So I always knew that my ThinkPad's display sucked. But now I have scientific proof! (Long-winded post about my job follows.)

I was running some timing tests at work on the code for a psychology experiment. This involves quickly presenting a stimulus to the subject, then measuring reaction time and synchronizing with EEG readings. Nothing very complicated, but accurate timing is essential; you don't want another variable confounding your results. So I have this photocell device that sends a signal to a Mac through the EEG hardware when there is really actually physical light coming off of the screen, not just when I told the computer to display something.

Now CRTs, of course, are actually scanning (with an electron gun) the display at 75 or 85 Hz or something, not constantly pumping out light to the whole screen. This means 12 or 13 (or whatever) ms between refreshes. So if you want something to appear for 100 ms, you have to fudge a little, and make it so that you get maybe 8 or 9 scans. You can synchronize your presentation to the refresh so this is feasible. But how long the subject really perceives it ultimately depends on how "flickery" the monitor is at a given refresh rate.

LCDs, on the other hand, do not have this problem. They actually do produce light constantly -- their refresh rate is pretty much always only 60 Hz or less, but you get the same(ish) output over the entire cycle, instead of in blips that give the illusion of a constant image (in fact, as Matthew points out, they can continue to hold the same image for even longer periods between refreshes. Can't wait to try out that patch). And luckily for us, 6 cycles are in theory exactly 100 ms (in practice, it's close enough).

Or so I thought. I could not, no matter how many times I banged my head against it, get consistent results between our laptop (also a ThinkPad) and any external display. For reasons I do not entirely understand (probably scheduling issues), on CRTs I was getting one extra cycle, so I had to reduce the time by one to compensate. This was done for published studies years before I arrived at the lab, so (for the sake of not introducing additional variables) there is an extremely large disincentive to go back and fix it the Right Way, whatever that is. Our laptop's internal display was producing results that were skewed like those of a CRT. I thought for a minute I was going nuts from testing all day, but it kept happening.

Curious, I swapped out the lab's T60 for my faster T61 and (to rule anything else out) killed all my other processes and wrote a fresh PyEPL script that did nothing but repeatedly blink some text for 100 ms. Here is, on the external VGA port, a cheap old 15" Dell LCD I pulled from the server room:

(These images are from the Mac, which is still using System 9.something. I had to remember Command-Shift-3 to take a screenshot and use StuffIt to get them off of it. Srsly. I can't remember the last time I used StuffIt.)

And here is a CRT, showing the refreshes: (The DIN3 line is the actual signal, and the others are integrated over some number of milliseconds to compensate for scanning and attempt to get a usable number. Of course, this is done in hardware, not synchronized to anything else.)

Now, this is the one that blew my mind. The ThinkPad's internal LCD:

You can see that 60 Hz is spaced much farther apart, and that there are 6 cycles as intended, but. It's acting like a CRT! On the refresh, it puts out enough light to trigger the photocell, but inbetween, nothing. I ought to be noticing flicker and/or getting headaches all the time, if this is truly the case. (I guess I did get glasses for and because of staring at computer displays all the time. Should update my hackergotchi.) I have to admit I do not understand what is going on here at all. But it seems like making any assumptions about a display is (as the voice in my head was telling me all along) a very bad idea. Perhaps someone can email me some clue. But at least I have some hard data to validate my subjective opinion of the ThinkPad's internal LCD.

Anyway: I still recommend the ThinkPad, even if the display is awful. I have a nice external monitor connected through DVI to the dock at home, and I mainly use a laptop so I can get out of the house and go hack outside or in a coffeeshop (not ideal lighting). The build quality, keyboard/trackpoint, performance, and the fact that Debian (testing or unstable, anyway) Just Works are all higher up on the list for me. I do wish I could afford the new lighter one, though.