  Background-oriented schlieren (BOS) is a powerful technique for flow
visualization. Nevertheless, the widespread dissemination of BOS is impeded by
its dependence on scientific cameras, computing hardware, and dedicated
analysis software. In this work, we aim to democratize BOS by providing a
smartphone based scientific tool called "Pocket Schlieren". Pocket Schlieren
enables users to directly capture, process, and visualize flow phenomena on
their smartphones. The underlying algorithm incorporates consecutive frame
subtraction (CFS) and optical flow (OF) techniques to compute the density
gradients inside a flow. It performs on both engineered and natural background
patterns. Using Pocket Schlieren, we successfully visualized the flow produced
from a burning candle flame, butane lighter, hot soldering iron, room heater,
water immersion heating rod, and a large outdoor butane flame. Pocket Schlieren
promises to serve as a frugal yet potent instrument for scientific and
educational purposes. We have made it publicly available at doi:
10.5281/zenodo.10949271.
