL semi empirical models (Fontenla et al. [CITE], ApJ, 518, 480). We demonstrate the inappropriateness of using these lines (full disk integrated) as indicators of quiet sun irradiance modifications without considering their intrinsic variations due to active regions. Instead, their different sensitivity to the presence of active regions can be exploited in order to discriminate the background quiet sun variations from the magnetic region contributions.