Photon Counting Image Sensor Multi-Threshold Saturation Correction
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Solution Overview
Problem
Conventional photon counting type image sensors face challenges in accurately counting photons that enter within a short time period, leading to saturation and loss of linearity in high-luminance areas, as multiple photons can be miscounted as a single event due to the dead time of the sensor.
Innovation Solution
The implementation of a photon counting type image sensor with multiple pixels, each equipped with a light-receiving element, multiple comparators, and counters that compare output voltages with different threshold voltages, allowing for the determination of count-saturation states by calculating differences in count values obtained from these comparisons.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional photon counting type image sensor uses a single threshold voltage for photon detection, then the device complexity is low, but measurement precision deteriorates in high-luminance areas due to count saturation
Solution Approach 1:
The single threshold voltage detection is segmented into multiple threshold voltage comparisons. Each pixel circuit is divided into multiple comparator units, each comparing the APD output voltage against a different threshold voltage. This segmentation allows the system to detect different photon count ranges simultaneously, preventing count saturation and improving measurement precision across varying luminance conditions.
Solution Approach 2:
The detection system transitions from a one-dimensional single-threshold approach to a multi-dimensional multi-threshold approach. By introducing multiple threshold voltage levels as an additional dimension of detection, the system can distinguish between different photon counts that would otherwise be indistinguishable in a single-threshold system, thereby extending the linear detection range.
2Measurement precision
If the image sensor operates in Geiger mode with high reverse bias voltage, then measurement precision for low-luminance areas is improved, but reliability deteriorates due to count saturation in high-luminance areas
Solution Approach 1:
Instead of using a single high threshold that would miss low-luminance photons, the system applies multiple threshold voltages ranging from low to high. Each threshold captures a portion of the photon signal spectrum, ensuring that low-luminance photons are detected by lower thresholds while high-luminance photons are accurately counted by higher thresholds, maintaining linearity across the full luminance range.
Solution Approach 2:
The system uses the count values from multiple threshold comparisons to determine whether count saturation has occurred. By comparing the relationships between count values at different thresholds, the system can identify saturation conditions and apply correction, providing a feedback mechanism that maintains reliability across varying luminance conditions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables accurate identification of count-saturation states and corrects count values for saturated pixels, allowing for improved linearity and accuracy in high-luminance conditions by distinguishing between different photon counts.
Implementation Method 1
use an avalanche phenomenon occurring when avalanche photodiodes (APDs) are operated in Geiger mode to measure the number of incoming photons
Implementation Method 2
When a photon enters the APD in this state, the avalanche phenomenon occurs in the APD and a large current flows in the APD
Data Source
AI summary
An image sensor comprises a plurality of pixels, each pixel including: a light-receiving element that outputs an output voltage that varies in response to a photon entering; at least one comparator that compares the output voltage with a plurality of mutually-different threshold voltages and outputs a single signal each time the output voltage varies so as to exceed either of the threshold voltages; and at least one counter that counts a number of signals output by the comparator upon comparing the output voltage with the plurality of threshold voltages and outputs a count value, for each of the threshold voltages.