In their paper, the authors argument that skin on our forearm or on the hand is inherently more intuitive to touch and interact with than a small artificial display area lacking tactile feedback. They designed and integrated a fully self-contained on-skin projection wristwatch, dubbed LumiWatch, featuring a custom-made 15-lumen scanned-laser projector combined with a 10-element 1D ToF depth-sensing array to determine a user's finger position on the skin when interacting with the projected display.
Designed around a Bluetooth 4.0 and WiFi-capable Qualcomm APQ8026 system-on-chip sporting a 1.2GHz quad-core CPU and a 450MHz GPU, the 50×41×17mm prototype ran on Android 5.1 and could operate one hour in continuous projection mode (drawing 2.7W at maximum brightness) on a 740mAh, 3.8V lithium-ion battery incorporated in the casing. Also integrated in the LumiWatch were 768MB of RAM, 4GB of flash memory, an inertial measurement unit and an ambient light sensor.
The researchers designed the 25.8×16.6×5.2mm projector module from three lasers (RGB) with a pair of MEMS mirrors operating in a raster-scan mode to project a 1024×600 image at 60Hz across a 39º×22.5º field of view.
A tricky part of this research was to calibrate and rectify the projected graphics onto the forearm as well as correctly perform continuous 2D finger touch tracking on the skin with coordinated interactive graphics.