CMOS Sensors : Operation of CMOS Sensors

Operation of CMOS Sensors

a) Pixel sensor acts like a charge bucket; accumulates electron charges the same way as water bucket stores water

b) Charge is converted to voltage & amplified at the pixel.

c) Individual CMOS microwire carry voltage from one pixel at a time, controlled by the pixel select switch

d) To output video signal, following steps are followed

1. All pixel select switches are turned ON. This outputs voltage of each pixel to column circuit.

2. Column select switches are turned ON from left to right. In this way, signal voltages of each pixel in the same row are output in order.

3. This is repeated for all rows from the top to the bottom in order, signal voltages of all pixels can be output from the top-left corner to the bottom-right corner of the image sensor.

e) These signal voltages are output to the signal processor of the camera.

CMOS SENSOR TYPES

Difference between types of CMOS sensors is generally due to the number of transistors (affecting fill factor) that are present for each pixel. A portion of the pixel sensor that is actually sensitive to light is called fill factor.

a) Rolling Shutter type

This has got limited number of transistors and therefore has a high fill factor. However, lines of pixels are exposed at different times and therefore, movement in the target gives a distorted image.

b) Global Shutter type

The number of transistors is high in this case resulting in a low fill factor. But, all the pixels are exposed at a time and thus the movement artifacts associated with rolling shutter type sensors are removed.