## 1 Introduction  
Nowadays, fast and accurate methods for assessing important parameters of finished pharmaceutical preparations are needed. Determining the stability of a drug formulation is one of the crucial parameters to ensure the efficacy of ongoing pharmacotherapy throughout the shelf life of a medicinal product [1]. Such methods, including spectral ones, are being implemented in other areas of science, and are still being improved which leads to the creation of compact devices that allow measurements to be taken under almost any conditions. In addition, these techniques are fast, inexpensive, and simple to perform, which are undoubtedly their advantages. Any opaque boundary surface partly absorbs and partially reflects incident light [2]. Two characteristics of the resulting reflection are observed, i.e., the specular character when the laws of reflection are met and the diffuse character otherwise. The most common type is directional reflection (so-called shiny reflection) or directional-diffuse reflection. This is an intermediate situation. For very smooth surfaces, with micro-irregularities much smaller than the wavelength of light, measuring the intensity of light reflected from the tested object at a specular angle allows for determining the roughness and other surface parameters [3]. The directional reflectance of a surface is defined as the ratio of the total energy reflected into the hemisphere from which the reflection occurs to the energy incident on the surface under examination. There are available data on the use of the described phenomenon in the cosmetics and pharmaceutical industry, e.g., concerning counterfeit drugs [4]. According to the International Conference on Harmonization (ICH) guidelines, generally, a single batch of a pharmaceutical product is tested for photostability for regulatory purposes. When tests give unclear results, a maximum of two subsequent batches should be analyzed. These tests should not be performed during the use of the product or for products not covered by overarching photostability guidelines. Therefore, in the absence of guidelines for assessing photostability during drug usage, the pharmaceutical industry is faced with the problem of how to design and conduct such an evaluation to ensure photostability [1]. Among active pharmaceutical ingredients (APIs), vitamins and antibiotics may be particularly radiation-sensitive. Clindamycin is a semi-synthetic antibiotic belonging to the incosamides and is a derivative of Incomycin, a naturally occurring antibiotic extracted from _Streptomyces Incolensis_. Over time, it almost completely replaced the original one due to its increased antibacterial activity and better absorption from the gastrointestinal tract [5]. Clindamycin has a wide range of uses, including treating infections of the teeth, bones, and joints, and the genitourinary system. It is also used to treat diabetic feet and prevent endocarditis [6; 7]. The mechanism of action of clindamycin is to inhibit ribosomal translocation in bacteria, resulting in reduced protein production. It is most effective against aerobic Gram-positive bacteria and some Gram-negative bacteria [8]. Despite the passing years since its discovery, clindamycin is still on the WHO list of essential medicines. Data from 2020 showed that it was ranked 125 th among the most commonly prescribed drugs in the US [5]. Penicillin is a \(\beta\)-lactam antibiotic and is among the first-line drugs to treat those infections of orthopedic and soft tissue which are caused by penicillin-sensitive _Staphylococcus aureus_ (_S. aureus_) and _Streptococcus_ species (_Strep._ spp.) [9]. Phenoxymethylpenicillin (_Penicillin_\(V\)) remains stable in gastric secretions so it can be used orally. Penicillin V is highly effective against Gram-positive granulomas such as _S. pyogenes_, _S. pneumoniae_, and _S. aureus_ that do not produce penicillin, and it is the drug of choice in upper respiratory tract infections [10].