A foundational study of device grip and locations for back of device interaction and
fingerprint sensing.
Overview
I conducted several studies to understand how people grip phones of different
sizes, and how they might interact with different parts of the device. One study specifically
considered how people interact with the back of a smartphone, while the other addressed where a
fingerprint sensor might be located if it were integrated with the phone’s display.
Methods
The methods used in this study required some creativity to capture the physicality
of the device interactions. In one study, I designed a paper wrapper that entirely covered the
phone. Participants applied “Color Wonder” paint to their hands (the paint is clear until it touches
the special paper so people do not have to worry about making a mess) and gripped the phone in a few
different ways. Then I applied another color of paint to just the index finger so participants could
indicate where they could comfortably move their finger on the back of the device. The paper
wrappers were then scanned and I wrote a Python script to identify and compute touch regions. In
another study, participants were given paper prototypes of phone displays with different options so
they could indicate different interaction regions for fingerprint input, while thinking aloud. This
study also explored a lot of perception questions about different smartphone authentication methods
through interview and ratings.
Outcome
For the grip study, results were shared with engineering and product teams and
inspired additional development and research on edge touches on devices. The second study, on
fingerprint locations under a display, was useful because it provided very quick answers to
questions from engineering teams about possible fingerprint locations. A related study which
required significant hardware and software development was conducted later by a colleague, and the
results from that study very closely corresponded to my results which were achieved using much
faster methods. Overall these studies provided important insight in a domain that was new to the
team and wider company at that time.
This picture shows a participant trying out their grip on a phone, while the paper-wrapped phones
sit on the table waiting to be used in the paint task.
This shows the scanned and processed images resulting from the participants' grips using paint. The
distributions of grip (red) and position for a possible interaction region (blue) are shown on a
small, medium, and large phone.
An example of a participant's paper prototypes showing where they would like to place a fingerprint
sensor if it was integrated with the phone display. The three versions apply different constraints
to where the participant could place the sensor.
Results of the paper prototype sensor location study showing the preferred locations across all
participants.