I have encountered very strange text in flat fields captured with the Lumenera Lu165. The camera is perfectly usable, but i still wonder where this might come from. Due to the perfect sharpness, it has to be a structure on or in the CCD sensor itself.
The flats were captured without any optics, by placing a large blurring surface in front of the camera and illuminating that with a light source. Due to this very large apparent light-source, no dust-particles are visible any more (i know some are there), so we only see variations in pixel sensitivity.
Each image shown is the sum of hundreds of individual imagesi of very short exposure, so no shot noise is visible any more and dark current should not be relevant either.
The image shows the full flat field, with a gaussian contrast enhancement. The strange text is clearly visible above the center. The text is not visible in non-enhanced image.
A crop of the text, mirrored left to right. Contrast enhanced to be more readable. Also, the histogramm of the small region shows, that the actual variations are rather small.
So, this apparently reads as "301P RETICLE". And what might that mean? The best explanation seems to be that this is a part of the lithography process.