adjusting backlight on lcd panel brands

For the video display developer LCD panels are available in many sizes and resolutions, they are also available with many choices of maximum brightness. The following considers the topic of LCD panel brightness, the choices, the methods for adjusting brightness and some brightness adjustment scenarios.

LCD panels are generally rated as to their maximum brightness level which is expressed in Nits, it is equal to Candela/sqm (cd/m2), and this will be at a particular color temperature as noted in the specification, usually 10,000 K. In terms of a practical understanding, the following is a rough guide:

Outdoor displays range from a low end of 700 nits to typically 1,000 or 1,500nits and up with 2,000~2,500nits and even up to 5,000nits seen with some models. This may include standard LCD panels, custom LCD panels as well as custom cut LCD panels.

Virtually all LCD panels have a LED backlight these days, these are powered by an LED driver board. Brightness control via the driver board will be by one of two methods:

PWM (Pulse Width Modulation): This varies the duty cycle of the backlight “on time” – it is predominant in modern LCD panel LED backlight designs to enable support for digital brightness controls.

Analog: Uses a simple variable voltage to adjust brightness, for example this might be a dial or slider type potentiometer / variable resistor. To see how to enable analog backlight adjustment visit: https://www.digitalview.com/blog/brightness-adjustment/

One of the advantages of LED for the backlight is the range of adjustment that is possible, however it is important to note that the range varies significantly from model to model. Some industrial panels can be turned to very low light levels making them suitable for use in special environments such as at night. Lower cost panels limit the range of brightness to what might be required for typical usage, whereas panels with full range dimming from full off to full on require more complex backlight drivers.

Backlight lifetime: Many LCD panels have a backlight lifetime rating of 50,000 hours (typically measured to half brightness), this can be extended by running the LED backlight at a lower brightness level. Some panels may only offer 30,000 hours as a lower cost solution while other panels may offer up to 100,000 hours for high end applications.

An LCD panel backlight may be constructed so the LED’s are mounted directly behind a light guide diffuser, or they may be mounted along one or more edges of the light guide.

Active backlight: This is a function of some LCD panel backlights to automatically adjust the backlight brightness in response to the image. For more advanced systems there is an LED array making up the LED backlight, this adjusts the brightness in areas localized to the image being shown. This can greatly enhance the brightness across the display and is being used primarily with video, for example on consumer TV sets. It is not useful to all image types, for example a spreadsheet or content like maps or data is not likely to benefit.

Local dimming: Some LCD panels with direct LED may support local dimming so the LED’s are dimmed in response to the image close to them. This will not be at the same resolution as the LCD panel itself but will help greater contrast over the display by enhancing the brightness in bright areas of the image and darkening the image in dark parts of the image.

Both of the above techniques are likely to be more beneficial to certain types of content than others. For example a movie is likely to benefit more than a spreadsheet.

For the LCD monitor manufacturer it is important to consider that any covering over the LCD panel will reduce the brightness. For example the protective glass over a digital signage display, or a touch screen, or a semi-silvered mirror. So if a specific brightness is required the measurement should be taken with these in place.

There are various relatively low cost brightness meters available, typically in the couple of hundred dollars range. It is difficult to comment on the accuracy of these but we have found them to be within 5% of each other, though more importantly they do appear to be quite consistent in measurement so good for measurement comparisons. For more accurate measurement there are light meters from companies such as Minolta that can be calibrated, the cost may run into several thousand dollars.

Examples of light meters costing a few hundred dollars include SpyderX by Datacolor (needs a PC), a handheld meter is the SM208 by Sanpometer (search SM208 meter). Note: Many light meters, including smartphone apps, will be meters used for photography and not give readings in nits (or candelas). LCD panel specifications are typically measured using nits.

PWM and Analog: Most Digital View LCD controllers support PWM and Analog as a method for adjusting the backlight brightness level (this is noted in the column headed “Other” on the controller board summary table: https://www.digitalview.com/controllers/lcd-controllers-home.html. Also see https://www.digitalview.com/blog/brightness-adjustment/ for a guide to using a dial or slider type variable resistor to adjust the backlight.

DPMS (Display Power Management System): The backlight will be automatically turned off after a period if there is no valid video signal being received.

Ambient light sensor: The backlight is adjusted for brightness or powered off depending on ambient light conditions. This uses a light sensor attached to the LCD controller board, see https://www.digitalview.com/blog/light-sensor-app-note/ for more details.

The specifics of the backlight control are documented separately for each LCD controller model (product summary here) in the product manual available for download on the product page.

Note: There are two ways to adjust the perceived brightness of a LCD panel or LCD monitor, the backlight and the black-level. Very often, particularly in the past, the monitor brightness setting adjusted the black-level, this adjusts the LCD but not the backlight.

Color, color temperature etc: In addition to adjusting the brightness other settings may be adjusted as well. For example the color temperature or for example a switch to green monochrome for night vision.

Night-safe lighting (update) : Dual-rail backlights can also be supported. These special backlight enable normal brightness and extreme low level brightness with custom night-safe lighting. Contact us for details.

Note: We have a blog on methods for implementing an ambient light sensor with Digital View LCD controller boards to automatically adjust the backlight or system power, see: Ambient Light Sensor

Update March 2019: Most of the above remains unchanged except for the increased availability of high bright LCD panels of around the 1,000 nit to 2,500 nit range. AUO for example has a number of large size LCD panels with 1,500 nit brightness for the digital signage market. Tianma has panels under 20″ with 1,000 nit to 1,500 nit brightness for various outdoor applications.

The other change is that high bright panels are now increasing edge-lit, this makes the panels thinner and these panels tend to use less power than the previous models. One of the benefits for monitor designers is easier heat management and reduced overall display system costs.

