To understand all of these effects, one turns to various types of spectra to decipher how the molecular and photonic degrees of freedom (DOFs) are coupled to each other and their surroundings. The simplest realization of this is the linear spectroscopy in which the system encounters a measurement photon at some frequency hω. For the molecule-cavity hybrid system, there are multiple types of linear spectroscopy that can be performed: (i) reflectance, (ii) absorption, and (iii) transmission. In the experiment, the reflectance R and transmission T spectroscopies are collected directly. The absorption A is then calculated as A% = 1 -T % -R% as an indirect measurement. Our focus here is the transmission spectroscopy whose intensity is proportional to the photon number inside the cavity, T ∝ ⟨ â † â⟩ .