Re: More on pixel shifting

Posted by Joachim Walter on Oct 02, 2006; 4:15pm
URL: http://imagej.273.s1.nabble.com/More-on-pixel-shifting-tp3701425p3701428.html

Dave,

if the front- and the back-surface of the emission filter are not
parallel ("wedge"), the filter will act as a prism and slightly tilt the
beam off its original direction. This tilt is translated by the tube
lens into a lateral offset on the detector. A different filter in the
filter-wheel will have a different wedge (or none) and so will lead to a
different offset on the detector. This is one of the reasons, why you
can get a pixel-shift between different color channels (chromatic shift)
seen through different filter sets.
Tilting the filter as a whole will not change the position of the image
in an infinity-optical system. In contrast, in a finite optical system
you will change the position of the image by tilting the filter.
So it's not a tilt _of_ the filter, but it's a tilt _in _the filter
(wedge) that causes the pixel shift. That also means you could not
correct the shift by tilting the filter, but you have to buy zero pixel
shift filters to get rid of it.
Sadly, if you use "zero pixel shift filters" does not mean that you will
have zero pixel shift. This is because the magnification of the
objective+tube-lens varies slightly over the spectrum (I think this has
also been posted, recently). To make things worse, the shift produced by
this effect varies over the field of view. E.g., if the magnification of
two channels differs by only 0.1%, and you take a 1000 pixels wide
image, the shift between the two channels on the left side and on the
right side of the image differs by 1 pixel.
For most objectives this effect is particularly prominent when you use
blue-violet dye (e.g.DAPI) together with a green/red/IR dye.

Hope this helps,
Joachim

David Knecht schrieb:

> I am still trying to understand pixel shifting.  I came across this
> old post by Jason Kirk when this came up before:
>
> "The Leica DM-IR/BE is an infinity corrected system, therefore an
> emission filter being slightly off-camber in the filter wheel would
> not effect the optical path since light passing the filters is
> parallel.  The most probable reason for seeing a shift AFTER the
> dichroic in the emission path is not the barrier filter, but is if the
> tube lens (recombining element) is somehow off axis. This is the only
> lens element remaining. "
> The implication of Jason's post (and no one seems to have contradicted
> him) is that one should not see pixel shifting in a properly aligned
> infinity corrected microscope due to filters.  I then queried Semrock
> about why they would sell Zero Pixel Shift filters if this were indeed
> the case.  They essentially disagreed as to the effect of infinity
> systems on pixel shifting.  Can someone clarify this issue?  THanks- Dave
>
>
> Dr. David Knecht
> Department of Molecular and Cell Biology
> U-3125
> 91 N. Eagleville Rd.
> University of Connecticut
> Storrs, CT 06269
> 860-486-2200
> 860-486-4331 (fax)

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