Archive for May, 2012

How To Measure : Status

Tuesday, May 29th, 2012

It has been a long time since we met, I know, but time to simulate, measure, evaluate and write is becoming more and more a problem.  Sorry about that !  Writing material for the blog is just a side project, and unfortunately has a low priority.  But on the other hand takes quite a bit of my time.  Those of you who run their own company are familiar with these kind of issues I guess.  Actually having no time to write blog stuff is probably a luxury problem …

Last week I prepared quite some data and on a short term I expect more information to come about temporal noise measurements !

Maybe some other interesting news : Harvest Imaging is going to expand.  Not in number of people, but in office space.  Till now I am running Harvest Imaging from my home office, and you should take this literally.  I do have my office in my house.  I am using all space available in the house, and sometimes even space that is not available.  To further accommodate with the growth of Harvest Imaging, new office space was urgently needed and is now acquired, about 800 meters from my home.  I attach here a picture of the apartment that will be becoming part of Harvest Imaging.  At this moment the building is still under construction, it will be ready after the Summer holidays.  Harvest Imaging will be located in the apartment under the black roof at the right hand side of the picture.  The balcony right is connected to my new office, the balcony left is connected to the measurement lab.  Once the building is ready, I can go back to my work as in the older days.

kantoor

Albert, 29-05-2012.

How raw RAW can be !

Sunday, May 13th, 2012

Coming June 5th is going to be again a great day for Neil Young fans.  On that day Neil will release his new album called “Americana”.  The title is referring to the old American folk songs that he recorded on this new  album.  But the greatest news is that Neil saddled up the Horse again.  After almost 10 years of recording and playing with other musicians and bands, Neil has recorded his new album with his original band, Crazy Horse.  It is good to hear that the Horse is back
!  The pure, raw guitar rock-and-roll of the band can be heard on Neil’s website (
www.neilyoung.com) in the song “Oh Suzannah”.  Just 2 guitars, bass and drums, and there they go.  That’s the sound of Neil Young and Crazy Horse.  The song tells you how raw and simple rock-and-roll can be.

After releasing the new album I really do hope that Neil saddles up the Horse again for a tour !  To keep a horse in ideal shape, you have to frequently ride the animal !  I hope we can rely on Neil to saddle up the Horse and ride it through Europe.

The rumour is also spread that Neil is recorded a second album. It is not known whether this is with or without the Horse.  Neil is unpredictable in what is going to be the next step, and that makes it so attractive to be a fan of his music.  Thanks Neil !

Albert,
13-05-2012. 

 

How to Measure : Fixed-Pattern Noise at Saturation

Tuesday, May 1st, 2012

 

There is one important fixed-pattern noise component left in the discussion about how to measure the FPN with or without light input, being the FPN at saturation.  These days most of the pixels have a built-in anti-blooming drain, e.g. lateral or vertical anti-blooming in CCDs or anti-blooming via the reset transistor in CMOS devices.  In all cases the anti-blooming characteristics of the pixels is relying on a “parasitic” transistor that opens at the moment that the pixel is going into saturation.  Because all these “parasitic” transistors differ in threshold voltage, all pixels have different anti-blooming characteristics, resulting in a (large) FPN component at saturation.  To measure this component of the FPN, the devices need to be driven into saturation, and the variation along the pixels can be characterized.  By itself, this is a very simple measurement.

Figure 1 shows the output characteristics of an imager (= average output signal of all green pixels) as a function of the exposure or integration time.  The input settings are indicated in the figure.

120420_blog_1

 

Figure 1 : Average output signal of all green pixels as a function of the exposure time.

In Figure 1, three curves can be recognized, respectively for a setting of the on-chip analog amplifier equal to 2, 1 and 0.5.  As can be seen, the curves for a gain equal to 2 and 1 saturate at 4095 DN.  This indicates that the ADC is determining the saturation level of the signal and not the pixels themselves.  The situation is different for a gain equal to 0.5.  In that case the saturation level is within the range of the ADC, indicating that the sensor itself is saturating and not the ADC.  In this example, the saturation at a gain equal to 0.5, is 2963 DN.

Of interest is the behavior of the FPN as a function of the same exposure time, as shown in Figure 2.  Before calculation of the FPN, the defect pixels were removed from the data set, otherwise the defects will influence the measurement results to a large extend.

120420_blog_2

 

Figure 2 : FPN as a function of the exposure time.

As was the case in Figure 1, also here the FPN behaves differently depending on whether the ADC or whether the sensor is determining the saturation level.  Two major cases can be distinguished :

-       When the ADC determines the saturation level (for gain equal to 2 and 1) : the FPN initially increases in absolute value because of the PRNU, reaches a maximum and then collapses to reach a level of 0 DN.  The latter refers to the fact that the saturation level of the ADC does not introduce any FPN in the case of saturation (apparently this example is using a sensor with a single ADC on-chip),

-       When the sensor itself determines the saturation level (for gain equal to 0.5) : the FPN initially increases as well, due to the PRNU, but after a first linear increase, the FPN jumps to a very large value around 105.6 DN.  For exposure times greater than 0.25 s, more and more pixels are saturating and more and more pixels contribute to a large FPN value generated by the anti-blooming transistors.  As can be seen, the final value of the FPN in saturation is 105.6 DN, being equal to 105.6/(2963 – 819) = 4.94 % of the pixel saturation value.

Finally the FPN versus average signal is shown in Figure 3, the figure summarizes both curves already shown in the two previous ones.

120420_blog_3

 

Figure 3 : FPN versus average signal for various settings of the analog gain.

From this curve the following data can be extracted :

-       FPN in dark for the various gain settings is respectively 100.867 = 7.36 DN, 100.566 = 3.68 DN and 100.268 = 1.85 DN,

-       The PRNU, independent of the gain setting, is equal to 10-1.68 or 0.0209 = 2.09 %,

-       Saturation level is equal to 103.52 = 3311 DN for the gain equal to 2 or 1, and saturation level is equal to 103.33 = 2138 DN for a gain = 0.5,

-       In the latter case the FPN at saturation is equal to 102.024 = 105.6 DN or 4.94 % while for the other gain settings, the FPN at saturation is equal to 0 DN.

That concludes the discussion on measuring fixed-pattern noise(s), in dark and with light on the sensor.  Next time the measurement of the temporal noise components will start. 

Albert, 01-05-2012.