In an arXiv preprint, Prof. Stanley Chan of Purdue University writes:
The one-bit quanta image sensor (QIS) is a photon-counting device that captures image intensities using binary bits. Assuming that the analog voltage generated at the floating diffusion of the photodiode follows a Poisson-Gaussian distribution, the sensor produces either a “1” if the voltage is above a certain threshold or “0” if it is below the threshold. The concept of this binary sensor has been proposed for more than a decade and physical devices have been built to realize the concept. However, what benefits does a one-bit QIS offer compared to a conventional multi-bit CMOS image sensor? Besides the known empirical results, are there theoretical proofs to support these findings? The goal of this paper is to provide new theoretical support from a signal processing perspective. In particular, it is theoretically found that the sensor can offer three benefits: (1) Low-light: One-bit QIS performs better at low-light because it has a low read noise and its one-bit quantization can produce an error-free measurement. However, this requires the exposure time to be appropriately configured. (2) Frame rate: One-bit sensors can operate at a much higher speed because a response is generated as soon as a photon is detected. However, in the presence of read noise, there exists an optimal frame rate beyond which the performance will degrade. A Closed-form expression of the optimal frame rate is derived. (3) Dynamic range: One-bit QIS offers a higher dynamic range. The benefit is brought by two complementary characteristics of the sensor: nonlinearity and exposure bracketing. The decoupling of the two factors is theoretically proved, and closed-form expressions are derived.
Pre-print available here: https://arxiv.org/pdf/2208.10350.pdf
The paper argues that, if implemented correctly, there are three main benefits:
1. Better SNR in low light
2. Higher speed (frame rate)
3. Better dynamic range
This paper has many interesting technical results and insights. It provides a balanced view in terms of the regimes where single-photon quanta image sensor provide benefits over conventional image sensors.
Why does quanta image sensor have Better dynamic range?
ReplyDeleteIt seems that as long as the number of photons is very low, it will be saturated, so the dynamic range should be very low.
From the point of view of just a single "bit plane" you are correct, the QIS pixel will saturate with just 1 photon count. However, you can accumulate many binary frames captured at high speed both spatially and temporally to improve dynamic range. (The paper gives some nice results about what binary frame rate is needed to achieve DR improvement over conventional CCD/CMOS camera pixels.)
ReplyDeleteAnother solution is to include a counter below each pixel. This has been done at low resolution for sensors used in nuclear particle detectors but could be built in silicon now. At low flux rates, the pixels count events and at higher flux rates, where the pulses can't be reliably separated, the readout mode switches to integration. Note that the scanning is not of the pixels themselves but of the stored data in the accumulators. This avoids the need to design in a frame rate.
ReplyDeleteThe dynamic range low end is set by the false count rate and the high end by the shortest exposure that saturates the pixel. It is not necessary for all pixels to change modes at the same time so the scene dynamic range would likely be limited by system light scattering.
Let's see how is the dynamic range of this setup...
ReplyDelete