Compact and fast depth sensor based on a liquid lens using chromatic aberration to improve accuracy

Depth from defocus (DFD) obtains depth information using two defocused images, making it possible to obtain a depth map with high resolution equal to that of the RGB image. However, it is difficult to change the focus mechanically in real-time applications, and the depth range is narrow because it is inversely proportional to the depth accuracy. This paper presents a compact DFD system based on a liquid lens that uses chromatic aberration for real-time application and depth accuracy improvement. The electrical focus changing of a liquid lens greatly shortens the image-capturing time, making it suitable for real-time applications as well as helping with compact lens design. Depth accuracy can be improved by dividing the depth range into three channels using chromatic aberration. This work demonstrated the improvement of depth accuracy through theory and simulation and verified it through DFD system design and depth measurement experiments of real 3D objects. Our depth measurement system showed a root mean square error (RMSE) of 0.7 mm to 4.98 mm compared to 2.275 mm to 12.3 mm in the conventional method, for the depth measurement range of 30 cm to 70 cm. Only three lenses are required in the total optical system. The response time of changing focus by the liquid lens is 10 ms, so two defocused images for DFD can be acquired within a single frame period of real-time operations. Lens design and image processing were conducted using Zemax and MATLAB, respectively.