Mobile visualization of density fields using smartphone background-oriented schlieren

The conventional background-oriented schlieren (BOS) technique is an image-based technique that can calculate the density field in fluids using two static images [i.e., an undistorted background image (reference image) and a distorted background image due to the density change in fluids (target image)]. This paper proposes the smartphone BOS (SBOS) technique, which offers the measurement of the density gradient using the high-speed imaging feature of the smartphone being carried with a moving observer. The conventional BOS with a fixed camera visualizes the density gradient by comparing the reference image and the target image. In contrast, SBOS can obtain the time difference of the density gradient field. A reference image in SBOS is a target one at a previous time step. The movement of the smartphone is canceled using a registration technique for image accurate alignment. Three demonstrations are conducted to perform SBOS. First, in a static situation, the density field of heated air by a gas burner is visualized by comparing between SBOS and conventional BOS. Second, the local displacement of density field and the error displacement is estimated quantitatively when the smartphone is moving. Third, SBOS using an embossed wallpaper to visualize the density field is performed in the mobile condition. These achievements suggest that SBOS is an effective system to visualize the density field using only the smartphone, and is expected to be a useful tool such as a preliminary experiment in the laboratory and a teaching tool for general smartphone users.