by Evolution of Camera Sensors
The earliest cameras utilized film to capture images but over time digital camera sensors replaced film. The initial digital image sensors were based on CCD (charge-coupled device) technology which transfers electric charge from pixel to pixel on the sensor. Later CMOS (complementary metal-oxide-semiconductor) sensors became popular as they are more power efficient compared to CCD sensors. Both CCD and CMOS sensors rely on a global electronic shutter to capture images which leads to image distortion issues caused due to rolling shutter effect.
Introduction of Global Electronic Shutters
Conventional CMOS and CCD image sensors use a global electronic shutter where the whole Shutter Image Sensors area is exposed simultaneously for a controlled duration. This leads to image distortion if the scene has fast moving objects. The objects appear skewed if they move during the exposure time. Some cameras try to minimize this by shortening exposure times but it reduces light gathering ability in low light scenes. To overcome rolling shutter drawbacks new global electronic shutter designs were introduced.
Advantages of the Shutter Sensor Design
To address rolling shutter limitations, researchers came up with the shutter sensor design where each individual pixel has a separate electronic shutter. When light strikes the shutter sensor, it is absorbed within the photodiode of each pixel. The electronic shutter of the pixel then transfers the electric charge collected during exposure to the charge storage unit located within the same pixel. This parallel charge transfer and readout avoids image distortions due to object movements. As charge from each pixel is read independently, the shutter sensor delivers distortion-free images even for very fast moving subjects. Its global exposure capability makes it ideal for applications like self-driving cars, drones and military uses.
Working Mechanism of the Shutters
In a conventional sensor, photodiode, charge storage, amplification and other circuitry are separate units within a pixel. But in a shutter image sensor, the electronic shutter, photodiode and charge storage components are tightly integrated within the pixel footprint itself. This allows independent and parallel operation of shutters across all pixels. The individual shutters expose the photodiode for a controlled time duration depending on illumination. Then the charge is transferred to the storage unit and readout happens parallelly without mixing signal from different exposure times.
Advancing Shutter Sensor Technology
Initial shutter sensor designs demonstrated the concept but had constraints like low pixel counts, large pixel sizes and high power consumption. Steady technology advancement is addressing these limitations. Smaller fabrication process nodes allow shrinkage of pixel sizes and tighter integration of components within the pixel. 3D stacking and back-side illumination improve light detection efficiency. Improved circuit designs and fabrication techniques have significantly brought down the power usage to match conventional sensors. Key players are developing high resolution shutter sensors for commercial applications. Mass manufacturing will make shutter sensors affordable alternatives to legacy sensors in near future.
The novel shutter image sensor breaks away from conventional sensor design constraints to eliminate image distortions caused due to rolling shutter effect. Its independent pixel level electronic shutters and parallel global exposure ensures distortion-free imaging for fast moving objects. Continuous technology advancement is making shutter sensors more viable by overcoming early limitations like pixel counts, sizes and power usage. With maturity, shutter sensors have the potential to become preferred choices for imaging applications which demand highest motion clarity and minimum distortions. Their global shutter capabilities open up use cases not possible with existing sensors.
