[sane-devel] Reflecta ProScan 7200

Michael Rickmann mrickma at gwdg.de
Sun May 1 18:21:57 UTC 2011

Hi Jan,

Am 01.05.2011 01:01, schrieb Vleeshouwers, J.M.:
> Michael&  Noah,
> Well, I got it to work:
>   $ scanimage -L
>   device `pie:libusb:001:005' is a PIE      SF Scanner film scanner
> Thanks for the help, I'll start experimenting with it!
> Yours,
> Jan
I am glad you were successful!

I have good and not so good news about our scanners. To find out 
something about the texp values in the dc-write command I started 
experimenting with an old empty, but a bit dirty slide. The first thing 
I saw were smooth, greyish vertical bands on a white background. I think 
that they are the result of not having done proper shading correction. 
 From the genesys backend I understood that shading correction is needed 
to tune each ccd element between a white and a dark value in addition to 
setting the general gain and offset. Then I had a look at a scan of the 
same slide with cyberview, perfect white background. However, the 
corresponding snoop showed the same greyish bands. So, I guess under 
Windows shading correction is done in software as we assume already for 
gamma. In principle the 13 lines read for calibration are white but they 
also have vertical greyish bands. In the calibration lines the mean R, G 
or B value is about 55000 with roughly 2% standard deviation.
Then I started changing the texp and gain values. The mean color values 
of the calibration lines was independent from that also when reading a 
second time after having changed texp or gain. The mean values varied a 
bit but perfectly stayed in the range of their standard deviations. I 
think that we have to know exactly how to calibrate as we can not see 
the effect before scanning the image. For shading this must be something 
like a simple * TARGET / SHADE_VALUE operation as we only get one value 
per ccd pixel.
Next, I tried to find a relation between the texp values and the color 
values of the scanned image (300dpi, 16 bit, quality mode) still using 
the empty slide. My conclusions:
1) The three colors have to be treated separately.
2) The relation between texp setting and detected illumination can be 
described with a linear equation.
3) There is a minimal exposure time. With texp values below 0x0c00 you 
easily run into an undersaturation, especially in the blue channel.
I do not know what this leads to. A few thoughts: From a theoretical 
point of view in the equation "intensity_value = a * _texp + b" both a 
and b should change with the extinction of an pixel. A purely white 
image should give us the values which we need in our calibration 
function. Scanning an image twice with different texp should allow for 
calculation of the extinction of its pixel representation. How do 
different resolution settings affect the coefficents when ccd pixels are 
binned (or not)? I guess time will show.
You find my stuff, images of shades and the spread sheet, at 
http://wwwuser.gwdg.de/~mrickma/sane-proscan-7200/status-010511 .

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