Pixel: an optical fingerprint reader needs light, so tint matters more than thickness

An ultrasonic reader — the Samsung flagship type — sends a sound pulse through the glass and reads the echo. It needs an unbroken medium, so it is defeated by thickness and by air gaps. Thin glass and full adhesive are the fixes.

An optical reader — which is what Pixel uses — illuminates your fingertip using the display itself and photographs the reflection. It does not care much about thickness, because light passes through a slightly thicker sheet perfectly well. What defeats it is anything that dims, tints or scatters the light on the way out and back.

So the recommendations invert. On a Galaxy, a thin clear glass is essential and a privacy film is the problem. On a Pixel, a slightly thicker glass is largely harmless — but a tinted, privacy or heavily matte protector is the thing that will break your unlock, because it starves the sensor of the light it depends on.

Privacy filters. Their entire function is absorbing light that is not travelling straight at you. That is precisely the light an optical sensor is trying to collect and return. This is the worst combination for a Pixel.

Heavily matte or anti-glare. Diffusion scatters the reflected image the sensor is trying to photograph. A light matte finish is usually tolerable; an aggressive one often is not.

Tinted or blue-light protectors. Any coloured layer reduces and shifts the light reaching the sensor. Mild tints are usually fine, stronger ones are not.

What is fine: ordinary clear tempered glass, including slightly thicker sheets that would cause problems on an ultrasonic phone.

Whatever you fit, re-enrol your fingerprint afterwards. The sensor stored your print as read through bare glass; with a protector on, the optical path has changed. Delete the saved prints and add them again with the protector in place. This resolves a large share of "the sensor stopped working" reports on Pixel specifically.

Enrol the same finger twice as two separate entries, at slightly different angles. Optical readers benefit from this more than ultrasonic ones because they are matching an image rather than a 3D structure.

Finally, check for a bubble directly over the sensor. On an optical reader that shows up as an unlock that works intermittently rather than not at all, which makes it easy to misdiagnose as a software problem.