Inertial measurement unit (IMU) has long suffered from the problem of integration drift, where sensor noises accumulate quickly and cause fast-growing tracking errors. Existing methods for calibrating IMU tracking either require human in the loop, or need energy-consuming cameras, or suffer from coarse tracking granularity. We propose to augment indoor inertial tracking by reusing existing indoor luminaries to project a static light polarization pattern in the space. This pattern is imperceptible to human eyes and yet through a polarizer, it becomes detectable by a color sensor, and thus can serve as fine-grained optical landmarks that constrain and correct IMU’s integration drift and boost tracking accuracy. Exploiting the birefringence optical property of transparent tapes – a low-cost and easily-accessible material – we realize the polarization pattern by simply adding to existing light cover a thin polarizer film with transparent tape stripes glued atop. When fusing with IMU sensor signals, the light pattern enables robust, accurate and low-power motion tracking. Meanwhile, our approach entails low deployment overhead by reusing existing lighting infrastructure without needing an active modulation unit. We build a prototype of our light cover and the sensing unit using off-the-shelf components. Experiments show 4.3 cm median error for 2D tracking and 10 cm for 3D tracking, as well as its robustness in diverse settings.