What is the reason...

[Tab]

What is the technical reason for the cause of eyestrain when looking at CRT's opposed to LCDs? What makes the CRT's image hurt or strain our eyes?

 

[CTho9305]

Flickering? Perhaps a not-perfectly-flat/rectangular image?

 

[andyman7]

flickering
try lowering your refresh rate on your crt down to 60 (or less if its possible)
headaches will most likely ensue, at least for me

 

[Lithium381]

refresh rate.....

 

[AndyHui]

Refresh rate is the first thing.

Actually, from what I understand the problem with CRTs is, is that you are looking at a "virtual image" that is swept across the screen, as opposed to a static "real image" that an LCD presents. That's probably a terrible way of putting it... Perhaps someone else can explain it better.

 

[Peter]

Refresh rate, yes, both too low and too high. The second thing is something nobody believes until they try. Go as high as you need to so you don't see the screen flicker (best tested not by looking at the screen, but by displaying a white surface, standing back a bit, and looking past the screen not at it) - but don't go above 90 Hz. Oddly enough and for reasons not yet understood, eye strain and reading performance get worse at very high refresh rates.

 

[buleyb]

The idea in CRTs is that if you post an image, it fades fast, but you post it again before the eye notices the fade, then the image looks constant. Unfortunately, while we see a constant image, our eyes can perceive the problem but it doesn't register as anything to us.

As Peter points out, looking next to the screen, but not at it helps perceive it, because peripheral vision is more sensitive to optical tricks like this.

In LCD land, the posted image never fades, but is turned off before posting another.

 

[Tab]

I know I can change it, thanks. I just want to know the medical reasons as to why it hurts our eyes. I guess that static image makes sense to me

 

[Jeff7]

Originally posted by: Peter
Refresh rate, yes, both too low and too high. The second thing is something nobody believes until they try. Go as high as you need to so you don't see the screen flicker (best tested not by looking at the screen, but by displaying a white surface, standing back a bit, and looking past the screen not at it) - but don't go above 90 Hz. Oddly enough and for reasons not yet understood, eye strain and reading performance get worse at very high refresh rates.

I've not heard anything about high refresh rates causing problems....though, excactly how high is "high" here? I run my main system's monitor at 120Hz, with no ill effects. When I switch to my secondary system, at 85Hz, I initially see flickering, but I acclimate to it in a few seconds.

 

[ingenuus]

I'm not sure about exactly how or what causes monitor related eye problems (refresh rate, reflections, lack of blinking).

However, I recently read that backlight luminance of LCDs is generally controlled by pulse width modulating (PWM) the lamp current. i.e. the Hz remains the same (about 250Hz) but the width of the "on" state is varied to achieve the desired brightness.

So, essentially, LCD lamps actually "flicker" on/off... or at least between "on" and a lower state of being "on", since @ 250Hz a cycle lasts 4ms and it takes about 10ms for the gas plasma in the lamp to die out completely (no light).

I don't know if this has a similar effect on the eyes as a high refresh rate with CRTs which was mentioned by a previous poster.

I read one comment a while ago where someone complained that LCDs cause their eyes to be more dry than CRTs. Though this might be more related to their not blinking enough or maybe this is dependent upon the specific LCD brightness setting?

Has anyone heard of or experienced this before?

 

[Tab]

Interesting, but do all LCDs have backlights?

 

[CTho9305]

Originally posted by: Tabb
Interesting, but do all LCDs have backlights?

Any you can see when the room isn't very bright do. All LCD monitors do (AFAIK). Reflective ones would be hard to read most of the time.

 

[zetter]

Also with the glass screen on a CRT screen you get all sorts of reflections from objects and light sources around the monitor which you don't get off a TFT screen which can also add to eye strain.

 

[Peter]

Originally posted by: Jeff7

Originally posted by: Peter
Refresh rate, yes, both too low and too high. The second thing is something nobody believes until they try. Go as high as you need to so you don't see the screen flicker (best tested not by looking at the screen, but by displaying a white surface, standing back a bit, and looking past the screen not at it) - but don't go above 90 Hz. Oddly enough and for reasons not yet understood, eye strain and reading performance get worse at very high refresh rates.

I've not heard anything about high refresh rates causing problems....though, excactly how high is "high" here? I run my main system's monitor at 120Hz, with no ill effects. When I switch to my secondary system, at 85Hz, I initially see flickering, but I acclimate to it in a few seconds.

