[Elphel-support] mage Noise problem

[Andrey Filippov]

>
> Visually the noise pattern looks completely static - ...


Alexander, what you see is really sensor "fixed-pattern noise" - the
different dark signal from the sensor. The pixels that seem white are really
what is called "warm pixels" - the ones that have significantly higher dark
current than the others (some chip defects). For short exposures this effect
is not that visible as even increased dark current does not provide enough
charges. At short exposures only "black"(always 0) and "white" (always
saturated) pixels are important.

When you close the iris (or the lens) with default camera settings it
increases exposure to maximal (default limit is 0.5 sec - it can be
adjusted), so you probably made those images with 0.5 sec exposure. If you
send the monochrome JPEG image (set mode to "mono" in user interface)  and
set 100% image quality you will get virtually lossless file with Exif header
that includes multiple parameters, including exposure time and analog gains.

Actually for monochrome sensors I would recommend using that 100% JPEG in
"mono" mode over "raw" - it will be much faster and there will be no
compression loss - 100% in Elphel cameras means literally  100% (as
suggested by JPEG) - all quantization coefficients are equal to ones, so no
quantization is actually performed on the image data. Color sensors benefit
from JP4 mode that preserves Bayer color mosaic while having good
compression ratio.

If you really plan to work with lonfg exposures you can use background
subtraction - either on the host computer or using FPGA code that allows to
subtract fixed patters during image acquisition.

BTW, here is the command to convert the camera raw files to PNG (or others)
with Imagemagick:

convert -size 2596x1940 -depth 8 gray:image1.raw image1.png

Andrey
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