The difference between global exposure and rolling shutter exposure

Minyou global exposure usb camera module with 1MP HD video

Exposure is the process of image sensor exposure. During the exposure process, photons are collected and converted into charges by CMOS Sensor; and after the exposure, CMOS Sensor removes the charges in a certain way.

The three factors that affect exposure are: aperture, sensitivity and shutter.

Aperture: Aperture is a light hole formed by overlapping small blades inside the lens;

Sensitivity: Also known as ISO value, it refers to the sensitivity of the camera's photosensitive element to light;

Shutter: The shutter is a device used by the camera to control the time that light shines on the photosensitive element.

There are two common electronic shutter methods: global shutter and rolling shutter. Global shutter is achieved by exposing the entire picture at the same time.

Rolling shutter:

Usually used for CMOS sensor line-by-line exposure.

At the beginning of exposure, the sensor will scan and expose line by line until all pixels of the photosensitive component are exposed. There is a certain time difference between each line.

Global shutter:

Achieved by exposing the entire scene at the same time.

At the beginning of exposure, the sensor starts to collect light at all pixels, and then at the end of exposure, the light collection circuit will be cut off. Then the sensor reads out a complete image.

Global exposure and rolling shutter exposure:

The more common exposure methods of industrial cameras are global exposure and rolling shutter exposure, which are actually named after rolling shutter and global shutter. So what is the difference between them?

Global exposure:

First, let's start with the simpler global exposure. That is to say, after the aperture is opened, the entire image chip is exposed at the same time. Therefore, the exposure time is related to the mechanical switching speed. Since it is related to mechanical movement, there is a theoretical minimum exposure time, so the advantages and disadvantages are very obvious:

Advantages: All pixels are exposed at the same time

Disadvantages: There are limitations on exposure time, there is a minimum exposure time, and moving objects can be photographed

Rolling shutter exposure:

The rolling shutter exposure method may be related to the concept of rolling shutter. The length of the rolling shutter exposure time depends entirely on the opening size of the rolling shutter and the movement speed of the rolling shutter. In other words, the faster the rolling shutter moves, the smaller the rolling shutter spacing, and the smaller the exposure time of its sensor. Therefore, the rolling shutter exposure method can have a shorter exposure time.

Advantages: shorter exposure time

Disadvantages: progressive exposure, rolling shutter effect, not suitable for shooting moving objects

< Rolling shutter exposure >

Rolling shutter effect

When the industrial camera is shaking, or the object is moving at high speed, the progressive scanning speed is not enough, and the shooting result may appear "tilted", "wobbly" or "partially exposed". This phenomenon of misalignment caused by different exposure times of the upper and lower parts of the picture is defined as the rolling shutter effect.