How do In-Display Fingerprint Sensors work?

As phones nowadays are moving towards an all-glass design from the front as well as back, there's no more room to fit a physical fingerprint sensor onboard the device to compromise the glass sandwich design. And hence the industry has moved towards a unique solution for the problem, incorporating a fingerprint sensor under the display of your smartphones. Have you ever wondered how these fingerprint sensors work scanning your finger through the glass display soo accurately and making life easier for us? Well if you have, then let's find out how these scanners actually work.


Working Of In-Display Fingerprint Sensors

          The working of In-display fingerprint sensors is actually quite simple and not that complicated as you might think. This simple process can be explained within three steps along with this diagram below:

The above diagram shows the working of an optical in-display fingerprint sensor. The three main components involved in this process are:
  • Light Source
  • Image Sensor
  • Processing Circuits
          Now, to understand the working of it let's break down the process in smaller parts starting with light emission. As we mentioned above, the sensor used here is an optical sensor and hence there's going to be involvement of light. So there's a light source like LED which radiates light rays on the surface of your finger through a glass which is basically your display glass in this case. 

          The light then reflects i.e. bounces backs from the surface of your finger again through the glass display to the receiver at the other end. This gets us to the next stage which is the receiving stage. At the receiver end, we have a simple set-up of a lens and an image sensor. The lens acts as a magnifier whereas the image sensor is a CCD or CMOS sensor just like the one seen in a typical digital camera. 


Now the task of the image sensor is to accurately read/scan the magnified image of the surface of your finger, and send it further to the sampling circuit and processor unit which is basically the processor of your smartphone. The sampling circuit samples the input i.e. it divides the surface of your finger into small parts or samples and feeds it to the processor. Now processor is the component where all the magic happens. The processor has your original fingerprint stored into its memory. Now it just compares the samples with the original fingerprint impressions and if it matches then boom, the phone unlocks and you're in! The processor accurately matches every minute details of your fingerprint(all the ridges and valleys).

Limitations/Disadvantages 

          The first thing coming to our mind while talking about the disadvantages is security. But there's nothing as such to worry. The optical fingerprint sensor is as secure as the capacitive ones we are used to in our standard smartphones. The actual disadvantage of these sensors is speed. In-Display fingerprint sensors tend to be a bit slower than capacitive ones and I'm sure now we know why! The light radiating from the source takes time to reach the surface of our finger and bounce back to the sensor. Still, the slight delay is not that much noticeable but yes, surely slower than capacitive ones. 

          Now another thing is that as we saw above, the surface of the finger needs to be lightened up, hence we need a light source. Only AMOLED displays have the capacity to light up individual pixels and hence In-display fingerprint sensors can only be implemented in phones with AMOLED displays. Now you'll how is that and disadvantage? Well AMOLED displays are costlier than the regular LED panels used in budget smartphones, and hence this total setup adds up a lot to the overall costing of the smartphone making it costlier than usual. This will be all when it comes to the limitations of this technology.

Actual In-Display Sensor Module


The above image shows the sensor module installed beneath the display of a smartphone. Below is the Synaptic's Clear ID Sensor.


          The Synaptic's Clear ID Sensor is most commonly used high-resolution optical fingerprint sensor. This sensor was used in the world's first smartphone with an in-display fingerprint sensor which was the Vivo X21. This sensor takes high resolution and accurate images of the finger.

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