Canon Patent Application: Photon Counting Sensor

Canon has applied for what I believe is their first patent application on a photon counting image sensor.

This patent is applying for methods to reduce the dark current surrounding the photon counters (Single Photon Avalanche Diodes or SPAD's in the document) thus improving the sensor's dynamic range.  A photon counting sensor has the ability to count individual photons making it the ultimate low light sensor.  Canon describes some challenges surrounding making such a sensor with a wide dynamic range and their proposed solution.

In recent years, application in a broad field is expected about the semiconductor device which can detect the feeble light of a single photon level. What is called photon counting that calculates precisely the luminance of the input light treated as a continuous value as a discrete value of the number of photons becomes possible conventionally by using a photo detector which the signal with which it corresponds to a single photon especially enlarges rather than the noise at the time of signal reading. 

As an example of a photo detector which realizes photon counting, an avalanche photodiode (it is hereafter written also as "APD") is mentioned. APD can amplify the signal charge quantity excited with the photon to several times - about 1 million times by using the avalanche amplification phenomenon which occurs by the strong electric field induced by the pn junction part of the semiconductor. By using the high gain nature of this avalanche amplification phenomenon, the signal of feeble light can be read, and it can amplify sufficiently more greatly than a noise, and can realize the luminance resolution of a single photon level. 

While this is simply a patent application, it had broad ramifications long term for image sensors.  However, this may never end up in production on a camera, but maybe for industrial or automotive purposes, however, it shows us a glimpse into Canon's research.

Japan Patent Application: 2018157387