## 1 Introduction  
Stains in occlusal pits and fissures can completely mask early enamel demineralization and hence confound visual detection of incipient, remineralizable caries states. The clinical standard of visual and radiographic examination for caries detection shows sensitivities of only 49.1% and 36.0%, respectively, for occlusal lesions in Germany [1]. Due to the poor performance of these techniques, some dentists may resort to tactile examination or probing, which can cause traumatic enamel defects in lesions [2; 3]. Occlusal caries refers to curious lesions originating from a tooth's occlusal surface. Due to the jagged cusp-and-fissure relief of occlusal surfaces, dental biofilms (plaque) can more easily persevere despite mechanical plaque removal (tooth brushing). In particular, fissures are often so narrow that a tooth brush's bristles cannot penetrate to and hence clean the fissure bottom, allowing plaque to remain and mature. Bacteria in dental plaque secrete acids, demineralizing the tooth's enamel and causing dental caries. When this demineralization is confined to enamel, the lesion can be treated non-invasively by remineralization techniques. However, if gone unnoticed, the lesion progresses, penetrating into dentin at the fissure bottom (Figure 1). Then, restorative therapy is required. Hidden caries are a form of occlusal caries where the lesion originates in the fissure bottom and spreads laterally, undermining a superficial layer of sound enamel. Therefore, the lesion is unlikely to be detected by visual inspection, even by experienced clinicians. The inadequacy of conventional techniques underlines the need for novel techniques with improved incipient lesion detection and caries-monitoring performance [4; 5; 6]. Demineralization causes changes to the enamel spectrum in the visible (VIS) and near-infrared (NIR) region due to an increase in pore volume [7]. On this basis, spectral imaging has been used previously in vitro for lesion detection, primarily focusing on NIR wavelengths beyond 1000 nm [8; 9; 10; 11; 12]. NIR wavelengths below 1000 nm have the advantage of being detectable with moderately priced silicon sensors at higher resolution, as opposed to the more costly and lower-resolved indium gallium arsenide (InGaAs) sensors required for higher wavelengths [13]. Hyperspectral imaging (HSI) was originally developed by NASA as a remote sensing technique [14]. It allows for the acquisition of two-dimensional spatial (morphological) images, while providing chemical information as an additional third, spectral dimension (Scheme 1). Figure 1: (**a**) The fissure is a predilection site for occlusal caries as its narrow anatomy hampers mechanical plaque removal. (**b**) Various lesions often originate from the bottom of the fissure and are characterized by demineralized enamel. If detected at this stage, non-invasive remineralization therapy can be performed. (**c**) Going unnoticed, the lesion progresses, infecting dentin and (**d**) eventually causing cavitation. Here, remineralization is no longer sufficient; restorative therapy is needed. As light enters a material, it is repeatedly scattered, absorbed and reflected, and conclusions on a material's composition can be drawn from its light-scattering and reflectance properties [15; 16]. HSI detects the reflected light and has thus evolved to become a diagnostic technique in numerous medical fields, including cancer detection [17; 18; 19; 20; 21], heart disease [22] and retinal pathologies [23; 24]. In dental caries detection, HSI relies on changes in enamel's light reflectance due to biochemical and morphological alterations induced by a curious lesion [7]. HSI shows promising potential for early lesion detection in a clinical setting, with short image acquisition times and noncontact, nonionizing imaging [11; 12]. Unlike previously applied spectral imaging approaches that focus on specific wavelengths, here, hyperspectral imaging considers a wide range of wavelengths and combines spatial (morphological) and spectral (chemical) information [13].