Measuring the value of light—not the amount
SPAD sensors are a type of image sensor. The term “image sensor” probably brings to mind the CMOS sensors found in digital cameras, but SPAD sensors operate on different principles.
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.
Breaking through the pixel count ceiling
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.
Specifically, on conventional SPAD sensors, structural demands made it necessary to leave some space in between the different sensing sites on neighboring pixels. The aperture ratio, which indicates the proportion of light that enters each pixel, would therefore shrink along with the pixel size, making it difficult to detect the signal charge.
However, Canon incorporated a proprietary structural design that used technologies cultivated through 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.
Unprecedented high-speed and high-precision distance measurements
The SPAD sensor that Canon developed has a time resolution as precise as 100 picoseconds2, which enables extremely fast information processing. This makes possible capture of the movement of objects that move extremely quickly, such as light particles. The sensor can also utilize its “high-speed response” feature to conduct high-precision distance measurements, including three-dimensional distance measurements.
While the Time-of-flight (ToF) method, which involves directing light at a subject and measuring the time taken for it to be reflected back onto the sensor, makes possible precise distance measurements, this method could not be used because the extremely fast speed of light necessitated a light sensor capable of extreme high-speed responsiveness. Canon’s SPAD sensor can detect returning light in nanosecond3 units or less, achieving what previous light sensors could not—making ToF measurements a reality.
Enriching possibilities in imaging
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 nanoseconds3 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.
Such phenomena include instantaneous natural phenomena or chemical reactions that previously could not be captured accurately, or the damage that occurs to objects when they fall or collide with something else. There are many potential applications for an image sensor that enables the detailed capture of such events, including greater understanding of natural phenomena and assess product safety and durability.https://api01-platform.stream.co.jp/apiservice/plt3/NjI4%23MTI4Nw%3d%3d%23280%23168%230%233FE620D1E400%23OzEw%23
Enables capture of slow-motion images of highs-speed phenomena that occur nearly instantaneously
Potential applications in AR, VR, and driverless vehicles
By making distance measurement via the ToF method possible, Canon’s SPAD sensor enables ultra-high-speed image capture at a high resolution of 1 megapixel. This facilitates accurate three-dimensional distance measurement, even in complex scenarios where multiple subjects overlap.
In the fields of AR (augmented reality) and VR (virtual reality), which involve superimposing virtual images on top of real ones, being able to use the SPAD sensor to speedily obtain accurate three-dimensional spatial information enables more precise alignment of positions in real time. There are also high expectations for the application of SPAD sensors in solving one of the greatest challenges in designing driverless vehicles: the measurement of distances between a vehicle and the people and objects in its vicinity.
Enriching the future of society by turning dreams into reality
The successful development of Canon’s 1-megapixel SPAD image sensor also means that 3D cameras capable of recognizing depth information can now do so at a resolution of up to 1 megapixel. One highly anticipated application of this capability is in the high-performance “eyes” of robots and devices that society will rely on in the future.
But before this, it was considered unlikely that a 1-megapixel resolution on a 3D camera could be realistically achieved.
Canon’s research and development efforts increase the possibility that yet-unknown services and products that many people would have never dreamt of, yet hold the potential for great impact, may someday become reality.
- 1 Among SPAD sensors. As of June 23, 2020. Based on Canon research.
- 2 1 picosecond = 1 trillionth of a second.
- 3 1 nanosecond = 1 billionth of a second