There are a few techniques that can be employed to make the LED backlight brighter, but if your goal is to compete with (or against) the direct sun light, there are better methods than to increase the brightness of the backlight.

“We need the brightest LED backlight possible for our new LCD design and we need to make sure that the LCD is sunlight readable.” This is a common request and the battle begins when engineers employ such techniques as PWM (pulse width modulation), reduction or elimination of current limiting resistors and overdriving the rated power limits with the final goal to make the backlight brighter.

Emissive & non-Emissive LCD modulesEmissive displays do not require a backlight since they produce their own light; an example of this is an OLED (Organic Light-Emitting Diode). OLED’s are readable in both dark and daylight environments.Non-Emissive displays require a backlight to illuminate the LCD when used in low or no ambient lighting conditions. Character, Graphic and Segment displays fall into this category and are the subject of this article.

What is a NIT? The brightness of a backlight is measured in "nits". As a general rule, one nit = the light produced by one candle. (It"s a bit more technical than that, but the idea works out the same.)

The majority of LCD display backlights are LED (Light Emitting Diode) and in most cases require the same power that is applied to the LCD logic (the power that drives the LCD only).

LED backlights require a current limiting resistor to reduce the driving current reaching the backlight. The lower the resistor value, the brighter the backlight.

Reducing the resistor value shortens the half-life of the LED backlight. The normal half-life of a LED backlight can range from 50K hours to 70K hours, but when overdriven the half-life can drop to 20K hours or less. This may not be an issue if the product has a short lifespan or if the backlight is rarely on, such as a cell phone.

The customer needs to decide on the tradeoff between making the LED backlight brighter and a dealing with a shorter half-life.Note: Half-life of the LED is the amount of time (in hours) for the LED to become half as bright as when it was first turned on. Half-Life is not when the backlight will burn out, but when it dims to half the brightness of when it was first turned on. MTBF (Mean Time Between Failures) is the amount of time before the backlight fails.

A second option to increasing the LED brightness is to replace the Transflective polarizer with a Transmissive polarizer.Note: Each LCD contains two polarizers, the front polarizer (facing the user) is always Transmissive; the rear polarizer is selected by the user.

One disadvantage of the Transmissive polarizer is that the display is difficult to read when the backlight is off. Transmissive is not recommended for battery powered product since the backlight must always be on.

Choosing a backlight system for LCD screen displays is a major consideration. It will determine a lot about your experience of the display and requirements during production. Different backlight options provide widely different effects in the contrast and brightness of the display. Also, depending on which backlight option you choose, it will affect some or all of the following: the cost of the overall product; how many products you will have to order due to manufacturing constraints; and how environmentally friendly the component parts are that make up the product.

The word LCD has been used to describe many display technologies. Often people believe that LCD screen displays are the same as a CRT (Cathode Ray Tube), an LED Display (Light Emitting Diode) or a Plasma display. This is not the case! Let’s discuss what an LCD is and what it is not.

Think of the liquid crystal display (LCD) as a window blind. Positioned in one direction the blinds allow light to pass through, or turned another direction they block the light. Just like a window blind, the LCD does not create its own light, it only blocks or allows it to pass through.

As you know, you can adjust the blinds to alter the amount of light desired. When fully closed, the blinds block light completely; when open, all light passes through; and when angled, partial light comes in. An LCD works similarly to this, with one significant enhancement: an LCD has the ability to block light in some areas and allow light to pass in other locations of the glass. An example of this is the display used on a gas pump. The customer sees numbers where the light is blocked, and a clear area where the light is allowed to pass through.

LCD’s are not CRT’s, LED’s, nor are they Plasma displays. Each of these types of displays produces their own light and are called emissive displays. Emissive displays require more power than an LCD.

Emissive displays have a distinct advantage in that they can be seen clearly at night whereas LCD’s cannot. However, the solution to this problem of low-light visibility is to install a backlight behind the LCD. Backlights do require more power than the LCD itself, but they can be turned on only when necessary. Many products that are powered by batteries will have the backlight dim or shut off after a certain amount of time. This can be seen on cell phones and watches. Consequently, even though a little more power is used for the backlight than used in a stand-alone LCD, because it is not constantly on, the LCD’s with backlights wind up using less power than their emissive display competitors. LCD screen displays using backlights become the clear choice.

This option is the most popular for products that have a lower power budget. Products that run on battery need to conserve power and the lowest powered backlight available is to have no backlight at all.

The Amazon Kindle is a perfect example. The Kindle makes use of a display technology called ‘e-paper’, which looks more like a printed page than any other device on the market currently. This specific e-book reader does not contain a backlight. Because it omits the backlight it can operate up to one month without recharging. Imagine, you could take it on a cruise to Fiji and back and never have to worry about recharging it!

Thinking back to your product, not all products can omit a backlight; in fact it may require one. If it does need a backlight the most popular option is an LED.

A light emitting Diode (LED) is a semiconductor that produces light when current is passed through the device. Light is created from the energy conversion that takes place in the LED die. The advantages of an LED are:LED lights are much more rugged and can handle shock much better than other types of lights.

LED backlights are made up of an array of LED’s. They come in a variety of colors including red, green, yellow, amber, blue, white and R/G/B (Red/Green/Blue). From the R/G/B trio any color in the rainbow can be made.

The majority of the LED backlight colors will operate with a half-life of 50K to 70K hours. (Remember, half-life is when the light will be half as bright as when it was first turned on. This is not when the LED will burn out.) Blue and white LED’s do have a shorter half-life than other colors. Presently, they are rated at 30K hours. That means that if you turned them on today and left them on, in 3.4 years they would be half as bright as they are today! One thing to take into account is that as technology improves, the lifetime of the LED’s will become longer, which will also increase the half-life. Below is a photo of a blue LED backlight.

