BenQ Flicker-free technology white paper

June 11, 2013

BenQ Europe Meerenakkerweg 1-17 5652 AR Eindhoven Tel: +31 88 888 92 00 Fax+31 88 888 92 99 BenQ.eu

Simon Baker, TFTCentral.co.uk

BenQ Flicker-free technology white paper With the increasing reliance on technology, personal computers and growth of electronic communications, more and more people are using their displays for long hours and on a daily basis. With the wide availability, reduced costs and diverse range of options, LCD monitors are now the norm for most people, whether at home or in the office. The thin profile, energy savings, large screen sizes, attractive designs and lower heat output make them a popular alternative to traditional CRT based displays. With pro-longed use, the issue of user comfort becomes more and more of an issue and users are keen to ensure they avoid any possible discomfort or health issues. There are several factors which can influence the suitability of a display for long usage periods. Screen brightness plays an important role, with an overly bright display being a common cause for eye fatigue, headaches and other similar problems. Similarly a display which is too dark can lead to eye strain, so it’s important to find a balance which suits an individual’s ambient lighting conditions and usages. The configuration and setup of a display can also have an impact, where the gamma, white point (colour temperature) and overall picture quality come into play. Obtaining a well configured and comfortable display can often require calibration, either through software “by eye” methods or through more advanced calibration tools. Flicker In general terms the human visual system perceives flicker where there is a significant change in the brightness of light reaching the eyes during short time intervals. The frequency of these shifts between lighter and darker light is defined by the number of times per second the change occurs. At around 3 shifts per second (3Hz) the changes in brightness are very noticeable as you might expect. While the very visible flicker of 3Hz may decrease with higher frequencies, visual disturbances are still very problematic up to around 20Hz. Above 20Hz the issues decrease slowly as the frequency is increased, until around 50Hz where the flicker commonly expands into an impression of constant light to the eyes for most users.

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The frequency of this transition point is called the flicker-fusion threshold. This threshold may of course vary by person and can also vary in peripheral fields of vision. Perhaps most significantly, monitor flicker has been attributed to issues with eye strain and headaches when using displays for many years. It is important to understand that monitor flicker in LCD displays is different from older CRT displays. These CRT’s refresh at a certain frequency from top to bottom as the cathode ray gun is fired across the screen, with only part of the screen illuminated at any one time. At low refresh rates the frequency is low enough that it can produce visible flicker for and can commonly lead to issues for the user. A refresh frequency of 72Hz or above is commonly considered suitable to eliminate flicker from CRT’s for most users (TCO 92). LCD displays are not refreshed in the same way as their image is constant and updates on a pixel by pixel basis when the image requires the change. While a 60Hz refresh rate on a CRT would be considered problematic to many users when it comes to flicker, most LCD monitors are designed to work at 60Hz but they do not produce flicker in the same way. Flicker on LCD displays is still a possibility and cause for concern for users, especially those who are using the screen for long periods of time. There are several factors which can cause flickering effects on an LCD monitor which should be checked: 1) Refresh Rate setting – the recommended and optimum refresh rate setting for most LCD displays is 60Hz. While they may commonly accept other settings, typically up to 75Hz maximum, the picture quality may be adversely affected when using settings outside of the recommended 60Hz. Users should always check their graphics card refresh rate to ensure a suitable setting is active. Some modern screens support 120Hz or higher refresh rates and so can be set to 120Hz+ instead without issue.

2) Cabling – If there are visible flickering issues or poor picture quality then it is always wise to check the cables being used. Where possible and available, a digital connection such as DVI, HDMI or DisplayPort should be used to obtain the optimum picture clarity. If analogue (VGA) is the only available option, ensure you run the monitors “auto configuration” option if available to obtain the best picture balance. Where there are concerns over picture quality using any of these options, try an alternative cable if

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available and check the connection at both ends.

3) Graphics Card – a useful step if you experience any issues with picture quality is to check and update your graphics card drivers and settings to ensure you have the latest and most appropriate software installed. Refer to your graphics card manufacturer for more information.

4) Interference – Interference from other nearby devices, electrical supplies and appliances may also cause issues with the picture quality of your display. Check the power connections, supplies and cabling and isolate the display from other electronics if possible to eliminate any issues caused by interference.

