"Not quite," Remus says, in response to Q's assertion that it sounds like a CT scan. "And the distinction is similar to the distinction between the brain, and the mind. CT scans show what's actually physically there -- or attempt to anyway."
"Dark Light Imaging# shows thoughts, dreams, fears, desires... and sometimes the physical, as well," Sirius says. "It's not quite as [emotionally] invasive as Legilimency, but it can still be very revealing." That's the whole point, after all. "Most of it is nonsense. Teenage girls and bored socialites with too much time and money patronise it the way Muggles do fortune tellers."
"It isn't nonsense," Remus says patiently, although his tone strongly implies that eyes are being rolled. "It's that we don't know exactly what we're looking at, which means that it's very difficult to know how to interpret it, and that means that -- "
"Only the Divination flakes give it any attention. For the most part. And even there, they only look at one image taken at one point in time," Sirius says. "You can't infer anything from them when the subject doesn't return for subsequent sessions. Most people don't voluntarily subject themselves to the type of analysis that can actually tell you something about the condition of their soul, since -- "
"It takes twenty-one days, and the image has to be recorded once a day for each of them so that you can separate day-to-day changes from what's there all the time."
"It takes forty-nine days, actually, and -- "
"Twenty-one for indicative inferences," Remus points out, cutting in.
"And another twenty-eight for a little more certainty. The technique itself isn't new -- people have been looking into this sort of thing for centuries -- but being able to record the image, and having objective criteria for ascertaining whether or not there has been major damage to the soul, is relatively new.
"About the accuracy... The criteria for analysing the condition of the soul were developed based on information mostly collected from people who... were not in a position to refuse the procedure## and who were also highly likely to be damaged in some way. I say 'reasonably accurate', because this results in a higher rate of false positives### than for most tests.
"The equipment can be arranged," Sirius continues, and then finishes with, "If need be," because he is not going to make anybody do anything, and he is also not going to be arranging for anything unless someone commits to twenty-one days of (ha) soul searching.
Am assuming that Sev has asked those questions, as well as these: "Is it similar in concept to the images of chakra circulation systems, or that old muggle concept of aura?" he asks, fascinated. "Who developed the imaging equipment, in your past? I've never heard of it, and I would have."
# Where I get stuck is coming up with the source for the light. On Discworld they use land eels or some such to generate Dark Light. So moving this forward requires (a) suggestions of what might generate the light [and by suggestions, I mean in meta, rather in thread -- Sirius certainly knows how DL is generated, I don't, and he won't tell me], OR (b) no questions being asked about the source of the light since they're wizards and know all about Dark Light and it's probably some trite variation of Lumos or some such, aurgh, aurgh, now my head hurts.
*cough*
Moving on...
## Patients and prisoners.
### It will err on the side of showing 'soul damage' when there isn't any, rather than not showing soul damage when there is some.