# Non-Contact Dermatoscope with Ultra-Bright Light Source and Liquid Lens-Based Autofocus Function
## 1 Introduction  
A dermatoscope is the standard instrument for a first examination of skin conditions. The whole field of dermatoscopy started with the use of epiluminescence microscopes for this purpose [1; 2; 3]. State-of-the-art dermatoscopy devices today are mostly contact based and often include a camera to capture digital images for documentation. Compared to a non-contact setup, a contact-based dermatoscope exhibits several disadvantages. State of the art contact-based dermatoscopes have built-in cameras to make the pictures digitally available. The skin contact may cause distortion of the skin geometry, which makes it more difficult to compare pictures made at different examinations. Also, the contact could suppress the perfusion hampering detection of small vessel structures. When thinking of infected lesions, the contact can also be painful and represents a larger invasion to the privacy of patients. A non-contact device circumvents all these problems. In addition, one can realise a larger field of view and the concept is much more compatible with automation approaches. Challenges of a non-contact design are the implementation of a variable focus and the realization of a lighting situation, which is independent from the surrounding light. In order to exploit the advantages of the non-contact design with regard to automation, some challenges need to be addressed. For example, to obtain sharp images, focus control is required. The preferred solution for easy handling or automation is an autofocus system. For documentation and post-processing, the images should also be digitally available. Whereas digital image acquisition is already state-of-the-art in contact-based devices [4; 5], the documentation is required to compare pictures from different examinations in order to identify changes in size or color of relevant skin areas. The size of a nevus is, for example, one criterion of the ABCDE (**A**symmetry, **B**order, **C**olor, **D**iameter, Evolving) rule of dermatoscopy to visually identify malignant melanoma [6; 7]. Therefore, it is necessary to record comparable images in non-contact mode without distortion of the skin geometry, as skin contact with standard devices can significantly distort the geometry and suppress blood perfusion correct assessment of skin lesions if often difficult [8]. As the hemoglobin of the blood, besides melanin, is one of the two dominant dye molecules in the skin, the suppression results in a color change of the skin area under study. The comparison of skin colors is not only important for skin cancer screening but also interesting, for example, for inflammatory skin diseases (e.g., erythema in the Psoriasis Area and Severity Index (PASI)). To enable to calibrate the camera sensor in a way that colors can be identified and compared between images made at different examinations and possibly under different ambient light situations, a bright light source is needed. A dermatoscope provides 2D information about light intensity and color of the imaged location, i.e., skin area. As this is the same information generated by the human eye, the resulting data is comparably intuitive for the dermatologists. In addition, to obtain information from deeper layers of the skin, cross polarization can be used (see Section 2.3) [9]. Other techniques like optical coherence tomography (OCT) [10; 11; 12] or high frequency ultrasound [11; 13] normally generating a 2D depth images are based on the optical thickness (for OCT) or the time of the propagation of the acoustic waves in the medium (for ultrasound), respectively. For these modalities, by scanning a 2D area, 3D information is generated. The resulting data, however, do not provide information about the color of the tissue and special training is required for their correct interpretation. Also, compared to dermatoscopes, OCT and high frequency ultrasound devices are much more expensive.