BenQ got back to me and provided me with their "Uniformity Test Tool". This is an excellent piece of software that, with my i-Rite Display Pro, measures the screen in 25 places (compared to i-Profiler's 9) and goes much closer to the edge of the screen. It assesses the screen at white (255, 255, 255), mid grey (127, 127, 127) and dark grey (63, 63, 63).
Their Pass Criteria are:
PV series: Delta E ≦4 & luminance uniformity should be lower than 10%
SW series: Delta E ≦6 & luminance uniformity should be lower than 25%
The screen is within specifications for white. Two cells fail for luminosity in dark grey, but I do not consider the variance to be significant given the low luminance values measured. However, five cells fail in the mid-grey (which is important for black and white work). All of the five fail on colour (delta E) (one marginally) and two for luminance (although another two are just under 25%).
This screen is much better than the first I received, but does not meet BenQ criteria. I ahve yet to hear back from BenQ is relation to the results of my measurements.
BenQ got back to me and informed me that the criteria are only valid when measured at gray 255 (white) and not at gray 127 (mid gray). I understand that at 255 white, the three colour channels are largely saturated and so there is little scope for large colour variations. Mid gray would be more sensitive to colour variations and is photographically more important. So I think BenQ's response is a fudge.
I took the readings from the BenQ uniformity tool, entered them in a spreadsheet and saved them as image files for easier viewing. In the black boxes are the 25 cells that related to the 25 positions in the screen, each for 255, 127 and 63, with the first being colour and the second delta E.
The first relates to differences in colour from the screen average (BenQ references everything to the central cell) as I think that this gives a better indication of uniformity. I used four classes: 0-50 (clear) then 50-100, 100-200 and >200 in steadily darker colours.
In the second I have used the delta E figures as generated in the BenQ uniformity tool, with yellow being greater than 3 (which I understand is the Eizo criterion), orange being greater than 4 (the BenQ criterion for PW displays) and red being greater than 6 (the BenQ criterion for my display).
Please note that this is much better than the first display I received from BenQ!!
Delta E is calculated through a complex formula to determine the distance between two colours in a colour space (see:
https://en.wikipedia.org/wiki/Color_difference).
But the ability to perceive differences doesn't just depend on the distance between the colours, but depends on the colours themselves. This is shown in the diagram below that I took from the Wikipedia reference, noting that the ellipses of tolerance contours are shown ten times their actual size.
So uniformity of luminosity is easily and objectively measured and my screen is okay.
But as I found out, colour is more complex. While I agree with Andrew, uniformity should be as high as possible, but of course there is a cost to this I believe that my screen does not meet BenQ's own criteria.
Perhaps the best way to judge the screen is the final image taken of the screen a few moments ago. All I have done is corrected for peripheral illumination and cropped to make the screen rectangular. I ahve not adjusted colour or contrast.
Is this good enough?