Capacitive Sensing vs. Optical Sensing: A Detailed Comparison

This article compares capacitive sensing and optical sensing techniques, two prominent methods used in biometric identification. There are four main types of sensing techniques used in biometric identification which include optical sensing, thermal sensing, capacitive sensing and ultrasound sensing. This page will focus on comparing capacitive and optical sensing techniques.

Capacitive Sensing

Here’s how capacitive sensing works:

• Fingerprints consist of tightly spaced ridges and valleys.
• As shown in the figure above, the sensor consists of a capacitive array.
• The capacitive array acts as one plate of a capacitor, while the finger acts as the other plate.
• Each pixel in the array is charged to a reference voltage and allowed to discharge with a reference current.
• The rate of change of potential at each pixel is proportional to the capacitance seen by the array.

The underlying principle is: I_ref = C * dv/dt

This principle is used in the design of capacitive sensing.

Optical Sensing

Let’s delve into the principle behind optical sensing:

• As shown in the figure above, the finger touches a light-emitting tactile-sense polymer.
• A photodiode array embedded in the glass detects the illumination.
• The image is captured and transferred for storage.

As shown in the figure above, sensing can also be achieved by projecting an image of the fingerprint onto a camera using total internal reflection.

Capacitive Sensor vs. Optical Sensor: Key Differences

Here’s a summary comparing capacitive and optical sensor types:

Capacitive Sensors:

• Advantages:
  • Greater miniaturization
  • Newer technology
  • Can be embedded into small devices
  • Relatively cheap
• Disadvantages:
  • Prone to dirt, as the finger touches the silicon directly.

Optical Sensors:

• Advantages:
  • Large sensing area (manufacturing large pure silicon chips is expensive)
  • More robust and longer life
  • Better image quality and higher resolution
• Disadvantages:
  • More expensive