At 4/8/2010 12:29 PM Thursday, Gary Schrock wrote:
>On 4/8/2010 12:01 PM, Chris Wolf wrote:
>>Not to divert the subject, but...I'm curious why "lcd monitors really just
>>don't cut it" for their experiments. The three main limitations I can think
>>of for LCD vs. CRT are that their color reproduction is often not as good,
>>their dynamic range is not as wide, and their viewing angle is more
>>restricted.
>
>It's been a while since I myself have looked carefully enough at
>lcd's monitors, but in the past, one thing we did do was pull out an
>oscilloscope and used a photodiode to do some measurements. I'm
>going from memory here, but as I recall, what we found was that
>while crt monitors do a fairly good job of going from black to grey,
>lcd monitors tended to have a longer period of rampup for the color
>to get to where it was supposed to. And ultimately it just proved
>to be too slow. I haven't had to worry about it much myself lately
>because I don't work in the lab that I used to have to worry about
>it personally anymore, so I've fallen out of touch, but from the
>information that I've had passed on to me by those that do still
>care, supposedly this is still something of an issue (plus some of
>the color reproductions issues too).
>
>David McFarlane might be able to chip in too on this, he was
>involved in doing these tests back when we did them, and he's far
>more meticulous of a note taker than I am.
And here I am. And I am running from memory myself, since I did
those measurements several years ago and don't have my notes
handy. By way of background, each pixel on a CRT lights up only for
the moment that the electron gun fires on it. So if you use an
oscillocope to look at the light intensity of a single pixel on a
CRT, with each screen refresh you will see a large spike that lasts
for no more than a millisecond or two. That means that most of the
time your CRT is black (e.g., at 60 Hz refresh, say 2 ms on and 15 ms
off), but your visual system averages it all out (remember,
traditional films run at a mere 24 frames/s).
By contrast, once a pixel on an LCD lights up it remains lighted
until told to go dark again, so if you look at light intensity of a
single pixel on an LCD you will see no spikes. Now in our case we
were concerned with how long it took for each pixel to reach full
intensity and then decay to zero intensity. As I recall (and this
differs a bit from Gary's statement), the LCD pixels reached full
intensity within a few ms, which suited us fine; however, the decay
was biphasic, an initial fast decay followed by a slow long tail that
lasted tens of ms (as I recall, even at 100 ms it had not quite
reached baseline). The desired experiments involved perception of
very brief ("tachistocopic") images on the order of 10 or 20 ms, so
this long tail did not suit the experimenter.
I am also sure that you could find a good article about this on
Wikipedia if you look around, which will do a better job than I can
here. Also, that was several years ago, and for all I know the newer
LCD panels have much better characteristics, but I have not gotten
around to looking at this in depth. But given that the technology is
made for consumers and not for scientists, I would have to measure
each LCD panel individually to vet it, whereas we know from long
experience that the technology of CRTs just works.
-- dkm
