I am building a computer for a friend and need some help. She is older lady with a vision problem and no computer experience. There is damage to one eye, after looking at the monitor for a very short time she gets a painful headache, due to the “flickering” of the monitor.
One of the replies;
1. A lower response time has nothing to do with flickering, or refresh rate. LCDs do not flicker when the screen refreshes, only the fluorescent backlight itself will flicker. For an absolutely flicker-free LCD (theoretically...), look at an LED screen.
2. A lower backlight setting on any given monitor will flicker more than the same monitor at full brightness. This means you need to take care to find a monitor with a low brightness rating, 250-300 cd/m^2 at the most. HDTVs are usually brighter than monitors, but not always.
CRTs do really flicker, because there is constantly beam "illuminating" the pixels, which writes each horizontal line from the top to the bottom of the image. And that can flicker at 60Hz. And that can be felt by the eye.
LCD displays are made up of many layers of transparent materials, an LCD panel and a backlight.
The LCD panel is a panel with lots of dots (pixeles) which vary their light transmition (from black-opaque to transparent). You can set its refresh rate from the computer, but normally you have to use a lower resolution than the maximum to be able to increase the refresh rate. Anyway, I don't think this is related to her problem, because the LCDs don't turn on and off, they just change the intensity (opaque, transparent, or in between) of the pixels. A 60Hz refresh rate means that the display is refreshing its information 60 times per second.
The backlight is a big white square light. Standard LCDs are lit by a compact fluorescent light (CCFL). Newer ones are lit by LED lighting.
Fluorescents need to flicker in order to work. There is no way to make them work without flickering. A magnetic ballast (like a brick inside home fluorescents fixtures) works at 60Hz (this means 60 times per second). An electronic ballast works at a much higher rate, I'm not sure but it might be around 20KHz (20 thousand times per second).
Very sensitive eyes can notice the difference between a 60Hz tube and a higher one. Even if you don't notice the difference, it is evident that after some hours of being inside a place lighted by magnetic-ballast-tubes, the sight gets more tired than with an electronic one.
I don't know what is her "hertz limit", but according to what the eye can normally perceive, no one should be ever affected by a 20KHz flickering tube.
My display shows 64.5KHz on the menu. That's the backlight frequency.
LEDs don't need to turn on and off to work. You can connect a LED light to a DC battery and it will work. But to be able to dim a LED, it's better to use a PWM dimmer instead of a standard dimmer. This means that instead of decreasing the voltage (which increases the temperature of the LED and reduces the lifetime), you turn it on and off, and you change the intensity by modifying the on-off time. Imagine the cycle lasts 1millisecond, then the LED can be 0,1ms on and 0,9ms off, or 0,9ms on and 0,9ms off, or in between.
LED baclkit LCDs, as any product with a dimming led, use this system. I made a simple PWM dimmer for my LEDs at home and when testing it, it shows to work at 20-22KHz. I don't know which is the frequency of the LED monitors, but I can say that it cannot be near a noticeable number (for the eye).
The only differences I can think of between LED backlit LCDs and CCFL backlit ones are:
- LEDs (as they use RGB LEDs) produce higher color gamut, better color reproduction.
- LEDs don't contain mercury, nicer to the environment.
- LEDs have a lower power consumption. But I'm not sure if LED monitors do consume less power, because they are brighter, which makes them be the same wattage compared to CCFL LCDs.
I think this woman, or maybe you, should ask an ophthalmologist about which is better.
regarding the dell g2410, i just received mine 3 days ago and i love. it's thin, lightweight(less than 9 lbs.) and is very energy efficient using only 20 watts
when in operation and only 0.15 watts while in standby mode thanks to it's
led backlighting which is far superior to ccfl backlighting weather it's on a hdtv
or computer monitor and most of all, led's have a much longer lifespan than
ccfl's thus making led products more economical than ccfl products. also led's
are used in other products such as flashlights, lightbulbs, and car tail lights
thus making led's the current technology and ccfl's the outdated technology.
I purchased a Dell G2410 about a month ago and, too, am pleased with the performance and 24" widescreen. Arrived quickly. Hope to go off-grid so chose this model particularly for the energy efficiency.
CCFL frquency is 60hz because thats the frequency of the AC power from your electrical socket.
But don't think of this like a monitor refresh rate. 60hz is actually a lot for a CCFL because the backlight doesn't turn off (black) when the current alternates. It simply dims a extremely extremely small amount before it turns on again. I am not sure if you can even percieve this difference.
LEDs on the otherhand uses DC power and would not have this effect. I have a LED monitor and im currently using a CCFL one but they both look the same to me.
May 6, 2010 4:20:55 AM
I think you should bang that old lady you are helping
May 6, 2010 4:26:00 AM
seriously, she is probably HAWT!!!!
May 6, 2010 4:27:11 AM
WOOT!! Granny Fanny!
November 10, 2010 4:11:48 AM
The the fluorescent backlight will flicker at least 120Hz in the US once at the max and once at the min of each cycle.
It the UK it flickers at 100Hz since are mains is at a lower 50Hz frequency.
Fluorescent light does not dim it is on or off, it works like lightening strikes.
I can see mains powered compact fluorescence and even LED lighting, when the voltage goes close to zero between max and min the lights go out.
Some people say that filament lights are stable because the filament does not have time to cool but this is not true of 100 Watt bulbs at 100Hz I find them worse than fluorescents using high speed ballasts.
Rectified AC is intermittent DC only full rectification with capacitors or tri-phasic supply produces DC.
Only continuous DC produces stable lighting (cars use DC batteries), computers have lots of capacitors in the PSU to fully rectify others have noticed that laptop displays cause less eye strain than the independent monitor but don't know why I would be interested in how rectified the relevant supplies were.