To make LED’s display correctly, they are placed behind the LCD screen display in an array pattern. The challenge is that LED’s, similar to a light bulb, project a beam of light which can show spots of light. These spots are called hot spots. This can be an issue with LED’s since they will make the display look like it has polka dots. Below is a photo of a LED behind the LCD glass. This problem will be solved with the use of a diffuser.

A diffuser is like a lamp shade and is placed between the glass and the LED’s. The goal is to disperse the light or make it more even. Below is a photo of a diffuser. At the bottom of the diffuser you will see the LED’s. This is where you can see hot spots. But as the light travels further into the diffuser, the light becomes more even. The diffuser solves the issue with hot spots and makes LED’s a very attractive option.

EL (Electro Luminescent) backlights, also known as ELP’s (Electroluminescence Panel), have been used as a backlight for LCD’s for several years. They are available in a range of colors with white being the most popular. EL technology makes use of colored phosphors to generate light. They require AC (alternating current) rating of 100VAC @ 400Hz.

In the last three to four years, EL backlights have decreased in popularity. There are several reasons for this drop in popularity.The EL backlight requires an inverter to convert DC to AC. The cost of the inverter increases the cost of the overall LCD Display.

The half-life of an EL is an estimated 3,000 to 5,000 hours. (Once again, half-life is when the backlight is half as bright as when it was first turned on.)

LCD screen display manufacturers now require an MOQ (Minimum Order Quantity) of no less than 500 displays for orders that include EL backlights. This MOQ number is likely to increase in the future as this type of backlight becomes less popular. Additionally, as the demand drops the price will naturally increase.

A Cold Cathode Fluorescent Lamp (CCFL) is similar to the long fluorescent light bulbs you see in the ceilings of offices. Below are photos of various types.

This technology has been in use for many years, but in the last few years the popularity of this type of backlight has decreased. There are a few reasons why this is so.Similar to EL backlights, this technology operates on AC.

The majority of LCD screen display manufacturers no longer offer CCFL as an option. There are too many negatives and not enough positives to their use. The suppliers that do offer this now require a very high MOQ (Minimum Order Quantity).

When choosing the type of backlight for your LCD screen displays it is important to keep in mind MOQ’s and future availability. Make sure you are choosing a technology with a future so that your product has one too!

Traditional LCDs use CCFLs, or cold-cathode florescent lamps, as their backlight. While cheap, they"re not as energy efficient as LEDs. More importantly, all contain mercury, and aren"t able to do some of the fancy area-lighting of which some LED backlit models are capable. Because of these issues and the falling prices of LEDs, CCFL backlit LCD TVs will disappear entirely very soon. In 2013

Most LED LCDs on the market today are edge-lit, which means the LEDs are in the sides of the TV, facing in toward the screen. In the image at the top, the LED strips are above and to the side of this exploded-view of an LCD panel. There"s a close-up view here (full article with more images

There are a few models that are have their LEDs arrayed on the back of the TV, facing you. These are less common, though are making a comeback in the form of cheaper, but thicker, mostly low-end LED LCDs. There are a handful of high-end TVs that use full-array LED backlighting in a slightly different way, which we"ll discuss later.

Because the light is brightest nearest the LEDs, it"s common for edge-lit LED LCDs to have poor uniformity. This is especially noticeable on dark scenes, where areas of the screen will appear brighter than others. Corners or edges can have what looks like tiny flashlights shining on the screen. Check out

Each manufacturer has a preferred method for edge-lighting, but some models may feature one type, while other models feature another type. Generally speaking, the fewer LEDs the cheaper the TV is to produce. Fewer LEDs also mean better energy efficiency, but LED LCDs are already so efficient that this is a tiny improvement. Unfortunately, specific details about where a TV"s LEDs are located (beyond "direct" or "edge"), the number of LEDs, and other useful information about the backlighting, are rarely listed on a TV"s spec sheet.

This design has all the LEDs along the bottom of the TV. Though manufacturers don"t like to reveal how many LEDs they use, this is likely the type with the least number of LEDs.

Though TVs of this style claim to have "local dimming" you can see how this is a pretty broad definition of "local." Even if each LED is dimmable independently (highly unlikely), you"re still only able to dim columns that stretch from top to bottom. Something like this:

As you can guess, this design has LEDs on the top and bottom edges of the screen. The local dimming here is a little better, where the zones can be slightly smaller areas of the screen, like this:

This is a less common method now, as it requires more LEDs than any of the other edge-lighting methods. The local dimming can get a little more accurate, but is still limited to large-ish zones. If we used our moon example image, the result with an all-sides edge-lit would look just like top and bottom. But with regular video (that has more light sources than just the moon), it will have a more zones to work with, sort of like this:

All Sides used to be the most common edge-lighting method. But as the light guides improved, and costs had to come down (to make cheaper LED LCDs), this method became fairly rare.

Nearly all "backlit" LED LCDs use this method. The LEDs are arrayed on the back of the TV, facing you, but there is no processing to dim them individually. They work instead as a uniform backlight, like most CCFL LCDs. The least expensive LED LCDs use this method, as do most of Sharp"s

This is the ultimate LED LCD, offering performance that rivals the better plasmas. Like the "direct-lit" TVs, these have their LEDs behind the screen (the image above for direct-lit works as a visual aid for this type as well). The full local-dimming aspect means the TV is able to dim zones behind the dark areas of the screen in fairly specific areas to make the image really pop, drastically increasing the apparent contrast ratio.

