Wikipedista:Strj32/Global shutter

Introduction

Global shutter is a type of image-capturing technology, where pixels on an image sensor are captured simultaneously. The shutter does not consist of any moving parts, hence it is an electronic shutter. As opposed to the more commercially used electronic shutter, rolling shutter, global shutter technology eliminates an undesirable artifact called rolling shutter distortion. This artifact is visible when capturing high-velocity subjects with a low frame rate. Sensor with rolling shutter exposes a pixel array by rows, most commonly from top to bottom, in contrast to global shutter, which exposes the pixel array as a whole. Global shutter technology was mostly CCD sensor technology until 2013, when Sony unveiled the first CMOS sensor with global shutter. CCD produces quality and low noise images but it lacks image capturing speed due to single analog-to-digital converter (ADC). Modern CMOS sensors benefit from higher frame rates, lower cost of production and great dynamic range. Sensor manufacturers merge the best qualities from both, for example Sony utilises CMOS sensor with addition of low-noise CCD structure into their Pregius and Pregius S sensors.

The global shutter technology has a wide range of applications throughout many fields. Such as photography, videography, biomedical engineering, machine vision, traffic monitoring, logistics, sport analytics and robotics adopt the global shutter technology. These industries make great use of high burst rate and no motion blur artifacts.

Principles and technology

Global shutter technology is a great step forward in digital image-capturing. Principle of exposing pixels simultaneously on the sensor instead of exposing rows or columns of pixels captures data most efficiently for scientific or professional purposes. It offers great possibilities for practical use due to precise synchronisation, compared to 10 milliseconds latency in rolling shutter. Key element of global shutter precision and high frame rate is ensured by the independence of any mechanical parts.

Global shutter with CCD sensor

CCD sensor provides image with a great quality, less noise and higher resolution, however their biggest drawback is the image acquisition time. Main difference between CCD and CMOS is their architecture. In CCD, charge is transferred across the whole sensor and stored in one corner. Then ACD assigns each pixel an individual value.

Global shutter with CMOS sensor

Most CMOS sensors have rolling shutter, but when Sony unveiled in 2013 CMOS with global shutter, it revolutionised the whole sensor industry.

CMOS sensor uses two different approaches. Most common is rolling shutter, where individual rows of pixels are exposed. The other approach is global shutter, which is capturing the whole pixel array synchronously. Biggest advantages of CMOS sensors is their cost efficiency.

Advantages of global shutter

Rolling shutter effect

No rolling shutter effect

This effect may happen if the object or the camera is in movement. Due to rolling exposure, picture looks unreal and distorted.

Flash photography

Global shutter is very useful in photography with flash. Rolling shutter does not expose the entire frame if the exposure time is very brief. It creates effect called banding. If rolling shutter exposes frame from top to bottom, the bottom half will be underexposed.

No mechanical parts

As previously mentioned, global shutter does not have any mechanical parts, hence making it less prone to malfunction.

Disadvantages of global shutter

Price

Compared to rolling shutter sensors, price is very high. Mainly because of complexity of global shutter sensors.

Reduced motion blur

In wanted scenarios, such as shooting a film, there is a desire for a natural motion blur. Global shutter reduces motion blur to minimum, which leads to un-film-like look.

Applications

Photography

Global shutter offers many possibilities in photography field. Mainly high speed photography such as sports, wildlife or portrait. In each of these genres, every millisecond counts

Videography

Professional videographers use global shutter cameras to get rid of the rolling shutter effect. If the camera or object is in movement, it will wobble and look unnatural.

Traffic monitoring

Important part of transportation coordination and surveillance is traffic monitoring. Usage widens from prevention, road safety, and monitoring for unlawful activities, such as speeding, red light running, road accidents and much more. Huge advantage of global shutter is in license plate recognition, where surveillance camera is synchronised with flash and can effectively capture license plate in speed over 120 km/h without any distortion. Sensors can be equipped with polarizing filters for better visibility and to reduce reflections.

Logistics

Logistics industry in recent years has rose due to widespread e-commerce gain. Lowering the workforce and rising online orders led to more automatisation in distribution centres. These centres use code recognition and sorting using global shutter for more effectivity, speed and cost efficiency.

Sport analytics

Global shutter helps with monitoring fast paced sports for coaching staff. Sports such as tennis, skiing, football and many more. Global shutter prevents rolling shutter artifact, so the footage can be fed to an AI for skeletal tracking.

3D scanning

This field is specific for CMOS global shutter cameras. Desired application is for scanning objects or automating inspections of a production line. Rolling shutter cameras are unable to capture the right footage due to distortion. Compared to CCD sensor camera, CMOS is cheaper and consumes less power.

Research and investigation

Global shutter finds application in science research, for example in microscopes with small moving organisms to capture the image seamlessly and without any artifact.

FPD manufacturing

Flat panel display manufacturing industry creates displays of all sizes. To ensure that the result is completed, workers need to check for any imperfections. For any glass imperfections, they use global shutter camera with polarizing filter to detect scratches.

Future

Global shutter technology has prospecting future, which is driven by continual innovation and progress. There are a few key future implementations such as smaller size, specific work occupations, better image quality, enhanced burst capture, better high dynamic range and better accessibility.