Archive for December, 2013

Merry Christmas and Happy New Year

Friday, December 20th, 2013

Good Bye 2013 !  The year is almost over.  And as I did in the foregoing years, also this time I would like to take a quick look back and see what 2013 brought to us.  Also now I can repeat that the year 2013 was again a great year for Harvest Imaging !  The year started with the move towards a new office space.  In the meantime all furniture, equipment and infrastructure is installed and in operation.  So most of the blogs you could read this year were “born” from my new office space.  This is especially true for the blogs that contained measurement data.

If I overlook the “products” of Harvest Imaging, I can split them up into three groups :

  • The training courses, in-house as well as public courses.  It is and remains amazing and sometimes hard to believe where all the people are coming from that attend the courses.  In 2013 I had a training almost every other week, and I just completed course number 150 !  It is very motivating to experience that so many young engineers step into the challenging but very rewarding world of imaging,
  • The consulting activities.  I hope that my readers do understand that I cannot elaborate on this because of confidentiality reasons.  But I can indicate that my expertise was used in the field of imaging technology as well as intellectual property related projects,
  • The new product of Harvest Imaging, being the organization of the Solid-State Imaging Forum.  The very first edition of this forum was organized this December, focusing on “ADCs for Imagers”.  It was really a success and the large attendance proofs that there is a need for this kind of in-depth information and knowledge exchange.

To conclude this overview of products, it is a pleasure for me to thank all my customers who brought business to Harvest Imaging, in one way or another.  It is great to experience your trust and confidence by consulting the expertise of Harvest Imaging.  Thanks very much !

2013 is an odd number, and it inherently translates into another International Image Sensor Workshop, this time in the USA.  My friends in the field, Boyd Fowler, Eric Fossum and Gennadyi Agranov, organized another great Workshop.  Location was Snow Bird in Utah, where all technical information was exchanged, distributed and absorbed (literally) at a very high level.  Although again the technical and scientific level of the Workshop was outstanding, the highlight for me was the “meet and greet” with Michael Tompsett, the real inventor of the CCD image sensor.  He gave a very impressive overview of his history in the CCD imaging world and clearly explained to the audience that the 2009 Nobel Prize for the invention of the CCD image sensor went to the wrong person.  Thanks to the chairs of the Workshop to take the initiative to invite Michael Tompsett !

To conclude, I wish all of you the very best for 2014, and hope that we will regularly “meet” through this blog.  Thanks for visiting the website of Harvest Imaging, hopefully see you next year.  Welcome 2014 !

Albert, 20-12-2013.


How to Measure Full Well Capacity (3)

Friday, December 6th, 2013

From the two foregoing discussions on the full well capacity, it could be learned that :

–       In the case the full well is determined/limited by the ADC, comparable results for the FWC can be obtained by means of linearity measurements as well as from the mean-variance method,

–       In the case the full well is not determined/limited by the ADC, the results obtained from the linearity measurements show larger full well values than the ones obtained from the mean-variance method.

To explain the discrepancy between the FWC data of the latter case, one should realize that when the average output signal turns into saturation, a few non-uniformity issues are simultaneously popping up :

–       PRNU or photo-response non-uniformities : the pixels with the highest sensitivity can reach saturation first,

–       Non-uniformities in saturation level, some pixels will saturate at a lower FWC than others,

–       It is not clear from the measurements which part of the pixel is causing the saturation : the pinned-photodiode, the floating diffusion capacitance, the output swing limitation of the source-follower, output limitation swing of the analog circuitry.  Moreover, all these limitations can interfere with each other, which makes the situation even more complex to understand and explain.

To find out what is going on, the fixed-pattern noise is measured, and some interesting results were obtained.  The analog gain is put to a low value, and the reference voltage of the ADC is set to a higher voltage (the reference voltage is defining the analog input voltage that corresponds to an output of all “1”s).  In this way the ADC is not limiting the output swing, neither defining the FWC.

The measurement results are shown in the figure 1 : the left axis indicates the average output signal of 100 x 100 pixels as a function of the integration/exposure time; the right axis shows the fixed-pattern noise obtained from these 100 x 100 pixels, also as a function of the exposure time.

Figure 1 : Average sensor output and fixed pattern noise as a function of exposure time for a window of 100 x 100 pixels.

Some interesting details can be revealed from the FPN data :

–       For very low values of the signal (exposure time < 1 ms), the FPN shows a kind of plateau, indicating the FPN in dark,

–       For moderate values of the signal (1 ms < exposure time <12 ms), the FPN linearly increases, determined to the PRNU, the latter is proportional to the average signal value,

–       For higher values of the signal, in the region where the output signal tends to saturate (12 ms < exposure time < 16 ms), the FPN grows faster and tends to saturate as well.  Most probably this is the effect of the pixels that saturate.  The FPN at saturation is larger than the PRNU and for that reason the FPN increases.  The FPN tends to saturate, because once all pixels are saturated, the FPN does no longer change,

–       For saturated values of the signal (16 ms < exposure time < 20 ms) the FPN gets a second boost.  It is not completely clear what is happening here (the camera and sensor are “unknown”), but most likely the double sampling of the reference and useful signal start showing some “black sun” or “eclipse” effects.  This results in a larger FPN,

–       For the largest exposure times (exposure time > 16 ms), all pixels are running in the “black sun” or “eclipse” mode, but apparently the sensor is provided with an anti-eclipse circuit which pins the column voltages to a fixed voltages.

The abovementioned explanation is based on a close observation of what the behavior of the output signal.  This is illustrated in Figure 2, showing the same results as the ones mentioned in Figure 1, but with an adapted scale on the vertical axis.

Figure 2 : Same data as shown in Figure 1, but with an adapted scale on the left vertical axis.

As can be noticed, the average output signal tends to reach saturation for an exposure time of (about) 17 ms, but then the average output signal decreases again for a longer exposure time.  From 20 ms onwards, the average output signal seems to be clipped to a particular value, so does the FPN.  A simple explanation for this effect can be the presence of an anti-eclipse circuit.

Anyone else has a better explanation ?

Albert, 06-12-2013.

Forum ADC’s for Imagers is completely SOLD OUT !

Wednesday, December 4th, 2013

The two planned sessions on Dec. 16-17, and Dec. 19-20, 2013 are completely sold out.  There is no need for further regsitration because more seats will not be added.  Thanks to all people who registered.  I will keep you updated about the feedback of the participants.  At that time I will also start with the preparation of a new forum in 2014.

Albert, 4-12-2013.