However, they basically don"t exist. The LG LM9600 wasn"t great last year, and LG has yet to announce any full-array local-dimming TVs for 2013. The only other local-dimming LED LCD was the Sony HX950, which was excellent, and is still current. In his review David Katzmaier called

The two biggest-selling TV makers in the U.S. are Samsung and Vizio, and neither has sold a full-array local-dimming LED TV for the last couple years. At CES 2013, Samsung"s only such TV announced was the insanely-expensive E420i-A1, saying "Sure, black levels get darker, but the trade-off in shadow detail is one I"m not willing to make," and concluded that its "local dimming does nothing to improve picture quality."

As I mentioned at the top, there"s no easy way to tell, just by looking at a spec sheet, what kind of backlight a TV has. By extension, there"s no way to tell how good its local dimming will be. Bad local dimming can, at worst, just be marketing hyperbole. At best, it does little to improve the picture. Good local dimming, however, can make a punchy image, with lots of apparent depth and realism. Or to put it differently, the best LCDs on the market have the best local dimming, allowing them to rival plasmas on the picture quality front. The better TV reviews, like ahem those here on CNET, will talk about all this, so you"re not duped into paying for a "feature" that"s little more than a check mark on a spec sheet.

Got a question for Geoff? First, check out all the other articles he"s written on topics like Send him an e-mail! He won"t tell you what TV to buy, but he might use your letter in a future article. You can also send him a message on Twitter: @TechWriterGeoff.

The contrast ratio (CR) is a property of a display system, defined as the ratio of the luminance of the brightest color (white) to that of the darkest color (black) that the system is capable of producing.

If the LCD contrast is too low, it is hard to read. Different applications have different contrast requirement. For normal reading, the contrast needs to be >2; for medical, the contrast needs to be >10, for welding helmet, contrast should be >1,000.

The higher the efficiency, the better of the LCD contrast . It is especially important for negative display. Change from 98% to 99.9% polarizer, the contrast can increase from 45 to over 1000 for negative LCD, but for positive LCD, the contrast increases from 7 to 10 for positive LCD.

Positive LCD to Negative LCD (When the LCD is used indoor or dark environment, The contrast will increase a lot, but it will not display well with ambient light only, it is also more expensive)

For negative display, black mask can block the light bleeding, the contrast can be improved. Black mask can be done either outside cell (low cost) and inside cell (high cost).

The view direction is the right direction marked with Φ which is with respect to the X-axis. The original location is the center point of the display panel surface, the Z axis is Normal, the X-axis is Horizontal and Y-axis is Vertical.

Normally it was defined 4 angles to correspond with 3, 12, 9, and 6 o’clock respectively. So, you can find the 6 o’clock or 12 o’clock parameter in the LCD datasheet.

Viewing Angle is the angle with respect to the Z-axis in a certain direction and marked by θ (θU means upper View Angle). LCD Viewing Angle describes the maximum watching angle, and it is one of the key indicators with the display module.

The LCD bias angle is the angle perpendicular from which the display is best viewed. (See Fig.2) This angle is determined when the display is designed and can be set at any angle or orientation. The orientation of the bias angle of LCD displays is often stated with reference to a clock face. If the offset is above the display, it is referred to as a 12:00 or Top view.

The LCD viewing angle is the angle formed on either side of the bias angle, where the contrast of the display is still considered acceptable. Generally, this contrast is specified as 2:1 for monochrome LCD and 10:1 for color LCD.

For example, assume the display is a 12:00 (topview) type. When the display is viewed from 25 degrees above the vertical, it will be at its maximum contrast and best look. If the viewer moves their eyes further above the display by an additional 30 degrees, they will see a contrast reduction, but the display will still be readable. Moving the view position any further above the display will reduce the contrast to an unacceptable degree.

Adjusting the contrast voltage, VL, effects the Bias Angle to some extent, but not the Viewing Angle. A top view 12:00 display can be optimized for a bottom view 6:00 viewing position by adjusting the contrast voltage. A 12:00 display set for a 6:00 viewing position will not have as great a contrast as a 6:00 display set for 6:00 viewing position and vice versa.

Generally, displays are optimized for straight-on viewing. Either a 6:00 or 12:00 module may be used, and the contrast voltage can be adjusted slightly to optimize the display for that viewing position. In the above example, the viewing angles of both 6:00 and 12:00 modules actually overlap the perpendicular (or straight on) viewing position.

Generally, a 10K ohm potentiometer is then connected between VDD and VSS in a single supply module, or from VDD to the negative rail in a dual supply module. The wiper of the pot is connected to the VL input of the module. (See Fig.3)

The LCD is positioned at the nominal viewing position and the pot is adjusted to obtain the desired LCD appearance. The voltage on the VL pin is now measured and a pair of resistors are chosen to produce this voltage in the production units.

By adjusting driving voltage and contrast is the most cost-effective way to improve the viewing angle. Different viewing angles need different driving voltage. It is compromising. In discussing the best viewing angle, we have to fix the voltage angle first.

– The higher the efficiency, the better of the contrast. It is especially important for negative display. Changing from 98% to 99.9% polarizer will do the work.

– Positive LCD to Negative LCD (When the LCD is used indoor or dark environment, the contrast will increase a lot, but it will not display well with ambient light only, it is also more expensive)

When a LCD is high density with the segments/icons or very crowded, some customers also complains the viewing angle or contrast are not good. The reason is for crowded display, the layout can be long and thin. The voltage drop along the layout can be big. The solutions are:

Want to find out more about LCD, OLED & TFT solutions? – Check out our knowledge base, where ypu can find tips on electronics operating temperature and differences between LCD and TFT!

