ADVANCES The Global (Shutter) Revolution

10 April 2018

SIG: Imaging Science

Main photo © Kyle Wood Creative Commons CC BY-NC-SA 2.0

Back side illumination (BSI) CMOS image detectors first appeared in technical uses such as security, microscopy and astronomy. Their design means that incoming light is captured more efficiently, some 50% - 90% more efficiently than previous CMOS chips, and they are now found in many smartphones as well as in cameras including Nikon's flagship D850. However, the design also comes with an odd price - the rolling shutter.


Rolling shutter effect ©  Soren Ragsdale Creative Commmons CC BY 2.0

Fast-moving objects, such as the aircraft propeller above, are shown in weird shapes. In the BSI chip, when you take a photo the image is taken from the chip line by line. This is because each row of pixels has its own A/D converter that changes the analogue light signal from each pixel into a digital value the camera can deal with. As the data are collected line by line, the image is scanned during the exposure. A very elegant video describing this is available by clicking here.

Now Sony, Canon, Nikon and other manufacturers are releasing details of BSI chips with global shutters. In this design, each individual pixel has its own A/D converter so there is no waiting for the data line by line. The whole image is gathered at one moment as each pixel signal can be converted simultanoeusly.


Courtesy Sony Corporation

In the test picture here, you can see how the rotating fan looks like a fan, not a series of stripes like the propeller in the top photo.

Of course, these are early days. The sensors being announced at the moment are about a megapixel or two in size - although this is close to adequate for HD video - as low-light video is certainly the target market for this technology today. However, it won't be long before a 20MP or bigger global shutter BSI chip is in your favourite brand of digital camera.