CCD-world: LED with very stable light fux

[Tom Droege]

The following was posted to CCD-world:

I have done some linearity tests on phototubes using a pulsed LED.  I still
have a graph on my Fermilab office wall showing 10E6 linear range.  But it
was done using neutral density filters, not good enough for this purpose.
I was using the PM to test the electronics I built, not the other way
round.  I do not know the linearity possible, but other tests have
indicated 1000-10000 to 1.  I used an LED driven by an avalanche transistor
and a short piece of cable.  One can make 1-2 ns wide, very high amplitude
pulses this way.  Trigger the avalanche transistor so that it goes at the
same amplitude.  The cable makes sure the same energy is going into the LED
on every pulse.  The same thing can be done using the Tektronix mercury
relay pulser (if you can find one).  If the LED is driven with a 60 or 100
volt, 2 nanosecond pulse then most of the things that vary are not changing
much with respect to 100 volts.  The LEDs work just fine.  You can put 1-5
amp pulses through them.  Another trick is to temperature regulate the LED
with a small TEC.  Put the LED in a block of aluminum and regulate the
block temperature. 

Of course feedback is a good idea.  The feedback sensor described below by
Roger could be used to regulate the HV supply to the cable.  A few switches
to gate the signal into the feedback loop and ...   I could make a career
out of doing this.  ;^)

Tom Droege 

At 06:43 PM 4/19/00 -0400, you wrote:
>The following was posted to CCD-world:
>
>Hi Reinhold,
>
>I had a similar requirement when testing CCD linearity by measuring
>signal as a function of exposure time with a pulsed LED.  The
>expereiment takes a long time if you can't generate fast stable
>pulses.  I was aiming for 0.05% accuracy of the integrated light in 1
>ms or greater pulse.
>
>I used an integrated photodiode-preamp from Burr Brown (OPT211) to
>servo control the LED intensity.  The photodiode/preamp has much
>better temperature stability than the LED and is almost free from self
>heating effects.  The LED is quite temprature sensitive, and thus
>varies in intensity depending on how long it has been on.
>
>One has to deliberately add some overshoot at turn-on to compensate for
>the fact that the turn off time of the LED is very much faster than the
>turn-on time for the servo controlled LED.  To acheve this circuit
>allowed adjustment of initial LED current.  To find the correct setting
>one compares the  output from 1 long pulse with the output in single
>CCD readout that integrates N pulses each 1/N th of the duration.
>
>The circuit is described in "How Linear are typical CCDs?", page 251 of
>the proceedings of the ESO workshop Optical Detectors For Astronomy,
>1996.   I bet you have a copy.
>
>Roger
>  
>
>
>> To: CCD-world <CCD-world at astro.ku.dk>
>> CC: "Dorn, Reinhold" <reinhold.dorn at eso.org>
>> Subject: CCD-world: LED with very stable light fux
>>
>> The following was posted to CCD-world:
>> 
>> 
>>  Hello CCD-world,
>> 
>>  I am working on a new designed CCD for curvature wavefront
>>  sensing, which measures the light at a location between the 
>>  the pupil plane and the image plane. In a real system, there
>>  is a membrane oscillating at a fixed frequency around 2 kHz and
>>  the CCD  measures the photons collected during the intrafocal 
>>  and extrafocal periods. From these signals we than calculate the 
>>  normalized difference to be able reconstruct the wavefont tilts.
>> 
>>  To simulate the optical setup and to test the performance of
>>  the CCD independently of the optical bench, I am locking for 
>>  very stable light source, preferred an LED. This LEDs should have
>>  stable rise and fall times in the range of 0 to 5 microseconds and 
>>  a very stable flux for a certain amount of time (i.e. 240 microseconds).
>>  Note that we try to move just a couple of electrons with the CCD and 
>>  therefore the LED flux should be extremely stable.
>>  
>>  The LED could then be driven by a clock pulse of the CCD controller 
>>  with and accuracy of 25 ns. 
>>  
>> 
>>   flux
>>    |
>>    |     /----\         /----\
>>    |----/      \-- ----/      \------ time
>> 
>>  Does anyone know of a manufacturer or any source which is 
>>  producing an LED with those requirements. Any help is welcome.
>> 
>>   Best wishes from Garching,
>> 
>>                        Reinhold Dorn
>> 
>>  --------------------------------------------------------------------
>>  Reinhold Dorn                          e-mail: reinhold.dorn at eso.org
>>  ESO-European Southern Observatory      phone : +49-89-32006-547
>>  K.Schwarzschildstr.2,D-85748 Garching  fax   : +49-89-3202362
>>  http://www.eso.org/odt                   http://pc002863.hq.eso.org
>>  --------------------------------------------------------------------
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