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The wide range of conditions over which LCD monitors are used means that it is desirable to produce displays whose luminance (brightness) can be altered to match both bright and dim environments. This allows a user to set the screen to a comfortable level of brightness depending on their working conditions and ambient lighting. Manufacturers will normally quote a maximum brightness figure in their display specification, but it is also important to consider the lower range of adjustments possible from the screen as you would probably never want to use it at its highest setting. Indeed with specs often ranging up to 500 cd/m2, you will certainly need to use the screen at something a little less harsh on the eyes. As a reminder, we test the full range of backlight adjustments and the corresponding brightness values during each of our reviews. During our calibration process as well we try to adjust the screen to a setting of 120 cd/m2 which is considered the recommended luminance for an LCD monitor in normal lighting conditions. This process helps to give you an idea of what adjustments you need to make to the screen in order to return a luminance which you might actually want to use day to day.

Changing the display luminance is achieved by reducing the total light output for both CCFL- and LED-based backlights. By far the most prevalent technique for dimming the backlight is called Pulse Width Modulation (PWM), which has been in use for many years in desktop and laptop displays. However, this technique is not without some issues and the introduction of displays with high brightness levels and the popularisation of LED backlights has made the side-effects of PWM more visible than before, and in some cases may be a source of visible flicker, eyestrain, eye fatigue, headaches and other associated issues for people sensitive to it. This article is not intended to alarm, but is intended to show how PWM works and why it is used, as well as how to test a display to see its effects more clearly. We will also take a look at the methods some manufacturers are now adopting to address these concerns and provide flicker-free backlights instead. As awareness grows, more and more manufacturers are focusing on eye health with their monitor ranges.

Pulse Width Modulation (PWM) is one method of reducing the perceived luminance in displays, which it achieves by cycling the backlight on and off very rapidly, at a frequency you can’t necessary detect with the naked eye, but which could lead to eye issues, headaches etc. This method generally means that at 100% brightness a constant voltage is applied to the backlight and it is continuously lit. As you lower the brightness control the perceived luminance for the user reduces due to a number of possible controlling factors:

1) Frequency –The backlight is cycled on and off very rapidly, and this cycling typically occurs at a fixed frequency (in Hz). How fast this cycling occurs can impact whether flicker is visible or perceivable to the user, with higher frequencies being potentially less problematic. PWM has been known to operate at low frequencies of 180 – 240Hz for example which are likely to be more problematic than higher frequencies ranging up in to the Kilohertz range (e.g. 18,000Hz).

2) Modulation –The modulation of the cycling has an impact on the perceived brightness, and this describes the difference between the luminance in an “on” and in an “off” state. In some examples the backlight is completely turned off during the cycle so it is literally being turned on/off rapidly across the full brightness adjustment range. In those examples the luminance output is controlled really by the duty cycle only (see point 3). In other examples the backlight is not always being completely turned off but rather the voltage applied to the backlight is being rapidly alternated, resulting in less extreme differences between the on and off states. Often this modulation will be narrow in the high brightness range of the display, but as you reduce further, the modulation becomes wider until it reaches a point where the backlight is being switched completely off. From there, the change in the duty cycle (point 3) controls the further changes in the luminance output.

3) Duty Cycle – The fraction of each cycle for which the backlight is in an “on” state is called the duty cycle. By altering this duty cycle the total light output of the backlight can be changed. As you reduce the brightness to reach a lower luminance, the duty cycle becomes progressively shorter, and the time for which the backlight is on becomes shorter, while the time for which it is off is longer. This technique works visually since cycling the backlight on and off sufficiently fast means the user cannot see this flickering, because it lies above their flicker-fusion threshold (more on this later).

Above we can see graphs of a backlight’s output using “ideal” PWM for several cycles. The maximum output of this backlight in the example is 100 cd/m2, and the perceived luminance for the 90%, 50% and 10% cases are: 90, 50 and 10 cd/m2 respectively. The modulation percentage is the ratio between the minimum and maximum luminance during the cycle, and is 100% here, so it is being completely turned on and off. Note that during the duty cycle the backlight is at its maximum luminance.

The analogue (non-PWM) graphs corresponding to these perceived luminance levels would appear as shown below. In this case there is no modulation. This is the method used for flicker-free backlights which we will discuss more a little later.

The main reasons for the use of PWM is that it is simple to implement, requiring only that the backlight can be switched on and off rapidly, and also gives a large range of possible luminance.

CCFL backlights can be dimmed by reducing the current through the bulb, but only by about a factor of 2 because of their strict current and voltage requirements. This leaves PWM as the only simple method of achieving a large range of luminance. A CCFL bulb is in fact normally driven by the inverter to cycle on and off at a rate in the 10’s of kilohertz and well outside the range of flicker visible to humans. However, the PWM cycling typically occurs at a much lower frequency, around 175Hz, which can produce artefacts visible to humans.

The luminance of LED backlights can be adjusted greatly by altering the current passing through them, though this has the effect of altering the colour temperature slightly. This analogue approach to LED luminance is also undesirable since the accompanying circuits must take into account the heat generated by the LED’s. LED’s heat up when on, which reduces their resistance and further increases the current flowing through them. This can quickly lead to runaway current use in very high-brightness LED’s and cause them to burn out. Using PWM the current can be forced to hold a constant value during the duty cycle, meaning the colour temperature is always the same and current overloads are not a problem.

While PWM is attractive to hardware makers for the reasons outlined above, it can also introduce distracting visual effects if not used carefully. Flicker from LED backlights is typically much more visible than for older 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. 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 W-LED backlighting units now.

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 the earlier analysis of the CCFL backlighting we know that false colour may be introduced as well, 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.