Other Causes of Monitor Flicker - Pulse Width Modulation (PWM) Perhaps the most common, yet not widely understood cause of flicker and related symptoms is the use of Pulse Width Modulation (PWM) in desktop LCD monitors. This technology is used in the majority of desktop monitors to control backlight dimming, and has been utilised for many years. Nearly all monitors offer the user a direct control over the intensity of the backlight unit through the brightness control in the On Screen Display (OSD), in turn allowing the user to obtain a suitable luminance for their requirements. To achieve this, in simple terms, PWM is a technique used to rapidly turn the backlight unit off and on to simulate a lower perceived luminance for the user, in theory at a level which should be undetectable for the user. At the maximum brightness setting (100%) this technique is not needed and the backlight is illuminated continuously. As the brightness setting is lowered, the luminance intensity is decreased using this PWM technique. Its operation is explained in the following section. This cycling of the backlight off and on is happening all the time, not only in changing images of games and movies, but when viewing static images for general day to day uses. Word processing, spreadsheets, email and internet browsing with their large bright backgrounds are perhaps most problematic when it comes to issues with PWM backlight techniques. PWM as a technology allows for wide adjustment ranges, helping to offer both high maximum luminance, and low luminance control for those who need to use the screen in darker ambient lighting conditions. PWM has been used for many years with success and offers an established, simple circuit design making

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it a cost-effective and simple route for manufacturers to utilise. PWM Operating Parameters

PWM cycling typically occurs at a fixed frequency, and the fraction of each cycle for which the backlight is on is called the duty cycle. By altering the duty cycle the total light output of the backlight is changed. As a user lowers the brightness setting, the duty cycle typically becomes progressively shorter, resulting in a reduced luminance. As a result, the lower the brightness setting, the longer the “off” periods are, and the more pronounced any flicker may become. The PWM operating frequency determines how many times per second the backlight is cycled on and off, with lower frequencies potentially being more problematic when it comes to flickering. Another factor is the difference between the brightness of the backlight during the on and off states, the amplitude or depth of the modulation. In some cases there may be extreme differences between the two states, and in others there may be a smaller modulation. Where a higher modulation is used, there is the potential for more noticeable issues with flicker due to the more pronounced difference in brightness levels. LED Backlighting While PWM dimming has been used for many years, it is only recently that LED (Light Emitting Diode) backlight units have become popular and incorporated into the majority of new displays produced. Older Cold Cathode Fluorescent Lamps (CCFLs) have been used for many years since LCD became a mainstream option for desktop monitors, but with the increased focus on the environment and energy

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saving, LED backlights have now taken over. Importantly, while PWM has been used with CCFL backlight units for a long time, the issues associated with its use have only recently become more commonly understood as the effect is more pronounced on LED backlight units due to their nature. PWM is used in the same way with both backlight technologies. However, flicker from LED backlights is typically much more visible than for CCFL backlights at the same duty cycle because the LED's are able to switch on and off much faster, and do not continue to "glow" after the power is cut off. CCFL backlights darken more slowly and smoothly and so flicker may be less noticeable to the user. In contrast the LED switching times are much quicker and so the difference between an “on” and an “off” period become more pronounced. This means that where the CCFL backlight showed rather smooth luminance variation, the LED version shows sharper transitions between on and off states. This is why more recently the subject of PWM has cropped up online and in reviews, since more and more displays are moving to LED backlighting units now. Visual Flicker In most cases the rapid cycling of the backlight on and off means that many users cannot see this flickering, because it lies above their flicker-fusion threshold. The higher the PWM frequency, the less likely it is that an individual will actually see visible flicker. PWM frequency varies greatly, from low frequencies such as 90Hz, common frequencies between 180 – 300Hz and even up to >2500Hz. Where the effect of flicker can really come into play is any time the user's eyes are moving. Under constant illumination with no flickering (e.g. sunlight) the image is smoothly blurred and is how we normally perceive motion. However, when combined with a light source using PWM several discrete afterimages of the screen may be perceived simultaneously and reduce readability and the ability of the eyes to lock onto objects. From analysis of various backlighting methods we know that false colour may also be introduced, even when the original image is monochromatic. Below are shown examples of how text might appear while the eyes are moving horizontally under different backlights.

