Updated: Dec 22, 2020
In the current age of AI, imaging systems are evolving to replace the human eye. Imaging is an exponentially growing mode of data acquisition and collection. Imaging is used to gather all kinds of data such as age, gender, height etc. of people standing at signal, using a vending machine, visiting a shop, crossing the road, passing by a kiosk, registering at some conference, ordering a coffee, boarding a flight etc. Apart from people, imaging also helps collect data on vehicles, packages, animals, birds, fishes, medicines, groceries and so on.
In this digital era, let’s keep our focus on digital sensors. Digital imaging sensors can be categorized into 3 types based on the method of scanning adopted to read the pixel values.
The scope of this article is to discuss on the three main types of CMOS sensors
1. Global Shutter
2. Rolling Shutter
3. Global Reset Release
The distinction is based on how the sensor gets exposed to the light, how it scans the pixels and integrates it before sending it out.
In this type of sensor, the entire image is captured at the same time, i.e., all the pixels on the sensor array are exposed simultaneously, thus enabling the ‘freeze frame’ capture of the scene.
Before the exposure begins, all pixels in the array will be held in a 'keep clean state'. At the start of the exposure, each pixel simultaneously begins to collect charge and is allowed to do so for the duration of the exposure time. At the end of exposure, each pixel transfers charge simultaneously to its readout node.
Thus, you can see that exposure and read out happens simultaneously. This enables us to capture fast moving objects or changing environment with good accuracy. The precise control on the start of the exposure also enables to synchronise multiple global shutter sensors to work in tandem.
In this type of sensor, the entire image is captured sequentially, i.e., pixels array is divided into a set of rows and they are exposed sequentially. Once all the rows get exposed, the values are integrated and then the readout also happens in a sequential manner.
In this case, unlike the global shutter sensor, all rows need not be in a clean slate before the capture. The sensor can work in an overlap mode. When each row has completed the read-out operations, it shall start the exposure for the next frame. There is no requirement for all the rows to be read-out.
Rolling Shutter or Global Shutter?
Global Shutter sensors enables you to have snapshot capture, precise control on exposure time, synchronize multiple sensors. Thus, for use cases involving image capture in motion or capturing objects in motion, global shutter sensors is a right way to go.
There are use cases where we need to capture static objects or just record video streams. In these cases, we can work with rolling shutter cameras. Due to simplicity in its architecture rolling shutter sensors can offer higher pixel at lower cost compared to global shutter cameras.
Global Reset Release
This family of sensors work towards bringing the best of both worlds. The cost effectiveness at higher resolutions of rolling shutter and capability to control exposure and sync of global shutter. In this architecture, all the pixels get exposed simultaneously, but the integration and read-out happen sequentially.
You shall be able to find sensors that are rolling shutter but with GRR mode. When you operate in rolling shutter mode, the camera shall continuously stream video out at the resolution of your choice. However, in GRR mode, the camera shall expect a continuous external pulse with the duty cycle determining the exposure time.
Regami Solutions shall be able to assist you with the selection of the sensor. Please feel free to chat with us or email us or call us to discuss your requirements.
At Regami, we have had the pleasure of working with imaging sensors from companies such as Sony, Omnivision, On Semiconductors. Please check out the below list of examples to give you an idea on the sensors available in the market in each category.
1. Rolling Shutter
a. Omni Vision – OV5640, OV8825, OV8865, OV13850
b. On Semi – AR0330, AR0522, AR1335, AR0230
c. Sony – IMX230, IMX214, IMX258
2. Global Shutter
a. Omni Vision – OV7251, OV6211, OV9282, OV2311
b. On Semi – MT9V024, AR0134
3. Global Reset Release
a. On Semi – MT9P031, AR0521
Please feel free to get in touch with us in case of any queries.
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