Monitor is flashing through camera but not in the naked eye

Question

Two monitors: Samsung T23C350 - Samsung 2243NW

Lets call the first A and the other B.

A is a LED monitor with a 2ms response time (in specs) with a native resolution of 1920*1080@60Hz

B is an older not-led one with 5ms response in specs with a resolution of 1680*1050@60Hz

In the attached images and video you'll see that A (on the LEFT) is flickering but this is not visible to the naked eye.

I took some pictures and videos with a camera and a mobile phone. This is also happening to all digital camcorders I've tested.

A has a setting response time with values normal, fast, fastest. No matter the setting, the effect is the same.

A on the left and B on the right. You can see the flickering, although both are @60Hz

A video where you can see both monitors. A is always the left monitor.

Another video where you can see what my digital camera sees.

Another thing is that I managed to get a picture where the left monitor is completely black whilst the right one is displaying fine. This proves that the monitor is continuously flashing.

My question is: Is this normal behavior and what is causing it? Am I affected by this fast flickering? I am using this as my primary monitor and as a programmer I have to keep my eyes on it for hours.

Answer 1

The "response time" settings on your monitor relates to the LCD panel itself. The problem you are seeing is with the backlight.

Your B monitor is CCFL, which uses a very high frequency to control the backlight brightness. LED monitors use Pulse Width Modulation to vary the total brightness, since varying the voltage to the LED would change the color response of the monitor. PWM allows full brightness for a specific period of time, then the LED array turns off for a specific period of time, with the average "on" time being the relative brightness of the panel.

LED PWM frequencies are usually 2 to 4 times the LCD refresh rate. More expensive monitors may be 8 times or higher. A low frequency is generally not a problem, unless the panel is not at 100% backlight output, then you may notice the strobing with your eyes. If you turn the backlight down to the lowest setting and you do not notice the strobing, it will not bother you.

This strobing effect has also been seen on newer GM and Dodge automobiles with PWM controlled rear brake lights that double duty as running lights. When the brake pedal is not applied, they are at half brightness due to PWM, and the strobing is quite visible since they use a very low frequency driver.

Answer 2

LCD

There are several sources of periodic changes in brightness that affect LCDs.

If your video frame rate does not match the frequency of these changes, the video will suffer from variations in light output.

If your still-camera exposure time is too short, it can also capture these variations.

Since the shutters used in liquid crystal displays for each pixel stay at a steady opacity, they do not flicker, even when the image is refreshed. The backlights of such displays typically operate in the range of 150–250 Hz. But, to save the crystals from detoriation caused by constant current, voltage is constantly reversed, which may cause flicker. "In a pixel on an LCD monitor, the amount of light that is transmitted from the backlight depends on the voltage applied to the pixel. For the amount of light, it doesn't matter whether that voltage is negative or positive. However, applying the same voltage for a long period would damage the pixel. For example, electricity decomposes water into oxygen and hydrogen gas. A comparable similar effect could happen inside the liquid crystals that are in the pixels. In order to prevent damage, LCD displays quickly alternate the voltage between positive and negative for each pixel, which is called 'polarity inversion'. Ideally, the rapid polarity inversion wouldn't be noticeable because every pixel has the same brightness whether a positive or a negative voltage is applied. However, in practice, there is a small difference, which means that every pixel flickers at about 30 hertz.

A LED monitor doesn't use a fluorescent backlight and so shouldn't be affected by any flicker associated with that technology but LED intensity is often adjusted using PWM (pulse width modulation). This should be at a sufficiently high frequency that it isn't noticeable to human eyesight, and should ideally be outside the range of frame-rates and exposure times of still images. However I suspect this is a potential cause for this phenomenon. Some commercial PWM chips for LEDs operate at around 100 Hz. LED output doesn't have the persistence of CRT output, so any change in the drive signal very quickly results in large changes in light output.

wikipedia

CRT

This is a problem where

1. the refresh rate of the screen does not match the scan rate of your video recorder
2. the exposure time of the camera is insufficient to average out the scan rate of the monitor

All CRT monitors continuously flash - this is a normal part of the way the screen image is refreshed. The video image is constructed in memory in the video card and is periodically transmitted to the display.