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It is important to remember that this is entirely due to the backlight, and the display itself is showing a static image. While the eyes are moving, such as when reading, it is possible to see the effects of flicker at several hundred hertz (Hz). The ability to observe flicker varies greatly between individuals, and even depends on where a user is looking since peripheral vision is most sensitive. Some users find they are very susceptible to the flicker from PWM, even at high frequencies, while others cannot see it in practice at all. Where PWM is used for backlight dimming the only solution really if one is affected by the flicker is to turn the brightness control of the monitor up. At 100% the PWM should eliminated completely, but there are then other issues with using a display at such a high luminance. Making alterations via software methods at a graphics card level can overcome this high luminance, but being digital adjustments they negatively impact the contrast ratio and appearance of the image. It should also be noted that the screen would be operating at the maximum backlight intensity, and therefore with the highest power consumption. Health Concerns Backlight flicker may or may not be perceptible to a given user, but there are still concerns which affect many people. Flicker and the use of PWM dimming methods, even where not directly visible to the user,

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have been linked to eye fatigue, eye strain, headaches and nausea. Again this varies significantly from person to person, but with the increased popularity of LED backlight units, it appears to be more of a widespread concern, and certainly now better understood in the industry. 100 - 120Hz flickering of fluorescent lights has in fact been linked to symptoms such as severe eye strain and headaches in a portion of the population, which is why high-frequency ballast circuits were developed which provide almost continuous output. Using PWM at low frequencies negates the advantages of using these better ballasts in backlights because it turns an almost constant light source back into one that flickers. An additional consideration is that poor quality or defective ballasts in fluorescent backlights can produce audible noise. In many cases this is exacerbated when PWM is introduced since the electronics are now dealing with an additional frequency at which power usage is changing. Concerns around flickering can affect any user, but may be especially problematic for anyone using a screen for long periods at a time. Web developers, writers, students, office workers and anyone who needs to sit in front of a screen for a long time may find they are more prone to issues associated with flicker and the use of PWM than casual users. How to Test for PWM Most screens will not list whether PWM is used for backlight dimming and in many cases the manufacturer may not even know. Fortunately there are some simple tests which can be carried out to establish whether PWM is used or not. There are also some more advanced tests used in the industry to more accurately measure PWM frequencies which are also explained. Of course, keep in mind that PWM may not affect every user, and many users will be perfectly fine using any screen with PWM with no side effects. It has been used for many years and only a small percentage of users are adversely affected. For those who have experienced problems in the past, or are keen to test if a given screen uses PWM for backlight dimming, the following simple tests are suggested. Keep in mind that PWM is generally only used below 100% brightness, and it may be more noticeable at the lowest brightness settings. 1) Basic Visual Tests – by setting up a fan in front of the screen you may be able to see where PWM is used, where the flicker is visible and the fan blades appear as shown in the image. This “strobing” effect can

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also be observed in some cases by waving your hand, with fingers parted, in front of the screen. While these are fairly crude tests, they can be quickly performed as a basic test for monitor flicker.

(Image from BenQ.eu)

2) Basic Camera Test – a more reliable, yet still very simple method is to use a camera to capture an image of the screen when using a certain test pattern.

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Display a simple all black image with a single 1 pixel wide white vertical line as shown below on the monitor

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Set the brightness to the desired setting. Suggested tests at 100%, 50% and 0%

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Set the camera shutter speed if possible to a slow setting. 1/8 of a second should be sufficient to capture a good test image

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If you cannot manually set the shutter speed then a picture may still be useable anyway

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Hold the camera approximately 2 feet in front of the monitor and perpendicular to (looking straight at) the front. Press the shutter button as you slowly move it horizontally across the screen (remaining perpendicular)

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You may need to experiment with moving the camera at different speeds to capture a useful image

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Adjust the captured image brightness so that the pattern is easily visible and crop the image if necessary

The resulting photographs should help you identify whether PWM is being used for backlight dimming. A classic PWM example might look like the following examples taken from the BenQ GW2750HM with LED backlight system. Shutter speed of 1/8 second:

Brightness setting:

100%

50%

0%

At a setting of 100% brightness PWM is not used and so the white line is a continuous, solid block. At 50% the line splits in the photograph with a short “off” period between each section evident by the thin black breaks. At 0% the “off” sections are longer and again the line is split, signifying a shorter duty cycle (the

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period for which the backlight is on). Using this technique we are turning a temporal effect into a spatial one by moving the camera during capture.

