Surface optical spectroscopy is of great interest, as it allows to acquire knowledge about optical properties of a surface without damaging the sample. In general, optical spectroscopy refers to several different optical measurement techniques, including DRS (differential reflectance spectroscopy) and RDS (reflectance difference spectroscopy). DRS and RDS play a non-negligible role in todays optical research, as both are highly powerful and versatile techniques for acquiring information on the reflectance of a surface in order to determine optical quantities of the sample. It is convenient to employ these two techniques during the deposition of molecules on top of a substrate. This approach reveals fundamental knowledge on the behavior of the growth process of the adlayer and provides important insights into the underlying physical principles. The fact that DRS and RDS are considered to be fundamentally different, despite both being spectroscopy methods, is a key argument for the motivation of this thesis. Therefore, the main subject of this thesis is the derivation of an analytical relation between these two optical spectroscopy methods and subsequently, an experimental proof of this relation. The aim is to show that DRS and RDS are not fundamentally different, but rather are even equivalent to each other to a certain extent. The first step of finding an analytical relation between DRS and RDS was to give a physical derivation of the DRS signal and the RDS signal, based on the well-known three-phase system of McIntyre and Aspnes. Their model represents an optical system, consisting of a substrate and an adlayer on top of the substrate, surrounded by an ambient phase. Based on these derivations, the DRS signal was put in relation to the RDS signal. Therefore, an analytical relation between DRS and RDS has been achieved successfully. Further, this relation represents the basis of the experimental part of this thesis, which pursues the experimental proof of the theoretical relation between DRS and RDS. To do so, several growth experiments have been carried out, whereas CoTMPP (Cobalt tetramethoxyphenylporphyrin) molecules have been deposited on Cu(110)-(2x1)O reconstructed surfaces, which have been prepared by exposing pristine Cu(110) samples to oxygen. CoTMPP is a metal-derivate of porphyrin, which has already been used for various applications in biology and chemistry, including solar cells and color sensors. The growth process was investigated with different optical setups, specifically polarization-dependent DRS at normal incidence as well as off-normal incidence (45) and RDS at normal incidence. In the case of polarization-dependent DRS at 45 incidence, it was not possible to find an experimental evidence of the theoretical expectation that puts DRS in relation to RDS. The reason may be that the evaluation in the case of DRS at off-normal incidence is linked to certain assumptions, which do not hold true over the whole energy range. Contrary, in the case of polarization-dependent DRS at normal incidence, it has been successfully shown that the experimental DRS and RDS data are linked according to the analytical relation. Further, this indicates that DRS and RDS are equivalent to a certain extent.