Is this a DC Restoration issue?
As mentioned in the CNET reviews for the 37PHD8UK (and as pointed out once before in this thread by someone else), CNET attributes this changing black level problem to a "DC Restoration" (aka Black Clamping) problem. As stated in their review:
"The Panasonic TH-37PHD8UK is a stellar performer with only one real exception. Black-level performance on this panel is first-rate, except for the fact that if floats blacks a little from dark to bright scenes, which is something we'd like to see Panasonic improve. In other words, the brightness of its "black" fluctuates depending on whether other areas of the screen become brighter. This flaw contributed to its Poor score in DC restoration in the geek box."
CNET defines DC Restoration as follows:
Geek box term. Measurement of how well a television is able to maintain a consistent color of black regardless of the brightness of the rest of the image.
CNET's "Geek Box" goes into further detail on how this is tested:
Example result: "All patterns stable"
Here we use the series of PLUGE (Picture LineUp Generating Equipment) patterns from the Avia disc to determine how well the TV is able to maintain a consistent color of black as the average overall brightness changes--essentially, whether black gets brighter or darker along with the rest of the picture. After setting the black level with the Black Bars + Log Steps pattern (1:3), we check whether the moving stripes change in intensity or remain stable when displaying other patterns.
Good: No change on black bars + half white (1:6) or Black Bars (1:4) (denoted by "All patterns stable")
Average: No change on black bars + half gray (1:5) (denoted by "Gray pattern stable")
Poor: Change on black bars + half gray (denoted by "No stable pattern")
Curiously, in CNET's original review of the 42PHD7UY (the model that I have), they listed DC Restoration as "Good - All paterns stable", which would indicate that in at least their review model, they did not see it. In the review for the consumer version, however (the TH-42PX50U), DC Restoration was listed as "Poor - no patterns stable". I have not done the PLUGE test they decribe yet to see if I can duplicate the problem using that test, but as I have mentioned in a number of earlier posts, I have seen the effect generally in my 42PHD7UY (and by the "effect" I mean blacks getting a shade lighter as a dark scene gets lighter)
I hope some others on this forum with more technical knowledge than (which would not be saying much) can confirm that this is the cause of the issue we are complaining of, and from there we might be closer to a confirmatory test, perhaps figuring out why the 50" sets don't seem to have the issue (or not as much), and perhaps whether it has anything to do with pixel shape. Even if we are no closer to a fix, at least we might be able to more accurately describe the matter as "the DC Restoration problem" rather then "black-level shifting problem", which for some reason still keeps getting confused with the screen protection/power supply "features"/theories, which (I think) we have determined are not the cause of this problem.