james@rover.bsd.uchicago.edu (05/08/89)
Well, barring the hopefully eventual creation of sci.med.physics (plug!) I thought this would be the next best forum to open comments on an idea that someone else may have already tried. I have a problem, in that I am digitizing x-ray films with a laser scanner, an optical drum scanner, and a ccd camera. The first two modalities obviously produce better images (re. dynamic range, resolution, and SNR), but the camera is the most economically feasible method. Many problems of local contrast enhancement and noise can be addressed by image processing methods, but the overall contrast range is limited by my camera and digitizer (8 bits). I am now trying a method for expanding the dynamic range. I capture two images, one at a low iris setting and one at a high setting, By taking histograms of these two images, I find the levels at which useful information gets "washed out" by either flooding at high intensities or quantum effects at low intensities. I take ranges of useful grey values in the two images, expand them to the same scale, and add the two images. I then do linear histogram equalization to normalize the contrast distribution (which is no longer linear). My results are ok, in that I can now see contrast in both the very bright and very dark regions of the film, but my normalization inherently enhances noise. I have tried clipping the histogram, but there still must be a better way. My next attempt will be with adaptive histogram equalization, but I am open to suggestions! James Balter James@rover.uchicago.edu "If the hat fits, slice it!"
jwi@lzfme.att.com (Jim Winer @ AT&T, Middletown, NJ) (05/10/89)
In article <3113@tank.uchicago.edu>, james@rover.bsd.uchicago.edu writes: > > I have a problem, in that I am digitizing x-ray films with ... > a ccd camera. ... Many problems of local > contrast enhancement and noise can be addressed by image processing methods, > but the overall contrast range is limited by my camera and digitizer (8 bits). > > James Balter On industrial ($1,500 up) and broadcast cameras ($15,000 up) there is an internal adjustment that is used to set the signal level. You can adjust the black level (knee at 7.5 IRE units?) and the maximum white level. Thus, you can set the dynamic range to match your needs exactly. You will also need a waveform monitor to set these levels properly. Unfortunately, I don't remember exactly which controls are used. I suggest that you contact your nearest (see yellow pages) professional/industrial dealer for Sony, Panasonic, JVC or Ikegami. (Note that some dealers are _box houses_ and some actually know something. A consumer dealer will be of no use as these controls are not present on consumer equipment.) Jim Winer ..!lzfme!jwi I believe in absolute freedom of the press. Pax Probiscus! Please do not email anything that requires a response outside AT&T. I receive email okay, but can rarely send a reply sucessfully. The opinions expressed here are not necessarily Those persons who advocate censorship offend my religion.