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How did we miss the best place for a fingerprint scanner?

The fingerprint sensor on the side of a phone addresses all the complaints about front/rear scanners. So why is it dead?

Published onJuly 15, 2018

Sony's Xperia Z5 with the side-fingerprint scanner

Last week’s poll gave us lots of interesting numbers where on a phone you like having the fingerprint scanner.

What surprised us the most was how passionate the fans of having a fingerprint scanner on the side of a smartphone were. The side scanner polled poorly, winning just 7 or 8 percent of the total vote, but the comments sections across the platforms were full of enthusiasm for it. A lot of you upvoted those comments, especially on the YouTube poll.

To be fair, the points made were really quite valid. So much so that it made us wonder, is the side actually the best and most natural place for a fingerprint scanner?

Why didn’t the sensor on the side of a smartphone get more traction with Android smartphones?

Pro: It doesn’t affect the bezel or design of a device, and it’s fast

The Samsung S8 was criticized for having a poor fingerprint sensor placement
The Samsung S8 was criticized for having a poor fingerprint sensor placement

The fingerprint sensor on the side is right there. It’s not on the front or back, and otherwise occupies unused space. All these efforts to shave the bezel down on smartphones, like under-display fingerprints scanners, or placing the scanner on the back, are compromises. Having a phone display occupy more of the front makes for a great looking device, and the side offers a perfectly accessible spot without needing a whole new look to work.

It’s also just as fast as a front or rear fingerprint scanner since it’s not under-display, which needs a little more time to unlock.

Pro: It’s where your fingers are anyway

Razer Phone

It’s a natural placement. Having the sensor on the side means instant access, because that’s where your fingers naturally hold the phone. When combined with the power button it means no reaching required. When your phone is lying flat, adding multiple fingerprints will ensure you can always unlock it no matter if you reach it with your forefingers or thumb.

Pro: Never obscured

LG G7 hands on

Maybe you place your phone on its front when you don’t want to see the screen flash up from notifications. Maybe you place it face up to make sure you don’t miss anything during the day.

Depending on where the fingerprint sensor is on your device, it will be obscured some of the time. That means you have to first pick it up to hit the sensor with your thumb or forefinger to unlock (or point it at your face if you’re using face detection). It’s a bit of hassle.

Having the sensor on the side means it’s never obscured, no matter which way you put your phone down. You can reach out and unlock it with a finger, without having to pick it up.

Con: Sony’s unmitigated disaster means you might’ve never tried one

The Sony Xperia XZ2 Compact.

Sony was the main player in the side scanner business. A lot of things went wrong with its Xperia line, but the side scanner wasn’t one of them. Despite this, its latest devices had fingerprint scanners on the back. That leaves the Razer Phone (courtesy of its Nextbit acquisition) and the upcoming Red Hydrogen One with side scanners.

Sony was the main flag-waver for side fingerprint sensors, which might’ve worked out, if the company hadn’t bafflingly decided to disable them in the U.S.

It’s never been explicitly revealed why Sony did this, which suggests the company can’t tell us. Sony’s answers became a little more specific last year, when it explained the probelm as either a patent or compliance issue. Given the limitation was U.S. only, it looks more like an odd contractual issue.

Sony's issues means that the U.S. market has had limited exposure to the side fingerprint scanner

Regardless, it meant the enormous U.S. market didn’t get to try an Xperia with a side fingerprint sensor. It’s no wonder polls we run see most people choose front or back, given how unlikely it is they even tried one to know.

There are one or two relevant practical questions regarding the use of side fingerprint sensors as well.

Con: Fingerprint security

The Galaxy S5 showing a fingerprint scan failure

The nature of the side bezel is to be thin, which makes fingerprint scanners on the side relatively narrow. That means a smaller slice of the finger is checked than with the larger fingerprint sensors we see on the front and back of phones. The design could have a potentially higher false positive rate than most manufacturers consider acceptable.

Researchers from New York University and Michigan State University last year confirmed those fears, exploiting the partial match to develop a “master fingerprint” that could unlock up to 65 percent of phones (in certain conditions). According to the researchers, the small size of the fingerprint scanners allowed significant leeway in detection, especially when multiple prints are used. This worked on both optical and capacitive fingerprint scanners

Read Next: How fingerprint scanners work: optical, capacitive, and ultrasonic variants explained

Con: Smartphone cases

Pixel 2 cases - MNML
Red has a bezel around the FP sensor

The second point is less technical but still a hitch: smartphone cases. People with Xperia smartphones found that putting a case on an Xperia sometimes made it hard for the scanner to get a read of their finger.

The opening around the scanner was often too narrow to read the finger very well, requiring a hard press to use. More carefully designed (and expensive) smartphone cases handled this more elegantly through bigger gaps around the scanner, but it was a complaint we heard from Xperia users that didn’t come from any other smartphone user with a case.

Did you ever try a phone with a fingerprint scanner on the side? Do you wish your next device would have one or have you accepted that under-the-display scanners and face recognition will win out? Have you heard the Samsung Galaxy S10 might have a model with a side-scanner? Sound off in the comments!

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