  Image sensors hold a pivotal role in society due to their ability to capture
vast amounts of information. Traditionally, image sensors are opaque due to
light absorption in both the pixels and the read-out electronics that are
stacked on top of each other. Making image sensors visibly transparent would
have a far-reaching impact in numerous areas such as human-computer interfaces,
smart displays, and both augmented and virtual reality. In this paper, we
present the development and analysis of the first semi-transparent image sensor
and its applicability as an eye-tracking device. The device consists of an 8x8
array of semi-transparent photodetectors and electrodes disposed on a fully
transparent substrate. Each pixel of the array has a size of 60 x 140 {\mu}m
and an optical transparency of 85-95%. Pixels have a high sensitivity, with
more than 90% of them showing a noise equivalent irradiance < 10-4 W/m2 for
wavelengths of 637 nm. As the semi-transparent photodetectors have a large
amount of built-in gain, the opaque read-out electronics can be placed far away
from the detector array to ensure maximum transparency and fill factor. Indeed,
the operation and appearance of transparent image sensors present a fundamental
shift in how we think about cameras and imaging, as these devices can be
concealed in plain sight.
