Input lag is the time it takes an image to appear on screen versus the time when the signal was sent by the computer into the monitor cable from your psychology experiment.
A traditional CRT monitor which if driven at 200Hz can display an image in 5 milliseconds, or one refresh. Typical TFT monitors and data projectors today might take over 10 times as long due to input lag. Input lag is caused by the quality and processing speed of the displays electronics. This can make presentation timing and synchronisation with other equipment a real issue.
What's more input lag varies between makes and models of display as the video below clearly illustrates. If you replaced one TFT monitor for the other on the same PC your experiment generator would continue to present visual stimuli as normal and it would record that it had done so at the time you requested. Unfortunately there is no way for your PC or experiment generator to know about the input lag on a given monitor. It only knows when it requested the image be shown and not when it physically appeared in the real world!
In reality as you can see input lag means that you are not presenting visual stimuli at the same time at all. Input lag also means that the image offset is shifted back by the same amount as the lag. If you are synchronising with other equipment such as EEG, MRI, eye-trackers etc. the image will drift still further out of sync. It will also mean that reaction times are likely to be measured as longer and more variable than they are in reality.
Input lag can mean that you are not running the experiment you thought you were and that your results might be questionable.
Input lag is the time it takes for a image to actually be displayed on a TFT panel
(this is not the same are panel response time). Here are two different panels
connected to the same graphics card via a splitter to illustrate the effect.
Input lag on data projectors can be even more problematic as the table below illustrates. When image processing is left at the default settings on many projectors, input lag can more than tripple as demonstrated with the Acer H9500 (shown in bold).