It is important to remember that this is entirely due to the backlight, and the display itself is showing a static image. Often it is said that humans cannot see more than 24 frames per second (fps), which is not true and actually corresponds to the approximate frame rate needed to perceive continuous motion. In fact, while the eyes are moving (such as when reading) it is possible to see the effects of flicker at several hundred hertz. The ability to observe flicker varies greatly between individuals, and even depends on where a user is looking since peripheral vision is most sensitive.

So how fast is PWM cycling backlights on and off? This seems to depend on the backlight type used, with CCFL-based backlights nearly all cycling at 175Hz or 175 times per second. LED backlights have been reported typically running from 180 – 420Hz, with those at the lower end flickering much more visibly. Some have even faster frequencies of >2000Hz so it really can vary. While this might seem too fast to be visible, keep in mind that 175Hz is not much faster than the 100-120Hz flicker observed in lights connected directly to the mains power.

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 that 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.

It is also important to distinguish the difference between flicker in CRT displays and CCFL and LED backlit TFT displays. While a CRT may flicker as low as 60Hz, only a small strip is illuminated at any time as the electron gun scans from top to bottom. With CCFL and LED backlit TFT displays the entire screen surface illuminates at once, meaning much more light is emitted over a short time. This can be more distracting than in CRTs in some cases, especially if short duty cycles are used.

The flicker itself in display backlights may be subtle and not easily perceptible for some people, but the natural variation in human vision seems to make it clearly visible to others. With the use of high-brightness LED’s on the rise it is becoming increasingly necessary to use short PWM duty cycles to control brightness, making flicker more of a problem. With users spending many hours every day looking at their monitors, shouldn’t we consider the long term effects of both perceptible and imperceptible flicker?

If you find PWM backlight flickering distracting or just want to see if reducing it makes reading on a monitor easier, I’d encourage you to try the following: Turn the brightness of your monitor up to maximum and disable any automatic brightness adjustments. Now use the colour correction available in your video card drivers or calibration device to reduce the brightness to normal levels (usually by adjusting the contrast slider). This will reduce the luminance and contrast of your monitor while leaving the backlight on as much as possible during PWM cycles. While not a long-term solution for most due to the decreased contrast, this technique can help to discover if a reduction in PWM usage is helpful.

A much better method of course would be to purchase a display not relying on PWM for dimming, or at least one which uses a much higher cycling frequency. Few manufacturers seem to have implemented PWM at frequencies that would limit visible artefacts (well above 500Hz for CCFL and above 2000 Hz for LED). Additionally, some displays using PWM do not use a 100% duty cycle even at full brightness, meaning they will always produce flicker. Several LED-based displays may in fact be currently available which do not use PWM, but until backlight frequency and modulation become listed in specifications it will be necessary to see the display in person. Some manufacturers promote “flicker free” monitors in their range (BenQ, Acer for example) which are designed to not use PWM at all and instead use a Direct Current (DC) method of backlight dimming. Other manufacturers such as Eizo talk about flicker free backlights but also list a hybrid solution for their backlight dimming, where PWM is used for some of the brightness adjustment range at the lower end. In fact it seems an increasingly common practice for a screen to be PWM free down to a certain point, and then fro PWM to be used to really drive down the minimum luminance from there.

An easy method of measuring the PWM frequency of a backlight would be ideal, and luckily it can be done using only a camera which allows manual control of the shutter speed. This can quickly and easily identify PWM frequencies in the lower range, but may not be suitable for high frequency PWM. It should be able to detect PWM up to at least 500Hz though, but anything above that may look like a solid block, suggesting no use of PWM, when in fact it might be just using a higher frequency. Further more complex methods such as our oscilloscope setup would be needed to validate flicker-free status for definite.

(Optional) Set the camera white balance by getting a reading off the screen while displaying only white. If not possible, then manually set the white balance to about 6000K.

Display a single vertical thin white line on a black background on the monitor (1-3 pixels wide should be fine). The image should be the only thing visible. Here is an example you may wish to save and use, show it full screen on your monitor.

Set the camera to use a shutter speed of 1/2 to 1/25 of a second. You may need to set the ISO sensitivity and aperture in order to capture enough light. Make sure the line is in focus at the distance you are holding it (lock the focus if needed).

Hold the camera about 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). You may need to experiment with moving the camera at different speeds.

Multiply this count by the inverse of the shutter speed. For example, if using a shutter speed of 1/25 of a second and 7 cycles are counted, then the number of cycles per second is 25 * 7 = 175Hz. This is the backlight cycle frequency.

What we are doing with this technique is turning a temporal effect into a spatial one by moving the camera during capture. The only significant source of light during the image capture is the thin line on the display, which is exposed onto consecutive columns on the sensor. If the backlight is flickering, different columns will have different brightness or colour values determined by the backlight at the time it was exposed.

A common problem when first attempting this technique is that the image is too dark. This can be mitigated by using a larger camera aperture (lower f/number) or increasing the ISO value. The shutter speed is not a factor in the exposure since we are using it only to control the total exposure time. The brightness of the image can also be adjusted by changing the speed at which the camera is moved, with a fast speed giving a darker image and more temporal resolution and a slow speed a brighter image with lower resolution. Another problem encountered is unevenly-spaced cycles in the final image, which is caused by the camera changing speed during exposure. Continuing to move the camera before and after the exposure helps to steady this. An image which looks particularly smooth may be due to it being out of focus. This can sometimes be helped by pressing the shutter button halfway to focus on the line target, then proceeding as normal.

