# Oral and Dental Spectral Image Database--ODSI-DB
## 1 Introduction  
The majority of dental and oral imaging is performed by X-ray-based techniques or by digital photography. While panoramic radiographs and computed tomography scans reveal anatomical and pathological structures of the teeth and alveolar bone, they expose the patient to ionizing radiation and possibly to non-risk-free contrast agents. The risks increase the barrier for imaging. Additionally, these imaging methods are suitable for hard tissue only. Soft-tissue diagnostics requires alternate methods. Digital photography is more suitable for soft tissue and avoids these risks while still providing useful diagnostic information, even of hard tissue (although this is limited to surface features). From data analysis aspect, these methods offer only a limited amount of information. X-ray-based techniques produce intensity graphs of transmitted radiation, i.e., grayscale images, and allow for analysis of spatial structures and their brightness only [1; 2; 3; 4]. X-ray techniques do not acquire any color information or provide comparable multichannel images. Digital photography has been designed to accommodate human color vision, and therefore acquires only a limited amount of color information. A digital photograph consists of only red, green, and blue color channels designed to approximately match the short-, middle-, and long-wavelength detecting cones of the human eye. The color of an object, however, is a more complex question. An illuminated object reflects different wavelengths at different strengths--it has a wavelength-dependent _reflectance_. Accurate evaluation of an object's color depends on measuring this reflectance spectrum. Spectral imaging [5; 6] increases the number of channels by imaging narrow contiguous wavelength bands, and can thus record the reflection spectrum of a sample. Generally, spectral imagingsystems may also extend the data acquisition range outside of the visible range (380-780 nm) to cover parts of the ultraviolet (10-380 nm) and infrared regions (780 nm-1 mm). The data files produced by these methods--X-ray imaging, digital photography, and spectral imaging--can, therefore, be divided into three categories: grayscale, color, and spectral images (Figure 1). The additional information provided by spectral imaging enables new data analysis and visualization methods compared to grayscale or color images. In the context of dental and oral imaging, healthy and diseased tissue may exhibit subtle changes in their reflection spectra. Accurate spectral imaging solutions can capture these spectra with high fidelity, while data-analysis methods may be able to differentiate these spectra. Successful data-analysis can allow the design of optimized optical imaging solutions for targeted diagnostic analyses. In the simplest cases, spectral images can be used to design systems that enhance the visibility of a targeted feature, for example, by applying different weights on the bands while computing a color image representation [7], or by optimizing contrast to produce optimal optical filters [8; 9]. The selected spectra could also be used in training targeted diagnostic imaging systems, like spectral filter array cameras [10], or in automatic diagnostic image segmentation based on deep learning [11]. Medical spectral imaging has been gaining attention, and research in, e.g., retinal disease [12], breast [13] and tongue [14] cancer diagnosis has shown promising results. Despite the amount of research performed and the rise in open-access principles, freely and publicly available medical spectral imaging datasets are still relatively rare. Examples of the few public medical spectral image datasets include images of brain [15; 16] and retinal [17] tissue. These publicly available image sets require contacting the authors for access. Using modern spectral cameras, we have collected a novel database of oral and dental spectral images as part of our DIGIDENT-project. The project aimed to research the suitability of spectral imaging in oral dental diagnostics.