## 1 Introduction  
Hexagonal boron nitride (hBN) is a wide-bandgap van der Waals material [1; 2] that features a remarkable chemical inertness [3; 4], making it an ideal capping or encapsulating material for two-dimensional (2D) crystals [5; 6; 7; 8; 9; 10; 11; 12]. In fact, hBN is currently widely used to protect air-sensitive 2D materials from oxidation and deterioration effects [13; 14; 15]. The use of hBN as a substrate or as a capping material has also been shown to have marked beneficial effects on the charge carrier mobility of 2D materials [5; 14; 16; 17], or on their light emission properties, leading to a significant narrowing of the exciton linewidth towards the homogeneous limit [8; 9; 18]. The top and bottom hBN encapsulation flakes may also form a microcavity-like structure that enables control of the radiative lifetime of excitons in 2D semiconductors through the Purcell effect by simply tuning the thickness of the hBN [19]. hBN can even have structural stabilization effects on mechanically-deformed 2D materials, as in the case of 2D-material bubbles for which hBN capping prevents their deflation when reducing the pressure of the substance trapped inside, e.g., when reducing temperature [20; 21]. This has enabled the achievement of remarkably high strainseven at cryogenic temperatures, providing a means to generate quantum emission in WS\({}_{2}\)[20]. hBN has also attracted wide interest for its intrinsic properties, such as its remarkable mechanical robustness [22; 23; 24; 25], which was exploited for the realization of high-quality mechanical resonators [26], or its capability to sustain the propagation of hyperbolic phonon polaritons [27; 28; 29]. Moreover, hBN hosts color centers [30; 31; 32; 33; 34; 35]--discrete states lying in the middle of the bandgap formed by crystal vacancies or impurities-- that are optically active even at room-temperature and whose zero-phonon line spans a wide range from near-IR [31; 36; 37] up to UV [32]. These properties are made even more relevant by the fact that the hBN crystal itself can be patterned to form photonic cavities with an extremely high quality factor that Purcell-enhances its quantum emission [37], or the emission of other 2D materials deposited on it [38]. Boron vacancies and carbon-related defects in hBN have also been shown to act as spin defects that can be optically read-out and manipulated, representing an ideal platform for quantum sensing [35; 39; 40; 41; 42]. Depending on the use case, hBN layers with different thicknesses are employed. For example, single or few layers are used when hBN acts as a gate dielectric [5; 43] or as a spacer in 2D devices [44; 45]; flakes with variable thicknesses, from monolayer up to tens of layers, are used as tunneling barriers between 2D materials in transistor and diode devices [46; 47; 48; 49]; flakes with thicknesses of tens of layers are used when hBN is employed to encapsulate other van der Waals materials and heterostructures [18]; and bulk hBN layers with thicknesses of hundreds of nm are selected for the realization of photonic cavities [37]. The remarkable versatility of hBN calls for quick and non-invasive methods to identify the layer thickness. Optical methods have indeed shown to be very effective for identifying monolayers or few-layer-thick flakes of van der Waals materials such as graphene and transition metal dichalcogenides (TMDs) [50; 51; 52; 53; 54; 55; 56; 57]. From very early studies, it was immediately clear that optical interference effects are of utmost relevance to determine the thicknesses of atomically thin materials [58]. For atomically thin flakes with just one or a few layers, cavity effects are indeed essential to make them visible using conventional light microscopy. SiO\({}_{2}\)/Si substrates were quickly shown to facilitate the observation of thin layers of 2D materials with sufficient optical contrast, thanks to Fabry-Perot effects involving the 2D material and the SiO\({}_{2}\) film.