GelPoLight: A Novel Visual-Tactile Sensor Based on Photometric Stereo With Point Lighting

Visual-tactile sensors based on photometric stereo (VTS-PS) have attracted much attention due to their sensitivity to subtle contact forces and their capability to provide real-time, high-resolution depth maps of contact geometry at a low cost. Conventional VTS-PS, such as GelSight, typically assume directional and uniform illumination to simplify the algorithm of photometric stereo. However, this assumption poses stringent requirements on the hardware side and limits the sensors in shape, size, and flexibility for various applications. Additionally, the configuration of directional lighting exacerbates the issue of cast shadowing. In this article, we analyze the benefits of point lighting over conventional directional lighting for VTS-PS and propose GelPoLight, a novel VTS-PS with point lighting, which can help handle or alleviate the aforementioned problems. GelPoLight does not assume ideal diffuse reflectance for the reflective membrane and considers the varying influencing factors of light intensity, light directions, and reflectance factors in the estimation of surface normal vectors. Accordingly, we developed a new calibration method for this approach. We also corrected image distortions caused by light refraction, which is a prevalent issue in visual-tactile sensors (VTSs). The final results demonstrate that GelPoLight can effectively generate the contact depth map, with errors of approximately 5% in the planar direction and 8% in the depth direction.