This test confirms that PWM is being used for backlight dimming in this example for settings below 100%. This method can also help you identify the frequency of the PWM being used as follows: •

Count the number of lines visible in the captured image.

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Multiply this count by the inverse of the shutter speed. For the example above, the shutter speed of 1/8 of a second shows 30 cycles. The number of cycles per second is 30 * 8 = 240Hz. This is the backlight cycle frequency.

At very high PWM frequencies it may not be possible to accurately capture the effect using the camera method here, as the changes are too fast and the line appears as a continuous solid block. Using an even slower shutter speed may help confirm.

3) Oscilloscope Tests – with the right equipment it is also possible to test a monitor’s backlight using an oscilloscope and photosensor setup. Monitor review sites including TFTCentral.co.uk use such a system to accurately determine the backlight dimming technique and the frequency of any PWM method.

Brightness Setting = 100%

A continuous current is applied to the backlight and so the backlight is continuously lit. The straight line confirms this.

Each horizontal grid represents a 20ms period in this scale

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PWM is clearly being used to cycle the backlight on and off. The upper peaks indicate when the backlight is turned on, while the lower troughs Each horizontal grid represents a 20ms period in this scale

are when it is switched off.


The PWM is even more pronounced due to a shorter duty cycle, with longer “off” (dark) periods and shorter “on” (light) periods, resulting in a lower luminance for the user. Each horizontal grid represents a 20ms period in this scale

Brightness Setting = 0% (zoomed image)

The PWM pattern becomes clearer as you zoom into the oscillograph at a smaller scale.


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The number of peaks can be counted within a measured time frame using an oscilloscope system to calculate the PWM frequency. In this example the PWM frequency is measured at 240Hz. Results were obtained from the BenQ GW2750HM with LED backlight system. CCFL Backlight vs LED Backlight

As we have already discussed, the brightness curve of a CCFL backlight is smoother than that of an LED backlight as there is some ‘glow’ from the backlight even after it has been turned off. The above image on the left is from a typical CCFL backlight, whereas the image on the right is from a W-LED backlight unit. You will see there are sharper and quicker changes in the brightness intensity as the backlight is cycled off using PWM in the LED backlight example, often leading to more noticeable flicker for the user.

Alternative Backlight Dimming Techniques Other options for backlight dimming do exist, although are not widely used. These include Direct Current (DC) control, which does not cycle the backlight off and on at all, but can be more complicated to implement. In some cases there is also difficulty controlling the colour in darker images and so DC backlight control is less common. In a study conducted by Kitasato University Japan, School of Allied Health Sciences, it was established that DC resulted in the lowest levels of flicker and eye fatigue, and was overall easiest to view for prolonged working conditions.

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Backlight Dimming Technique

Pros

Pulse Width Modulation (PWM)

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Cons

Wide adjustment range for

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Possible visible flicker for the user

monitor brightness/luminance

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Associated health concerns

Simple and cost-effective circuit

including eye fatigue, headaches,

design

nausea

Established technique used for

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many years

Even where flicker isn’t directly visible to the user, PWM may adversely affect the user

Direct Current (DC)

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No flicker

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Complicated circuitry

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Not widely used and less established than PWM for backlight dimming

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Trouble controlling colour in darker images in some cases

BenQ Flicker-free Monitor Range With the increased focus and awareness of monitor flicker and the associated medical concerns, BenQ have now introduced a range of ‘Flicker-free’ monitors. These monitors are designed to address any concerns users might have around eye fatigue and other health issues associated with flickering displays. They are based on a Direct Current backlight system where Pulse Width Modulation is not used. As a result, the main cause of monitor flicker is eliminated, making the Flicker-free range suitable for even the most demanding users. Oscilloscope tests conducted by TFTCentral.co.uk of the ‘Flicker-free’ BenQ GW2760HS confirm that PWM is not being used for backlight dimming as shown below:

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The straight line indicates a constant backlight illumination at maximum brightness setting as normal.


When reduced to 50% brightness, the straight line confirms no PWM is being used, and the backlight is not being cycled on or off.


Even at the lowest backlight setting, PWM is not used and the illumination is constant.