# One rotating photodiode and slit, what does it see per rotation? Curiosity.

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• 09-29-2013, 05:24 PM
Mustafa Umut Sarac
One rotating photodiode and slit, what does it see per rotation? Curiosity.
There are hundreds of photodiodes available at the market. If we get one and connect to electronics , put a slit in front of it and if we rotate it , what does it see ? Does it see one pixel high image or more ?

Umut
• 09-29-2013, 06:15 PM
polyglot
Depends on what you put in front of it and what axis you're rotating around, and how you're sampling the signal from the photodiode.

If you have a slit and no other optics, the diode will see a whole slice of the scene summed. If you then scan the slit across, the slice will of course change. Say you take a vertical slit and scan it horizontally, the image recorded by the diode would be the same as if you'd made a pinhole image and averaged everything vertically, i.e. you have an Nx1 pixel image. There is only 1 diode, so there can be only 1 pixel, multiplied by how many samples (in time) you took horizontally.

There are IR imaging sensors that work a little like this, though they have lenses for good focus, a single (cooled) extremely sensitive photosensor (looking at one very small point in the scene at a time) and then scan that in a circular pattern that slowly slides to one side. It's very slow and requires a static subject, but it removes all inter-pixel calibration issues; assuming you have a well-calibrated sensor then the result is of extremely high precision.
• 09-29-2013, 06:33 PM
Prof_Pixel
The web is full of references relating to photodiode rotational sensors. See for example: http://www.ti.com/lit/an/slaa289/slaa289.pdf
• 09-29-2013, 07:37 PM
gleaf
By definition with only a slit and photo diode each each revolution will provide one pixel. The rotation and slit are in control.
• 09-29-2013, 07:40 PM
Prof_Pixel
I certainly wouldn't call the electronic pulse a 'pixel'.
• 09-30-2013, 01:39 PM
Hexavalent
This has what to do with analog photography?
• 09-30-2013, 02:41 PM
Prof_Pixel
Quote:

Originally Posted by Hexavalent
This has what to do with analog photography?

Actually, the pulse created by such a system is definitely analog.
• 09-30-2013, 04:26 PM
Mustafa Umut Sarac
Is there a thin long photodiode which made from two solid semiconductor -pn- and would be able to scan a horizontal rotation with recording more than one tiny photodiode and is there a way to get reliable message from this complex signal with timing - more distance the semiconductor to cable , it would take more time to reach signal to the cable- ?
• 09-30-2013, 04:58 PM
Hexavalent
Well I suppose it is analog... just like the signal coming from a CCD.

Still don't see how it belongs on APUG.
• 09-30-2013, 06:02 PM
Prof_Pixel
Quote:

Originally Posted by Mustafa Umut Sarac
Is there a thin long photodiode .....?

Yes, there are linear sensors; but a discussion of such technology doesn't belong on APUG.
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