# Spectral Line Reflectance and Fluorescence Imaging Device for Skin Diagnostics
## 3 Design of the Prototype Device  
A block-diagram of the device is presented in Figure 2, and its design scheme is presented in Figure 3. Two laser modules--an RGB fiber-coupled module (_Elite Optoelectronics_, CN) simultaneously emitting three spectral lines (450, 523, and 638 nm, \(\approx\)20 mW power each) and a module emitting at 850 nm (RLDH850-40-3, _[PERSON]_, AT with a 800 nm long-pass filter) -- are used for 4-wavelengths illumination of skin via a visible - near infrared polarizer (#33084, _Edmund Optics_, UK). A spiral-shaped flat loop of side-emitting 400 micron silica core optical fiber (_Light Guide Optics Ltd._, LV) serves as the illumination source [28]. The fiber is SMA-terminated at both ends; one of them is used for the RGB laser input and the other one is used for the 850 nm laser input. In result, the examined skin area is uniformly illuminated by the four above-mentioned laser spectral lines. Four 405 nm, 40 mW laser  
Figure 1: Absorption features of three main skin chromophores at three fixed wavelengths [24; 26; 27]. diodes (DL-5146-101S, _Routhner_, AT) are square-placed inside the fiber spiral to enable the excitation of skin autofluorescence. The skin target area is round with a diameter of 10, 20, or 30 mm, depending on the used changeable conical nozzle, which is inside-covered by a black coating film (Spectral Black, _Actar_, IL). The device is fully self-sustained using four rechargeable Li-ion batteries (INR18650-35E, _Samsung_, KR) for power supply. A four-band RGB-NIR camera (MSC-RGBN-1-A, _Spectral Devices Inc._, CA) covered by a 420 nm long-pass filter, equipped with an objective lens (25 mm #67-715, Edmund Optics, US) and top-mounted orthogonally oriented VIS-NIR polarizer, is capturing an image of the targeted skin area with subsequent extraction of the four spectral line images. They are further stored in the embedded single-board computer (Rock960, _Vanrs_, CN) and wi-fi transmitted to an external computer for the calculation of four chromophore distribution maps using the previously developed algorithm [9]. Within a second, the laser modules are switched off, violet lasers are switched on, and an autofluorescence image at the G-channel of the camera is captured. This image is also transmitted to the external computer in order  
Figure 3: Design scheme of the prototype device. Figure 2: Block-diagram of the device. to discriminate seborrheic keratosis from skin melanoma and other pigmented lesions [29]. The second imaging step can be skipped or used alone, if necessary. The device is initiated by an operator pushing the START button and selecting the appropriate operation mode (SMSLI only, AF image only, combined mode) and exposure time for the RGB-NIR camera. Then, the camera and lasers are switched on, and the device is properly placed on the skin target area (monitored on the round display). After pressing the SHOT button, a micro-controller (STM32G071, _STMicroelectronics_, CH) synchronizes the proper illumination of the target to capture the image (or set of images), which is/are read by the single-board-computer. The recorded images can be seen on the display and/or are transmitted via the single-board computer's _wi-fi_ to the remote computer for the calculation of chromophore maps or performing other tasks. The recorded RAW images (\(2048\times 2048\) pixels) are stored on the computer hard disk. In order to get correct multi-spectral reflectance images, first, the RAW data are pre-processed. RAW images are converted into R-G-B-IR spectral line images (\(512\times 512\) pix.) using the _SpectralDevices debuyering_ algorithm and the available spectral sensitivity curves of four detection bands (Figure 4).