The response time should be a measurement of how quickly a pixel can switch from off to on and off again, ie, in other words how quickly a panel can transmit from black to white and to black again. Although the way response time is measured can vary between manufacturers the usual approach is to measure the time of transition between 10% to 90% and back to 10% of required brightness (sometimes referred to grey to grey or gtg). These transitions are referred to as rise time and fall time respectively.