@PM490 - you started me on looking further into this noise issue. Here’s a .dng capture (scanner settings as above) which basically only shows a rather dark frame. (Note that this is only a scan of a dark header film; a dark frame of a Kodachrome would be even lower in amplitude, but I do not have such a scan at the moment.)
Looking again on the red channel (enhanced to make structure more visible)
one instantly can verify your observation: the amplitude of the bands are increasing from left to right.
A wild guess would be that this is happening in the analog part of the read-out circuitry of the sensor, being somewhat reset once a full scanline is processed.
Actually testing the above (enhanced) image in more detail, here’s a plot of a vertical line (red) on the right (diagram “Intensities”)
and here the same, with a line on the left:
the difference is less noticable, but there are higher spikes in the signal on the right side of the frame.
Here’s another raw scan for experiments. It’s Kodachrome stock and shows a rather dark scene. The intensity plots do also not show a too much difference between left
and right borders of the scanned frame
In summary, the sensor noise we are talking about is certainly there, but I think it is especially visible to the human eye because of the correlation in the horizontal direction.
In the second .dng-file there is a tiny dust particle in the center top part of the frame. It gives a sort of reference of the performance of the optics/sensor/software-combination of the scanner used in comparision to the film grain at low exposure parts of a scene. I’d say that the spatio-temporal noise introduced by film grain overwhelms sensor noise by far.