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Canon Patent Application: Stacked DPAF SPAD sensor
Canon has published a couple of interesting sensor patent applications using Single Photon Avalanche Diode (SPAD) pixels. The difference between an single photon avalanche photo diode sensor and a normal photo diode based sensor is that an single photon avalanche photo diode as the name suggests tcounts the photons that it receives, instead of a regular photo diode which stores the amount of photons it receives in storage, commonly called a well. Now that sounds fantastic, each photon being counted would allow for some crazy low light photography. These sensors are primarily used for LiDAR (Light Detection and Ranging) and TOF (Time of flight) sensors, which are now commonly used on smartphones to assist with autofocus. It is typically a separate sensor specific for that purpose and is usually quite limited in resolution.
This brings us to this patent, Japan Patent Application 2021-013147. This patent application discusses a stacked DPAF SPAD Sensor. This means that it is not used for LiDAR or TOF purposes, but for other purposes. Atypically you need to count each pixel at the same time, so each pixel needs a counter. That creates a rather inefficient sensor design. Canon is proposing to stack the sensor so that they can create smaller pixels, and also support dual pixel autofocus. The lower stack has a comparator and a counter for each pixel. I'm not sure what Canon's thoughts are on the purpose of this sensor would be, but a sensor with DPAF and the ability to basically see in darkness could have interesting applications.
Canon as a method of describing the patent application discusses an embodiment of having a digital camera equipped with such a sensor. Yes, it's simply an embodiment, but the only reason for having Dual Pixel AF is to drive a lens of some sort. So unlike a LiDAR sensor, this is most certainly meant to capture some sort of imagery.
Whatever the case, don't be expecting this in your R1 camera ;)
Canon is one of the many commercial companies involved in SPAD sensor technology. Last year they published a paper on the miniaturization of SPAD sensors, that is well beyond my paygrade, but if you are interested it's downloadable below.
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