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Archive for August, 2009

What can you expect in the coming months from this blog ?

Thursday, August 6th, 2009

This blog started about one month ago and the main purpose is to share information and/or to stimulate discussion in the field of solid-state imaging.  After two general topic stories, I will soon start with items having more technical content, for example, the various noise sources in image sensors.  This plan of introducing more technical items does not imply that every future article published in this blog will have technical or scientific information.  Depending on my inspiration I will publish also non-technical stuff in between.

 A while ago I started working on a new course program : a two-day or three-day course with hands-on measurements on existing, off-the-shelf solid-state cameras.  The basic idea for this new course is to use the photon-transfer curve to do the evaluation of a sensor/camera.  The photon-transfer curve (also called “mean-variance method” or “photon-shot noise method”) was published for the first time in the 80’s by Jim Janesick (those days working at JPL).  Long time ago I had a discussion with Jim about his measurement technique.  He told me that he discovered more and new benefits of the photon-transfer curve (PTC) each time he worked on it.  To my own surprise I have to admit that Jim was right.  The PTC is a great evaluation tool, because basically you do not need to know what is inside the sensor/camera under test, and absolute light measurements are not needed.  All together, the PTC method is an easy way to get a lot of information (performance parameters and characteristics) about the sensor/camera under test.

During the preparation of the new course material I developed a noise model for a 4T CMOS image sensor.  The model describes the fixed-pattern noise as well as the temporal noise present in rolling-shutter pinned-photodiode pixels (PPD).  Next to the PPD, the imager is provided with column-level CDS, a global output amplifier and an on-chip analog-to-digital converter.  The model takes into account also the noise generated by these peripheral circuits.  By using this noise model I do have the possibility to change every single noise source in the hypothetical CMOS imager and to study its effect on the PTC. 

In the near future I will share this information with you through this blog.  In the very first publication I will focus on the dark current shot noise and its effect on the PTC.  The “P” of the PTC stands for “photon”, but the basic PTC idea can be successfully applied to the dark current present in the imagers and its shot noise component as well.  In other words, in the coming weeks I will describe how to apply the photon transfer curve without a single photon coming to the imager. 

Albert 2009-08-06