Monday, December 26, 2011

Recent Image Sensor Theses

There are few recently published image sensor theses:

"Pixel and Readout Circuit of a Wide Dynamic Range Linear-Logarithmic Current-Mode Image Sensor"
MS Thesis by Elham Khamsehashari, Aug. 2011
ÉCOLE POLYTECHNIQUE DE MONTRÉAL

"This thesis presents a current-mode CMOS image sensor operating in linear-logarithmic response. The objective of this design is to improve the dynamic range of the image sensor, and to provide a method for mode detection of the image sensor response. One of the motivations of using current-mode has been the shrinking feature size of CMOS devices. This leads to the reduction of supply voltage which causes the degradation of circuit performance in term of dynamic range. Such problem can be alleviated by operating in current-mode. The column readout circuits are designed in current-mode in order to be compatible with the image sensor. The readout circuit is composed of a firstgeneration current conveyor, an improved current memory is employed as a delta reset sampling unit, a differential amplifier as an integrator and a dynamic comparator."

"Single Shot High Dynamic Range and Multispectral Imaging Based on Properties of Color Filter Arrays"
MS Thesis by Paul M. Simon
UNIVERSITY OF DAYTON, May 2011

"This paper addresses the difficulty of generating High Dynamic Range (HDR) images using current Low Dynamic Range (LDR) camera technology. Typically, several LDR images must be acquired using various camera f-stops and then the images must be blended using one of several exposure bracketing techniques to generate HDR images. Based on Fourier analysis of typical Color Filter Array (CFA) sampled images, we demonstrate that the the existing CFA sampled images provide information that is currently underutilized. This thesis presents an approach to generating HDR images that uses only one input image while exploiting that underutilized CFA data. We propose that information stored in unsaturated color channels is used it to enhance or estimate details lost in saturated regions."

One must note that the DR extension is not that big and is based on the assumption that not all colors saturate simultaneously.

"Analysis, Modeling and Dynamic Optimization of 3D Time-of-Flight Imaging Systems"
PhD Thesis by Mirko Schmidt
Ruperto-Carola University of Heidelberg, Germany, July 2011

"This thesis covers four main contributions: A physical sensor model is presented which enables the analysis and optimization of the process of raw image acquisition. This model supports the proposal of a new ToF sensor design which employs a logarithmic photo response.
Due to asymmetries of the two read-out paths current systems need to acquire the raw images in multiple instances. This allows the correction of systematic errors. The present thesis proposes a method for dynamic calibration and compensation of these asymmetries. It facilitates the computation of two depth maps from a single set of raw images and thus increases the frame rate by a factor of two.
Since not all required raw images are captured simultaneously motion artifacts can occur. The present thesis proposes a robust method for detection and correction of such artifacts.
All proposed algorithms have a computational complexity which allows real-time execution even on systems with limited resources (e.g. embedded systems). The algorithms are demonstrated by use of a commercial ToF camera.
"

"Random Dopants and Low-Frequency Noise Reduction in Deep-Submicron MOSFET Technology"
PhD Thesis by Drake A. Miller
Oregon State University, March 2011

Quite significant RTS and 1/f noise reduction in image sensors has been reported:
"In the case of this research it was shown that once the noise source and mechanism was understood necessary steps could be taken to reduce the source of the noise. Two examples shown here are the impact of substrate bias and modification of the doping levels. Substrate biasing is a relatively straight forward approach to reducing the noise and has been show here to have this repeatable effect. With additional understanding of the percolation currents modification of the channel dopant profile can serve as an additional means for device noise improvement. Once understood, these relatively easy steps, as in the case of reducing the implant dose in the channel, verified the theory and model developed during this research and resulted in a superior performing CMOS image sensor
product.
"

4 comments:

  1. Current mode logic should certainly be pushed a little more in university courses. Some of the circuits are really quite interesting. Elham Khamsehashari and many authors in the current mode literature pose interesting points regarding linearity, dynamic range, noise and RC timing constraints. Although many of these circuits come at the cost of circuit power and area.

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  2. Current mode circuit has no particular advantages.

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  3. MS Thesis by Elham Khamsehashari are not available :(

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