Try it out the way I wrote above. Use the lowest refresh rate you don't see flicker when looking PAST the monitor. If you'd measure your speed and accuracy at, say, 85 and 120 Hz, you'll find that you're actually slower and making more mistakes at 120. The effect starts to kick in at 90 on average.

 

[buleyb]

Originally posted by: ingenuus
I'm not sure about exactly how or what causes monitor related eye problems (refresh rate, reflections, lack of blinking).

However, I recently read that backlight luminance of LCDs is generally controlled by pulse width modulating (PWM) the lamp current. i.e. the Hz remains the same (about 250Hz) but the width of the "on" state is varied to achieve the desired brightness.

So, essentially, LCD lamps actually "flicker" on/off... or at least between "on" and a lower state of being "on", since @ 250Hz a cycle lasts 4ms and it takes about 10ms for the gas plasma in the lamp to die out completely (no light).

I don't know if this has a similar effect on the eyes as a high refresh rate with CRTs which was mentioned by a previous poster.

I read one comment a while ago where someone complained that LCDs cause their eyes to be more dry than CRTs. Though this might be more related to their not blinking enough or maybe this is dependent upon the specific LCD brightness setting?

Has anyone heard of or experienced this before?

I suppose its possible, but I would put the likely category in with those that are bothered by the flicker of fluorescent lighting with electric ballasts. I do believe that some people have eye problems either product. Nobody sees the same way, and some people aren't sensitive to high or low refresh rates at all. As a whole though, I'd say LCD is a `cleaner` tech for your eyes.

 

[Jeff7]

Try it out the way I wrote above. Use the lowest refresh rate you don't see flicker when looking PAST the monitor. If you'd measure your speed and accuracy at, say, 85 and 120 Hz, you'll find that you're actually slower and making more mistakes at 120. The effect starts to kick in at 90 on average.

Slower, and more mistakes...like, in games you mean? All I can figure anyway. Forgive me if I missed something.
I'll have to try this once I get my PC hooked up again - doing some major cleaning and rearranging here. And I hate cleaning, so it goes pretty slowly. All I know is, my game performance seems more influenced by how long I've been awake - in my afternoon/evening hours, reaction time is exceptional, like assume I wake up at 9am. After 3pm, reaction time in games gets really good. Any other times though, it's more like this: (simulated, of course )
10:00:01.00: Ok, he's swinging the lightsaber at my head.
10:00:01.01: That's bad - (quick recognition time, but not reaction time)
10:00:01.50: Sending message to hand to react
10:00:02.40: Lightsaber impacts head, killing player.
10:00:02.50: Hand is reacting.

That's at 120Hz; I'll see what happens at 90Hz. I do use Vsync all the time, just FYI. Can't stand the tearing that I get without it.

 

[Peter]

In games, there is much less difference simply because of the dark background. I'm talking about actual work. In really fast paced games, you actually benefit from a high CRT refresh rate simply because the on-screen action gets updated more often per second.

 

[Tab]

Originally posted by: Peter
In games, there is much less difference simply because of the dark background. I'm talking about actual work. In really fast paced games, you actually benefit from a high CRT refresh rate simply because the on-screen action gets updated more often per second.

How? This doesn't make sense to me.

 

[glugglug]

Originally posted by: zetter
Also with the glass screen on a CRT screen you get all sorts of reflections from objects and light sources around the monitor which you don't get off a TFT screen which can also add to eye strain.

BINGO!

Noone seems to believe me on this, but having lights on in the room is BAD for looking at the screen, even if they are overhead, not behind you causing a really really nasty reflection.

This is also the biggest problem with 60Hz. Not only is it a bit low, but it's in sync (but not PERFECTLY in sync) with artificial room lighting which causes bad interference patterns because it is a bit off. This happens with 120Hz as well.

 

[Peter]

Actually, the reason why we have 60 Hz refresh rate as a default is exactly because the AC power runs at that frequency. VGA standard resolution is standard TV timing times two, and TV timing is in multiples of the AC power frequency. (Incidentally, guess why NTSC TV is 60 Hz and European standards SECAM and PAL are 50. We have 50 Hz power here.)

 

[ReiAyanami]

for me its the brightness, or the intensity of photons hitting my eyeballs, retina or whatever

for example i can work at my comp 8 hours straight with no eye strain if i turn off all lights and turn brightness dial to the lowest level possible, while leaving contrast dial at the highest setting. not ideal for playing video games but good for spreadsheets and stuff

 

[gsellis]

Sorry, but I think the refresh rate is a red herring. It is all about focus and contrast. Ask yourself why a TV does not seem to affect you the same way? What about a movie in a theater (24fps)? Persistance of vision usually fills in the gaps.