Depending on the monitor several additional effects may be visible. CCFL-based backlights often show different colours at the start and end of each cycle, which means the phosphors used respond at different rates. LED-based backlights often use a higher cycling frequency than CCFL-based, and more rapid camera movement may be needed to easily see them. Dark stripes between cycles mean that the PWM duty cycle has been reduced to such an extent that no light is emitted for part of the cycles.

Using our oscilloscope and photosensor equipment it is possible to measure the PWM frequency and patterns far more accurately. While the above photo method is certainly suitable for a casual user, an oscilloscope can reveal more detail about the PWM operation and will be featured in all our reviews moving forward. We measure the luminance output of the screen at brightness settings of 100, 50 and 0%. This allows us to easily identify the backlight dimming technique, and if PWM is being used we can work out its frequency and comment on modulation, duty cycle etc.

Asus PA248Q – W-LED backlight. At 100% brightness we see a constant luminance output and a straight line, as there is no need for the backlight to be cycled. At 50% you can see PWM controls the backlight on and off. The modulation is always 100%, but the luminance reduction is controlled by the duty cycle which becomes progressively shorter. You can see much shorter “on” peaks in the 0% brightness graphs. We measure the frequency at 180Hz which is fairly typical.

BenQ GW2760HS – W-LED backlight. At all brightness settings the luminance output is a flat line, showing no PWM is being used. This is part of BenQ’s flicker free range.

The oscilloscope graphs can also allow us to examine the behaviour of the luminance output. Above is a typical W-LED backlight dimmed to 0% where PWM is used. You can see the changes between on and off are very steep and sudden, as the LED backlight is able to turn on and off very rapidly. As we’ve already discussed this can lead to potentially more noticeable flicker and associated issues as the changes are more pronounced.

The oscillographs for a typical CCFL display using PWM at 0% looks like the above. You can see the transitions from on to off are less sudden as the phosphors don’t go dark as quickly as with LED backlight units. As a result, the use of PWM may be less problematic to users.

As we said at the beginning, this article is not designed to scare people away from modern LCD displays, rather to help inform people of this potential issue. With the growing popularity in W-LED backlit monitors it does seem to be causing more user complaints than older displays, and this is related to the PWM technique used and ultimately the type of backlight selected. Of course the problems which can potentially be caused by the use of PWM are not seen by everyone, and in fact I expect there are far more people who would never notice any of the symptoms than there are people who do. For those who do suffer from side effects including headaches and eye strain there is an explanation at least.

With the long term and proven success of a technology like Pulse Width Modulation, and the many years of use in CCFL displays we can’t see it being widely changed at any time soon to be honest, even with the popular move to W-LED backlit units. It is still a reliable method for controlling the backlight intensity and therefore offering a range of brightness adjustments which every user would want and need. Those who are concerned about its side effects or who have had problems with previous displays should try and consider the frequency of the PWM in their new display, or perhaps even try and find a screen where it is not used at all in backlight dimming. Some manufacturers are proactively addressing this concern through the use of flicker free backlights, and so options are emerging which do not use PWM.

This article is about backlights in liquid crystal displays. For the rear window of an automobile, see Car glass. For the lighting design practice, see Backlighting (lighting design). For other uses, see Backlight (disambiguation).

A backlight is a form of illumination used in liquid crystal displays (LCDs). As LCDs do not produce light by themselves—unlike, for example, cathode ray tube (CRT), plasma (PDP) or OLED displays—they need illumination (ambient light or a special light source) to produce a visible image. Backlights illuminate the LCD from the side or back of the display panel, unlike frontlights, which are placed in front of the LCD. Backlights are used in small displays to increase readability in low light conditions such as in wristwatches,smart phones, computer displays and LCD televisions to produce light in a manner similar to a CRT display. A review of some early backlighting schemes for LCDs is given in a report Engineering and Technology History by Peter J. Wild.

Simple types of LCDs such as in pocket calculators are built without an internal light source, requiring external light sources to convey the display image to the user. Most LCD screens, however, are built with an internal light source. Such screens consist of several layers. The backlight is usually the first layer from the back. Light valves then vary the amount of light reaching the eye, by blocking its passage in some way. Most use a fixed polarizing filter and a switching one, to block the undesired light.

An ELP gives off uniform light over its entire surface, but other backlights frequently employ a diffuser to provide even lighting from an uneven source.

Backlights come in many colors. Monochrome LCDs typically have yellow, green, blue, or white backlights, while color displays use white backlights that cover most of the color spectrum.

Colored LED backlighting is most commonly used in small, inexpensive LCD panels. White LED backlighting is becoming dominant. ELP backlighting is often used for larger displays or when even backlighting is important; it can also be either colored or white. An ELP must be driven by relatively highAC power, which is provided by an inverter circuit. CCFL backlights are used on larger displays such as computer monitors, and are typically white in color; these also require the use of an inverter and diffuser. Incandescent backlighting was used by early LCD panels to achieve high brightness, but the limited life and excess heat produced by incandescent bulbs were severe limitations. The heat generated by incandescent bulbs typically requires the bulbs to be mounted away from the display to prevent damage.

For several years (until about 2010), the preferred backlight for matrix-addressed large LCD panels such as in monitors and TVs was based on a cold-cathode fluorescent lamp (CCFL) by using two CCFLs at opposite edges of the LCD or by an array of CCFLs behind the LCD (see picture of an array with 18 CCFLs for a 40-inch LCD TV). Due to the disadvantages in comparison with LED illumination (higher voltage and power needed, thicker panel design, no high-speed switching, faster aging), LED backlighting is becoming more popular.

