[This is an invited blog post by Prof. Andreas Velten from University of Wisconsin-Madison.]
When we started working on single photon imaging we were anticipating having to do away with many established concepts in computational imaging and photography. Concepts like exposure time, well depth, motion blur, and many others don’t make sense for single photon sensors. Despite this expectation we still encountered several unexpected surprises.
Our first surprise was that SPAD cameras, which typically are touted for low light applications, have an exceptionally large dynamic range and therefore outperform conventional sensors not only in dark, but also in very bright scenes. Due to their hold off time, SPADs reject a growing number of photons at higher flux levels resulting in a nonlinear response curve. The classical light flux is usually estimated by counting photons over a certain time interval. One can instead measure the time between photons or the time a sensor pixel waits for a photon in the active state. This further increases dynamic range so that the saturation flux level is above the safe operating range of the detector pixel and far above eye safety levels. The camera does not saturate. [1][2][3]
The second surprise was that single photon cameras, without further computational improvements, are of limited use in low light imaging situations. In most imaging applications motion of the scene or camera demands short exposure times well below 1 second to avoid motion blur. At light levels low enough to present a challenge to current CMOS sensors results in low photon counts even for a perfect camera. The image looks noisy not because of a problem introduced by the sensor, but because of Poisson noise due to light quantization. The low light capabilities of SPADs only come to bear when long exposure times are used or when motion can be compensated for. Luckily motion compensation strategies inspired by burst photography and event cameras work exceptionally well for SPADs due to the absence of readout noise and inherent motion blur. [4][5][6]
Finally, we assumed early on that single photon sensors have an inherent disadvantage due to larger energy consumption. They either need internal amplification like the SPAD or high frame rates like QIS and qCMOS both of which result in higher power consumption. We learned that the internal amplification process in SPADs makes up a small and decreasing portion of the overall energy consumption of a SPAD. The lions share is spent in transferring and storing the large data volumes resulting from individually processing every single photon. To address the power consumption of SPAD cameras we therefore need to find better ways to compress photon data close to the pixel and be more selective about which photons to process and which to ignore. Even the operation of a conventional CMOS camera can be thought of as a type of compression. Photons are accumulated over an exposure time and only the total is read out after each frame. The challenge for SPAD cameras is to use their access to every single photon and combine it with more sophisticated ways of data compression implemented close to the pixel. [7]
As we transition imaging to widely available high resolution single photon cameras, we are likely in for more surprises. Light is made up of photons. Light detection is a Poisson process. Light and light intensity are derived quantities that are based on ensemble averages over a large number of photons. It is reasonable to assume that detection and processing methods that are based on the classical concept of flux are sub-optimal. The full potential of single photon capture and processing is therefore not yet known. I am hoping for more positive surprises.
References
[1] Ingle, A., Velten, A., & Gupta, M. (2019). High flux passive imaging with single-photon sensors. In Proceedings of the IEEE/CVF Conference on Computer Vision and Pattern Recognition (pp. 6760-6769). [Project Page]
[2] Ingle, A., Seets, T., Buttafava, M., Gupta, S., Tosi, A., Gupta, M., & Velten, A. (2021). Passive inter-photon imaging. In Proceedings of the IEEE/CVF Conference on Computer Vision and Pattern Recognition (pp. 8585-8595). [Project Page]
[3] Liu, Y., Gutierrez-Barragan, F., Ingle, A., Gupta, M., & Velten, A.
(2022). Single-photon camera guided extreme dynamic range imaging. In Proceedings of the IEEE/CVF Winter Conference on Applications of Computer Vision (pp. 1575-1585). [Project Page]
[4] Seets, T., Ingle, A., Laurenzis, M., & Velten, A. (2021). Motion adaptive deblurring with single-photon cameras. In Proceedings of the IEEE/CVF Winter Conference on Applications of Computer Vision (pp. 1945-1954). [Interactive Visualization]
[5] Ma, S., Gupta, S., Ulku, A. C., Bruschini, C., Charbon, E., & Gupta, M. (2020). Quanta burst photography. ACM Transactions on Graphics (TOG), 39(4), 79-1. [Project Page]
[6] Laurenzis, M., Seets, T., Bacher, E.,
Ingle, A., & Velten, A. (2022). Comparison of super-resolution and
noise reduction for passive single-photon imaging. Journal of Electronic Imaging, 31(3), 033042.
[7] Gutierrez-Barragan, F., Ingle, A., Seets, T., Gupta, M., & Velten, A. (2022). Compressive Single-Photon 3D Cameras. In Proceedings of the IEEE/CVF Conference on Computer Vision and Pattern Recognition (pp. 17854-17864). [Project Page]
About the author:
Andreas Velten is Assistant Professor at the Department of Biostatistics and Medical Informatics and the department of Electrical and Computer Engineering at the University of Wisconsin-Madison and directs the Computational Optics Group. He obtained his PhD with Prof. Jean-Claude Diels in Physics at the University of New Mexico in Albuquerque and was a postdoctoral associate of the Camera Culture Group at the MIT Media Lab. He has included in the MIT TR35 list of the world's top innovators under the age of 35 and is a senior member of NAI, OSA, and SPIE as well as a member of Sigma Xi. He is co-Founder of OnLume, a company that develops surgical imaging systems, and Ubicept, a company developing single photon imaging solutions.
