Rolling Shutter Image Sensors

Simple, Cost-Effective Imaging Solutions

Introduction

Rolling shutter image sensors use a method of image capture in which a still picture (in a still camera) or each frame of a video (in a video camera) is acquired by scanning across the scene rapidly, either vertically or horizontally. Essentially, only portions of the image of the scene are recorded at exactly the same instant. 
This process of image capture produces predictable distortions of fast-moving objects, or rapid flashes of light. 
The rolling shutter method can be either mechanical or electronic. The advantage here is that the image sensor can continue to gather photons during the acquisition process, thereby effectively increasing sensitivity. It is found on many digital still and video cameras using CMOS sensors. The effect is most noticeable when imaging extreme conditions of motion or the fast flashing of light.

Pixel Architectures

Front Side Illuminated (FSI) Sensors

In FSI CMOS sensors, there is a wiring layer on the silicon substrate that forms the photodiode. The benefit of this structure facilitates the formation of a light shield for protecting the signal charge temporarily stored in the memory area from leaked light. For this reason, conventional CMOS image sensors with global shutter function have adopted an FSI pixel structure. However, the wiring on top of the photodiode hinders the incident light, which creates an issue when attempting to miniaturize the pixels.

Back Side Illuminated (BSI) Sensors

Conventional CMOS image sensors mount the pixel section and analog logic circuit on top of the same chip, which require numerous constraints when mounting these large-scale circuits. These constraints include measures to counter the circuit scale and chip size; suppression of the noise caused by the layout of the pixel and circuit sections; and, optimizing the characteristics of pixels and circuit transistors.

Image sensor designers have created the BSI sensor architecture to help overcome these drawbacks, above. In this configuration, the pixel’s circuit section is moved below the photosensitive area (pixel section), to allow more incoming light. This arrangement also allows the creation of smaller pixel configurations. Consequently, this feature leads to smaller, more compact sensors which enable both smaller sensors and larger resolutions to work with smaller optical formats. 

Other benefits of this structure are less cross talk between pixels, with better response times and more accurate color. Photons that enter the sensor quickly hit the pixel section to ensure that they are captured and correctly detected. Additionally, the pixel’s fill factor and the ratio of the photosensitive area to the total pixel area is much larger. These features reduce the requirements of the pixel’s micro-lens design (i.e., no increased refraction of the light to send it to the pixel); improves its CRA (Chief Ray Angle), while bringing the Bayer filter mask closer to the pixel. All these factors further improve the sensor’s image performance.

Sony’s STARVIS Technology

Video systems for the surveillance of dark scenes require clean images with fine details and high contrast, despite low light levels. Sony’s STARVIS image sensor series is the result of world-class semiconductor engineering that is designed to fulfil the requirements of low light surveillance.

It features unprecedented sensitivity in a compact format and high resolution. These characteristics were created based on a new pixel architecture. Even in moonlight, STARVIS sensors enable imaging with low noise and wide dynamic range.

The resident broadcasting image quality allows both the imaging software and the control room staff to accurately analyze the video stream.

STARVIS backside-illuminated sensors with the rolling shutter feature a sensitivity of 2000 mV or more per 1 µm2 (color product, when imaging with a 706 cd/m2 light source, F5.6 in 1 s accumulation equivalent). They can reach an SNR of 1 already at 0.13 lux under Sony’s proprietary sensitivity index, SNR1s. For reference, a full moon on a clear night creates an illuminance on surfaces of 0.05 to 0.3 lux. Near-infrared enhanced models are also available to further improve image intensities for security applications using NIR lighting, for example.

STARVIS sensors provide resolutions from 2 MP to 61 MP. With pixel pitches starting from just 1.45 μm, a 4K resolution sensor gets as compact as 1/2.8 in optical format. Frame rates of up to 120 fps also ensure a high resolution in the time domain for smooth display, or detailed analysis. Sensor interfaces like MIPI CSI-2, sub LVDS, and SLVS-EC facilitate camera integration.

The most essential characteristic required for security cameras is a high degree of sensitivity.
Now for the first time, a back-illuminated pixel developed specifically for security camera has resulted in an outstanding improvement in sensitivity over our previous product structures, with light as bright as 0.5 lux. With the given combination of sensitivity, compactness, resolution, and speed, STARVIS is the first choice for low light surveillance.

Sony’s STARVIS 2 Technology

The STARVIS 2 is back-illuminated pixel technology used in CMOS image sensors for security camera applications. It features a sensitivity of 2000 mV or more per 1 µm2 (color product, when imaging with a 706 cd/m2 light source, F5.6 in 1 s accumulation equivalent). It also has a wide dynamic range (12 bit AD) of more than 8 dB as compared to STARVIS for the same pixel size in a single exposure. It achieves high picture quality in the visible light and near infrared light regions.

Sensor Functions and Features

DOL-HDR: Digital Overlap High Dynamic Range

Digital overlap HDR (DOL-HDR) delivers the data from three exposures on a line-by-line basis rather than frame by frame, resulting in improved picture quality, especially under poor light conditions, compared to multi-exposure HDR. DOL WDR function requires a dedicated image signal processor to control and processing. However, DOL WDR can provide many advantages such as improved characteristics in low-light environments compared to conventional WDR that uses multiple frames set output. 

Part of what makes this method special is that the sensor can extend the integration time of the dark image (the long exposure one) so that it consumes the unused integration time of the previous short exposure frame to capture more light and information. This extends the dynamic range that can be presented into the subsequent HDR images, adding more detail, and improving the low-light response.

High Dynamic Range (HDR) Movie Function

The HDR Movie function has two different exposure conditions set within a single screen shooting. Afterward, the appropriate signal processing is conducted for the captured image information under each optimal exposure condition. Additionally, this process generates an image with a broad dynamic range and enables a shooting with brilliant colors, even in bright environments.

Stacked CMOS Image Sensor

A stacked structure has been adopted by these image sensors; the pixel section where the back-illuminated pixels are formed, is layered over a chip (instead of the supporting substrate in conventional back-illuminated CMOS image sensors) where signal processing circuits are formed. One advantage is that large-scale circuits can be mounted on a small chip. Each section is formed on a separate chip; consequently, specialized manufacturing processes can produce a pixel section with high image quality and a circuit section with high performance; enabling higher resolution; multi- functionality, and a compact size.

PDAF: Phase Detection Auto Focus

PDAF function is the high-speed auto focus technology. This technology has these merits compared to conventional contrast-based Auto-Focus: PDAF provides amazingly quick and accurate focusing with every frame detection; permits enhanced focusing with fast moving objects; and, enables rapid focusing with different subject positioning and ranges. The rapid calculation of the phase differences in the data between frames is the key to the performance of Sony’s proprietary PDAF technology.

Custom Rolling Sensor Options

Looking for custom design, semi-custom sensors, or design services?

Our partner, Pyxalis, is there to help with unique offerings and key technology differentiations to create a sensor that is one of a kind. Their sensor designs leverage their many years of experience in the field of image sensor development with competencies in pixel designs, Analog and Digital designs, product characterization, and product industrialization and qualifications.

Want to learn more details about our custom RS options? – Check out our complete portfolio of Pyxalis sensors.

Rolling Shutter Sensors: Main Applications

Rolling Shutter Sensors are found in these application domains: high performance medical imaging, security, machine vision scientific applications, flight and aerospace, and, digital photography (consumer).

FRAMOS OFFERS A WIDE RANGE OF ROLLING SHUTTER SENSORS FOR YOUR VISION APPLICATIONS.