LED backlighting in color screens comes in two varieties: white LED backlights and RGB LED backlights.blue LED with broad spectrum yellow phosphor to result in the emission of white light. However, because the spectral curve peaks at yellow, it is a poor match to the transmission peaks of the red and green color filters of the LCD. This causes the red and green primaries to shift toward yellow, reducing the color gamut of the display.a red, a blue, and a green LED and can be controlled to produce different color temperatures of white. RGB LEDs for backlighting are found in high end color proofing displays such as the HP DreamColor LP2480zx monitor or selected HP EliteBook notebooks, as well as more recent consumer-grade displays such as Dell"s Studio series laptops which have an optional RGB LED display.

RGB LEDs can deliver an enormous color gamut to screens.additive color) the backlight can produce a color spectrum that closely matches the color filters in the LCD pixels themselves. In this way, the filter passband can be narrowed so that each color component lets only a very narrow band of spectrum through the LCD. This improves the efficiency of the display since less light is blocked when white is displayed. Also, the actual red, green, and blue points can be moved farther out so that the display is capable of reproducing more vivid colors.

A newNanosys, claims that the color output of the dots can be tuned precisely by controlling the size of the nanocrystals. Other companies pursuing this method are Nanoco Group PLC (UK), QD Vision, 3M a licensee of Nanosys and Avantama of Switzerland.Sony has adapted Quantum Dot technology from the US company QD Visionedge-lit LED backlight marketed under the term Triluminos in 2013. With a blue LED and optimized nanocrystals for green and red colors in front of it, the resulting combined white light allows for an equivalent or better color gamut than that emitted by a more expensive set of three RGB LEDs. At the Consumer Electronics Show 2015, Samsung Electronics, LG Electronics, the Chinese TCL Corporation and Sony showed QD-enhanced LED-backlighting of LCD TVs.

CCFL backlighting has also improved in this respect. Many LCD models, from cheap TN-displays to color proofing S-IPS or S-PVA panels, have wide gamut CCFLs representing more than 95% of the NTSC color specification.

There are several challenges with LED backlights. Uniformity is hard to achieve, especially as the LEDs age, with each LED aging at a different rate. Also, the use of three separate light sources for red, green, and blue means that the white point of the display can move as the LEDs age at different rates; white LEDs are also affected by this phenomenon, with changes of several hundred kelvins being recorded. White LEDs also suffer from blue shifts at higher temperatures varying from 3141K to 3222K for 10 °C to 80 °C respectively.Benq G2420HDB consumer display has a 49W consumption compared to the 24W of the LED version of the same display (G2420HDBL).

To overcome the aforementioned challenges with RGB and white LED backlights an "advanced remote phosphor" cockpit displays,Air Traffic Control displays and medical displays. This technology uses blue pump LEDs in combination with a sheet on which phosphorous luminescent materials are printed for colour conversion. The principle is similar to Quantum Dots, but the phosphors applied are much more robust than the quantum dot nano-particles for applications that require long lifetime in more demanding operational conditions. Because the phosphor sheet is placed at a distance (remote) of the LED it experiences much less temperature stress than phosphors in white LEDs. As a result, the white point is less dependent on individual LEDs, and degrading of individual LEDs over lifetime, leading to a more homogenous backlight with improved colour consistency and lower lumen depreciation.

The use of LED backlights in notebook computers has been growing. Sony has used LED backlights in some of its higher-end slim VAIO notebooks since 2005, and Fujitsu introduced notebooks with LED backlights in 2006. In 2007, Asus, Dell, and Apple introduced LED backlights into some of their notebook models. As of 2008Lenovo has also announced LED-backlit notebooks. In October 2008, Apple announced that it would be using LED backlights for all of its notebooks and new 24-inch Apple Cinema Display, and one year later it introduced a new LED iMac, meaning all of Apple"s new computer screens are now LED. Almost every laptop with a 16:9 display introduced since September 2009 uses LED-backlit panels. This is also the case for most LCD television sets, which are marketed in some countries under the misleading name LED TV, although the image is still generated by an LCD panel.

Most LED backlights for LCDs are edge-lit, i.e. several LEDs are placed at the edges of a lightguide (Light guide plate, LGP), which distributes the light behind the LC panel. Advantages of this technique are the very thin flat-panel construction and low cost. A more expensive version is called full-array or direct LED and consists of many LEDs placed behind the LC panel (an array of LEDs), such that large panels can be evenly illuminated. This arrangement allows for local dimming to obtain darker black pixels depending on the image displayed.

Using PWM (pulse-width modulation, a technology where the intensity of the LEDs are kept constant, but the brightness adjustment is achieved by varying a time interval of flashing these constant light intensity light sources

If the frequency of the pulse-width modulation is too low or the user is very sensitive to flicker, this may cause discomfort and eye-strain, similar to the flicker of CRT displays.

For a non-ELP backlight to produce even lighting, which is critical for displays, the light is first passed through a lightguide (Light guide plate, LGP) - a specially designed layer of plastic that diffuses the light through a series of unevenly spaced bumps. The density of bumps increases further away from the light source according to a diffusion equation. The diffused light then travels to either side of the diffuser; the front faces the actual LCD panel, the back has a reflector to guide otherwise wasted light back toward the LCD panel. The reflector is sometimes made of aluminum foil or a simple white-pigmented surface.

The LCD backlight systems are made highly efficient by applying optical films such as prismatic structure to gain the light into the desired viewer directions and reflective polarizing films that recycle the polarized light that was formerly absorbed by the first polarizer of the LCD (invented by Philips researchers Adrianus de Vaan and Paulus Schaareman),

The evolution of energy standards and the increasing public expectations regarding power consumption have made it necessary for backlight systems to manage their power. As for other consumer electronics products (e.g., fridges or light bulbs), energy consumption categories are enforced for television sets.

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