pintomollo
Posts: 1
Joined: Mon Apr 22, 2019 4:09 pm

raspistill uneven illumination

Mon Apr 22, 2019 4:20 pm

Hi all,

I'm trying to use the Pi Camera as an affordable microscopy solution. So I'm taking pictures using raspistill directly through the imaging tube of my stereoscope and while the picture is of good quality, it has this strange uneven "illumination" artefact in the middle (see example):
illumination.jpg
uneven illumination artefact
illumination.jpg (145.56 KiB) Viewed 391 times
I understand that the pink hue is due to some auto white-balance (I've tried them all, didn't go for the manual one yet tough. What, I don't understand is why this central part is more illuminated. I've reduced all gains to their minimum, and the illumination is very homogeneous in the scope itself.

I've also tried to acquire the same image using an olympus reflex camera, which gives perfectly normal results.

Any suggestions anyone? There are just so many parameters in raspistill :D

Cheers,
Simon

User avatar
schoolpost
Posts: 39
Joined: Sun Feb 19, 2017 10:47 am
Location: Canada
Contact: Website

Re: raspistill uneven illumination

Wed Apr 24, 2019 3:37 am

This an a product of lens shading, not white balance. Most lenses don't evenly illuminate the sensor, there's a gradual falloff.
To counter it, the image processing brightens up these parts of the image, but these corrections are specific to the built in lens, vs
what you are using with the microscope lens.

Read more here:
viewtopic.php?f=43&t=190586
-Csaba Nagy

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