In June 2020, Canon announced that it had successfully developed the world’s first 1-megapixel single-photon avalanche diode (SPAD) image sensor. Now Canon has published an article about it, says that this sensor will be utilized when ultra-high frames-per-second shooting speeds are needed. How much FPS? Up to 24,000 frames-per-second! This ability significantly enriches the possibilities in imaging.
As state in the document published by Canon: “Until recently, it was considered difficult to create a high-pixel-count SPAD sensor. On each pixel, the sensing site (surface area available for detecting incoming light as signals) was already small. Making the pixels smaller so that more pixels could be incorporated in the image sensor would cause the sensing sites to become even smaller, in turn resulting in very little light entering the sensor, which would also be a big problem”. As opposed to traditional CMOS image sensors, the SPAD (single-photon avalanche diode) features an electronic element at each pixel that receives a single photon and turns it into an “avalanche” of electrons, resulting in a relatively large electrical pulse, and thus elevation of sensitivity when capturing photos. However, the SPADs are much harder to be scaled to allow a high pixel count. Nevertheless, Canon has resolved it with this world’s first 1-megapixel SPAD sensor. Explore the SPAD vs CMOS slide below (by Canon).
Both SPAD and CMOS sensors make use of the fact that light is made up of particles. However, with CMOS sensors, each pixel measures the amount of light that reaches the pixel within a given time, whereas SPAD sensors measure each individual light particle (i.e., photon) that reaches the pixel. Each photon that enters the pixel immediately gets converted into an electric charge, and the electrons that result are eventually multiplied like an avalanche until they form a large signal charge that can be extracted.
CMOS sensors read light as electric signals by measuring the volume of light that accumulates in a pixel within a certain time frame, which makes it possible for noise to enter the pixel along with the light particles (photons), hence contaminating the information received. Meanwhile, SPAD sensors digitally count individual photon particles, making it hard for electronic noise to enter. This makes it possible to obtain a clear image.
Canon incorporated a proprietary structural design that used technologies cultivated through the production of commercial-use CMOS sensors. This design successfully kept the aperture rate at 100% regardless of the pixel size, making it possible to capture all light that entered without any leakage, even if the number of pixels was increased. The result was the achievement of an unprecedented 1,000,000-pixel SPAD sensor. Have a look at the slide below which demonstrates Canon’s challenges and solutions regarding the development of high pixel count SPAD.
The SPAD sensor that Canon has developed is also equipped with a global shutter that can capture videos of fast-moving subjects while keeping their shapes accurate and distortion-free. Unlike the rolling shutter method that exposes by activating a sensor’s consecutive rows of pixels one after another, the SPAD sensor controls exposure on all the pixels at the same time, reducing exposure time to as short as 3.8 nanoseconds and achieving an ultra-high frame rate of up to 24,000 frames-per-second (fps) in 1-bit output. This enables the sensor to capture slow-motion videos of phenomena that occur in extremely short time frames and were previously impossible to capture.
Utilizing SPAD technology with the elevation of pixel-count unleashes the possibilities regarding ultra-high FPS imaging. This SPAD sensor is premature to be used on high-speed video applications, however, the technology and knowledge can facilitate the development of a higher pixel count that may be good enough for high-quality video. Thus, I wouldn’t be surprised if Canon will announce in a couple of years its first ultra-high-speed EOS.
More Info About Canon's SPAD Sensor: Global Shutter and 24000FPS - Y.M.Cinema - News & Insights on Digital Cinema - YMCinema Magazine
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