You are "trying" to focus on text. The dpi levels on the current monitors make the text harder to focus on. If you add a refresh rate issue, it will compound it. If you play a game on the same monitor you read lots of text on, do you have the same problem? Not unless the refresh rate is really bad. Remember the early FPS games, Wolfenstein comes to mind, that would cause you to feel 'sick'. The game would strobe when you turned. After awhile, it made you feel dizzy. It all is about the clarity. When the 200+ dpi monitors come out, lets see where we are at.

You eyes are fatiguing because they cannot find the focus on the fine print blasted to you on a bright background. I have a gut feeling that san serif fonts are less fatiguing than serif fonts. Nothing to back it up, but...

So, it is more about the poor quality of the display and contrast (which refresh rate can make worse) than the refresh rate itself.

 

[buleyb]

Originally posted by: gsellis
Sorry, but I think the refresh rate is a red herring. It is all about focus and contrast. Ask yourself why a TV does not seem to affect you the same way? What about a movie in a theater (24fps)? Persistance of vision usually fills in the gaps.

You are "trying" to focus on text. The dpi levels on the current monitors make the text harder to focus on. If you add a refresh rate issue, it will compound it. If you play a game on the same monitor you read lots of text on, do you have the same problem? Not unless the refresh rate is really bad. Remember the early FPS games, Wolfenstein comes to mind, that would cause you to feel 'sick'. The game would strobe when you turned. After awhile, it made you feel dizzy. It all is about the clarity. When the 200+ dpi monitors come out, lets see where we are at.

You eyes are fatiguing because they cannot find the focus on the fine print blasted to you on a bright background. I have a gut feeling that san serif fonts are less fatiguing than serif fonts. Nothing to back it up, but...

So, it is more about the poor quality of the display and contrast (which refresh rate can make worse) than the refresh rate itself.

Ever had to read a lot on a TV? Same thing happens. It just happens that the stuff we look at on TVs doesn't effect us as much. And it has always been a thing to not stare at the TV because it has the same eye fatigue issues...

 

[glugglug]

Originally posted by: gsellis
Sorry, but I think the refresh rate is a red herring. It is all about focus and contrast. Ask yourself why a TV does not seem to affect you the same way? What about a movie in a theater (24fps)? Persistance of vision usually fills in the gaps.

The phosphors on a TV have a much slower response time than those used on a CRT because the manufacturers know it will never be used at a refresh higher than 30fps. So the pixels don't actually fade all the way to black in-between frames. If your refresh is too low relative to the response time of your monitor, it will flicker badly, even if we are talking about say a 5ms response time and 120Hz refresh, the flickering will be awful.....

Makes it kind of ironic that people are looking for LCDs with the lowest response time possible. Slower response time on a CRT would actually be desirable because it would eliminate flicker. (Basically you get a trade-off between flicker and ghosting). This will become more of an issue as 3D stereovision shutter glasses become more popular, as most CRT response times are slow enough to cause faint double images when used for this purpose, and the super slow response of a TV would make a soild double-image.

 

[Tab]

Originally posted by: glugglug

Originally posted by: gsellis
Sorry, but I think the refresh rate is a red herring. It is all about focus and contrast. Ask yourself why a TV does not seem to affect you the same way? What about a movie in a theater (24fps)? Persistance of vision usually fills in the gaps.

The phosphors on a TV have a much slower response time than those used on a CRT because the manufacturers know it will never be used at a refresh higher than 30fps. So the pixels don't actually fade all the way to black in-between frames. If your refresh is too low relative to the response time of your monitor, it will flicker badly, even if we are talking about say a 5ms response time and 120Hz refresh, the flickering will be awful.....

Makes it kind of ironic that people are looking for LCDs with the lowest response time possible. Slower response time on a CRT would actually be desirable because it would eliminate flicker. (Basically you get a trade-off between flicker and ghosting). This will become more of an issue as 3D stereovision shutter glasses become more popular, as most CRT response times are slow enough to cause faint double images when used for this purpose, and the super slow response of a TV would make a soild double-image.

So, since the image isn't changing as much it is easier on our eyes to focus on them? Our minds don't realize that the image is change every second, but our eyes do? I am not sure if I understand all of that above. Maybe I do? Hertz is just a cycle? What happens when we change the refresh rates on our LCDs? What does that do? Also, if 60Hz is the default for our moniters, why is that? I don't really understand electricity that much either.