Some monitors, including the now out of production Elo E666761 Touchscreen, used in the CES lab, draw their frames horizontal line, by horizontal line. This results in a lag between when an image appears at the top of the screen vs the bottom of the screen. Interestingly, this delay does not seem to affect a viewer's ability to clearly perceive a moving image (although if the synchrony of the drawing and flipping of video frames is not correctly handled by the operating system, screen 'tearing' is readily apparent -- this is not an issue in the CES lab).
Following a very similar procedure outlined here, the time it takes to draw a screen frame was measured at both the upper left hand corner as well as the bottom right hand corner of the screen. As in the experiment mentioned above a Matlab function that utilizes Psychophysics Toolbox, was used to generate periods of white and black screens. This is the code used to generate the images that were analyzed. As in the previous experiment, photo-transistor circuits were used to convert the local brightness of the screen to voltages. This signal was acquired using a BioSemi amplifier outfitted with a hand-made terminal that breaks out to BNC connectors. The latency of the BioSemi device (7.72 ms -- added post-hoc by the analysis code) is known and was accounted for. Dropped frames (an innevitability) were also accounted for.
Shown here is a small segment of the analyzed signal (corresponding to the voltage which is analgous to the brightness of the area of the screen we are measuring). The green vertical line is the time of the correct screen flip that we wish to be synchronized with. It is evident that there is a delay between when the upper left and bottom right hand corners of the screen tranisition from black to white.
Histograms of the latencies show very low jitter. This is especially true of the top left corner latencies. Also shown here is the difference between the latencies.
To summarize, the latency of the top left corner of the screen is 17.52ms and 29.66ms for the bottom right. We have verified that this 11.74ms period that it takes to draw a frame on these monitors is a linear one -- i.e., an image in the exact center of the screen will appear 22.66ms after the computer says it is time to draw a new frame. This assumption was verified by repeating this experiment with the second sensor at 1/4, 1/2 and 3/4s of the way down the diagonal of the monitor and seeing the same nearly linear delay.
The data for this experiment were gather on 1/27/2015.