Grey-to-grey response times can not be compared with the black/white response times previously given out by manufacturers.
When measuring b/w responsiveness, the time it takes a pixel to shift from black to white and back to black again is determined. There are two separate times within the b/w change that add up to the specified overall time: The "rise" time it takes from black to white and the "fall" time it takes back from white to black. Therefore, the response time specified represents the sum of rise and fall time. The b/w shift is defined in the standard ISO (1) 13406-2.
To the contrary, the grey-to-grey response time represents the average value established over several measurements from one grey shade to another grey shade. In doing so, only the time from starting gray to end grey is being measured but not the time it takes to change back to the initial grey value. Thus, there is only one reading per measurement and nothing to be summed up.
Unfortunately, no official standard as to how exactly the grey-to-grey time is to be measured has been defined by now. This way, manufacturers can basically measure and specify grey-to-grey response times as they see fit. A new, revised form of the European ISO 9241 standard, expected to be released this year, is supposed to bring about the changes needed in order for customers to be able to better form an opinion on how displays actually perform.
