Subjective tests reported by the BBC investigated viewer tolerance to sudden changes in overall brightness for HDR television, using the mean pixel display luminance as a measure of brightness as described in [2]. This measure has been shown to correlate well with subjective ratings of the overall brightness, but there may occasionally be a scene with a non homogeneous spatial luminance distribution where the measure does not fully correspond to subjective brightness. For the tests, the luminance behind the screen was 5 cd/m2, and the peak screen luminance was 1 000 cd/m2 [3]. Subjects were asked to rate the change in overall brightness between two still HDR images.
Figure 4 shows the overall results, with transitions from the first mean luminance A to the second mean luminance B categorised according to whether they are “not annoying”, “slightly annoying”, or “annoying”. Two regions are marked in the Figure with thick blue lines. The inner region, with mean display luminance levels of 5 to 80 cd/m2, contains only one possible “slightly annoying” jump, and so could be considered a suitable range for operation that will not cause viewer discomfort. The outer region, with mean display luminance levels up to 160 cd/m2, includes several slightly annoying jumps, and so could be considered an extended range for creative effect. Further experiments reported by the BBC show that this outer region can be extended down to 2.5 cd/m2, and production trials with a prototype meter suggest that this extended range is appropriate. – source
The Daejeon Chronicles 
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