J. Cosmet. Sci., 69, 9–17 ( January/February 2018) 9 Evaluation on an Optical Scanning Device for Skin Profi le Measurement JIUAI SUN, School of Medical Imaging, Shanghai University of Medicine and Health Sciences, Shanghai, China, 201318 Accepted for publication August 22, 2017. Synopsis This paper describes experimental evaluations of an optical scanning device for skin surface recovery using multiple light source photometric stereo method. The portable optical device based on the principle of six- light photometric stereo was developed and subjected to evaluation and advancement through clinical trials for the purpose of monitoring skin conditions. As the device can provide objective topographic data for the description of the skin surface condition, the evaluation processes are mainly applied on skin in vitro and in vivo and compared with a commercial product, PRIMOS, which has been so far considered as a standard device used for skin surface measurement. The results of the experiment show that the topography measured by the device is signifi cantly closer to that of the ground truth. Meanwhile, the new optical scanning device demonstrates better performance in measuring skin surface in vivo, superior to that of the PRIMOS. INTRODUCTION SKIN TOPOGRAPHY MEASUREMENT IN VITRO AND IN VIVO As one of the largest organs of human body, it is important to be aware of changes in the condition of the skin surface as an important indicator of human health. However, the skin surface in vivo has proved diffi cult to monitor and record because of its complex char- acteristics. For example, it is complex in structure, elastic in behavior, and easily shrunk or stretched when subjected to extreme changes in the external environment. It is also optically complex in terms of its light transferring properties. As such, few instruments are available to measure such an object accurately (1). Skin replicas were used as a simple and repeatable approach to record the microstructures of the skin without affecting skin function and structure. Essentially a negative skin rep- lica can be easily obtained by smearing a silicon rubber material mixed with a catalyst over the skin surface. This is left for several minutes before being taken off. Fine details in the form of the furrows and peaks of the skin relief can be reproduced exactly when this process is carefully undertaken. After the replica has been produced, measurement of skin Address all correspondence to Jiuai Sun at sunja@sumhs.edu.cn.

JOURNAL OF COSMETIC SCIENCE 10 topography can be achieved by applying either a contacting stylus or some sort of optical systems to the surface of the replica (2). These traditional measurement techniques applied to static silicone replicas of the skin have proved very useful in recording skin topography with satisfactory accuracy for use in analyzing skin microstructure and anisotropy. Although skin replicas have been used as a successful means to investigate the topography of the skin, there are a number of drawbacks associated with this kind of in vitro replica- tion technique. First, the silicon replica can reproduce the structure of the skin, but fails to copy the color information of the skin which is also most important in skin disease diagnosis. Second, it takes a relatively long time to collect the data, which makes it un- suitable for real applications with short time requirements, as may be the case when col- lecting data in a primary care scenario. Third, considerable operator skill is required to successfully copy the skin using the replica method. To obtain a qualifi ed replica, the sample should be taken on clean and dry skin under given specifi ed temperature and hu- midity on standardized positions of the body and with correct specifi ed mixture ratio of catalyst and paste. Finally, highly sensitive and/or damaged skin may not be durable to contact with the polymer replica materials directly. Clearly an appropriate direct observation of the skin surface is clinically desirable, par- ticularly given the aforementioned disadvantages of an indirect replica-based method. In a clinical setting, the most frequently used methods are direct manual observations with the naked eye together with the use of photography. Although these methods have been used for a long time, they tend to suffer from low accuracy and subjective judgment. Hence, more scientifi c techniques are required to be integrated in the inspection of the skin surface in vivo. Various types of video microscope have been developed to scan the skin surface, but few of them can be used with minimal patient inconvenience while in a clinical setting (3). The PRIMOS device uses a structured light source technique and has been commercialized with released specifi cations for the measurement of skin in vivo. The 3D data of the skin from this device are recovered by using a phase-shifting principle from a series of images with stripe lights projected onto the skin. It has been considered in dermatology as an ideal tool for the investigation and documentation of skin micro- structure and wrinkles (4). The temporal phase shift–based PRIMOS method is one of the most frequently used methods however its working principle, where the object is recov- ered using a serial projection of parallel black/white stripes, makes this technique unsuit- able for the recovery of heavily colored skin surface. Meanwhile, the resolution of this technique has been limited. Some experiments have pointed to an inability to record fi ne scale features as the fringe may not be able to reach deep valley features on living skin (5). In addition, the PRIMOS is a bulky device and does not lend itself to handheld use. It is almost impossible to use it for in vivo measurement of those skin lesions distributed arbi- trarily around the body. A NEW SKIN PROFILE MEASUREMENT DEVICE: SKIN ANALYZER Photometric stereo is an optical approach to recover the surface shape of an object using several images taken from the same view point but under different lighting directions and has been extensively used in industrial applications, especially for inspection of large-scale products such as tile, stones, and metal components. Because of the relative unrestricted availability of space in the design and construction of these optical systems,