CCD-world: LED with very stable light flux

[Phil Hobbs]

The following was posted to CCD-world:


I agree about using a LED, because as long as you don't let the die get hot, the spectrum of a LED is very constant over a wide range of
drive levels.

The best way to do this sort of job is with an integrating sphere and a feedback
loop.  Put a garden-variety indicator LED of the appropriate colour (green is
probably a good choice) in one port of the sphere, covered by a white baffle,
put a photodiode in a second port (no baffle needed), and take the output from a
third port.  Make sure that the total non-white area of the three ports is no
more than 5% of the interior surface area of the sphere, and 2% is better.


That 240 microsecond pulse is easily regulated with a photodiode in an op amp
feedback loop, and the integrating sphere will ensure that (apart from
photocurrent shot noise) the photocurrent is an exact replica of the output
light pulse.

Good luck,

Phil Hobbs <pcdh at vnet.ibm.com>
IBM T. J. Watson Research Center
PO Box 218, Yorktown Heights NY 10598



Reinhold Dorn <reinhold.dorn at eso.org> writes:

> 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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