## 4 Discussion  
The fungi could acquire the elements needed for growth from the deteriorating glass without an external supply of mineral nutrients [11]. Fungi have a great ability to metabolize a wide range of carbon sources and grow on every inorganic material easily [23]. Thus, the coinfection of different microorganisms and favorable environmental conditions  
Figure 4: The effect of fungal growth on light transmission through fungus-inoculated glass samples in the visible light spectrum. Figure 5: SEM micrographs of glass after 28 days of the test: (**a**) fungal growth on glass after 28 days; (**b**) sample photo taken after 28 days of the alcohol-purge test; (**c**) glass sample after six months of testing showing fungal spores; (**d**) glass sample after six months of testing showing the glass corrosion caused by fungi. play a significant role and could affect the biodeterioration and biocorrosion outcomes on the optical instruments. The main component of glass is SiO\({}_{2}\) and other metal oxides, i.e., Na\({}_{2}\)O, K\({}_{2}\)O, CaO, Al\({}_{2}\)O\({}_{3}\), B\({}_{2}\)O, BaO, ZnO, As\({}_{2}\)O\({}_{3}\), and La\({}_{2}\)O\({}_{3}\). Elements such as Mg, Zn, Co, and Cr are also added to change the glass properties, including the magnification coefficient, mechanical strength, refractive index, color, and opacity transparency [24; 25]. The glass surface of binoculars is also coated by antireflective layers, including the outer layer consisting of low-refractive-index dielectric materials that are environmentally resistant, such as MgF\({}_{2}\), Na\({}_{3}\)AlF\({}_{6}\), CaF\({}_{2}\), and LiF\({}_{2}\). The inner layer is usually a dielectric with a high refractive index and good adhesion to the glass surface, including ZnO, TiO\({}_{2}\), CeF\({}_{3}\), and ThO\({}_{2}\)[26]. Moreover, a lipid layer is coated around the edges of eyepieces to prevent dirt particles from spreading and sticking to the inner glass surface and the optical system. This lipid layer consists of long-chain hydrocarbons, fatty acids, and some alkaline metals such as Li, Na, K, Ca, Ba, Al, Zn, and Pb, together with dust and other organic contaminants, providing the primary growth medium to initiate microbial growth. Such nutrients are sufficient for airborne fungal spores to germinate and develop into hyphae. Once the hyphal growth occurs on the glass surface, it quickly corrodes the inner surface of eyepieces. _Aspergillus, Cladosporium_, and _Penicillium_ are the most prevalent fungi genera commonly found in indoor and outdoor environments, especially glass materials [4; 27; 28]. [PERSON] et al. identified six fungal genera, among which _Penicillium_ and _Cladosporium_ were the two predominant fungal genera isolated from stained-glass windows in Portugal [4]. In addition, _Cladosporium_ and _Phoma_ were also reported from glass samples collected at two Catalonian churches [9]. _Aspergillus, Penicillium_, and _Alternaria_ were found to contribute to the formation of pits in optical glass [16]. In the present study, _Aspergillus_ and _Penicillium_ were the two most dominant fungal genera, accounting for 75% of the total identified fungal strains. They are known as primary contaminants that harm historical and cultural heritages and cause glass corrosion [2; 9; 29]. Because of the ability to produce numerous spores easily dispersed in the air, _Aspergillus_ and _Penicillium_ are primary corrosive agents [30]. Earlier regional studies and reports on the frequent occurrence of these fungi in all areas of the northern provinces of Vietnam suggest the high load of these fungal spores. Therefore, chances of risk are increased for fungal contamination of optical equipment if no preventive measures are taken. Interestingly, fungal genera Byssochlamys, Curvularia, Phomopsis, Coprinellus, Perenniporia, Talaromyces, Pithomyces, Neopestalotiopsis, Pleospora, and Humicola were identified for the first time as harmful agents on optical instruments.