Smartphone sales may be stagnating, but one particular strand of technological wizardry behind them is not. Companies that make the sensors powering your phone's camera and facial recognition system are preparing for a mini boom.

The slowdown in global smartphone sales has made life tougher for semiconductor makers. Chips giants from Qualcomm Inc. to Samsung Electronics Co. Ltd. have all recently issued disappointing forecasts. Pricing for memory chips is close to an all-time low.

One bright spot is the market for 3D and camera sensors. Smartphone makers are either struggling to find major new innovations, or are holding them back for handsets that are 5G-enabled and can transmit heaps of data very quickly. Meantime, the likes of Apple and Huawei are pushing more incremental design improvements: Bigger displays and better cameras. The displays often require more sensors too, as fingerprint scanners make way for optical sensing systems.

This new upswell has as much to do with photography as facial recognition. Even as handset sales drop, more advanced and therefore higher margin image sensors are going into the handsets that are still sold.

It's a promise that is overdue. AMS had anticipated a far faster return on its significant investment in the technology. It mistimed its spending and the stock has suffered as a consequence. Infineon, Sony and STMicro have invested in a steadier fashion and benefit from the deep pockets that their other businesses afford them.

Time-of-flight gear works the way it sounds, by measuring how long it takes for a laser signal to bounce off an object and inferring its topography accordingly. It requires fewer components and is generally more robust, reducing the risk of breakage and therefore wastage in the manufacturing process.