adjusting backlight on lcd panel supplier

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.

Brightness level in LCD panels is controlled by pulse-width modulation (PWM). PWM is a simple turning the switch between supply and load on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the power supplied to the load is. The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power. Typically, switching frequency is set to about 200 Hz in most LCD backlight control units.

The term duty cycle describes the proportion of time when switch is on to the regular period of time. A low duty cycle corresponds to low power (low brightness in our case), because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on (max. backlight brightness).

Below are several examples of different duty cycles. That’s exactly the waveforms that you can see with oscilloscope connected to pin 4 of connector CON4 on our v2 PCB.

PWM waveform is generated by PIC12 microcontroller (U3) on our PCB. The duty cycles of PWM depends on voltage measured on its pin 7 (AN0) that is used as an input to internal analog to digital converter (ADC). We supply our LCD bundle with ambient light sensor connected to this pin. PIC12 measures voltage 10 times every second, and smoothly changes PWM duty cycle. Full cycle of brightness change takes 2 seconds to avoid screen flickers. Here is short video that shows how this works.

Now you can turn knob to control brightness of screen. Please, be aware that it has the same delay of 2 seconds to change brightness to new level. And finally, short video of manual brightness control in action.

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.

Typically, users have the ability to adjust the contrast of an LCD module. This application note describes characteristics and methods withregards to contrast adjustment, as well as design guidelines for proper operation. An example application circuit is shown at the end using a 16x4 character LCD.

The ratio of light to dark areas on an LCD is defined as the contrast ratio. This is one consideration for designers when adjusting the LCD bias voltage. This voltage is represented by: Vee, Ve or Vo. Focus LCDs will reference bias voltage as Vo throughout this document.

Vo is used to power the circuits that drive the LCD. This voltage sets the contrast level of the LCD. Any changes in this voltage will cause a visible change in the contrast, it’s important that it is regulated with less than 100mV ripple.

Contrast is not affected by the LED backlight, EL backlight, or of the type of polarizer such as reflective polarizer, transflective polarizer and transmissive polarizer.

This is the simplest method to use where a voltage divider or potentiometer is connected to Vdd (Figure 1). With varying resistance, one can adjust the bias voltage hence the contrast of the display. Once an appropriate resistance is identified, we will implement that in future displays. Note that for higher voltage displays (>20Vdc), use the dual supply option.

Another bias voltage supply method is the charge pump (Figure 2) DC to DC converter that uses capacitors for energetic charge storage to raise or lower voltage. Charge pump circuits are capable of high efficiencies, sometimes as high as 90–95%. The output can be either positive or negative.

The circuit in Figure 3 does the same job of varying bias but digitally using a microcontroller through pin 2(ADJ). A CTRL (enable) pin is also provided for turning on or off via the microcontroller. This shutdown signal can also be used to properly sequence the power to the display during power-up and power-down sequences.

The potentiometer can be replaced with a fixed resistance if the end user has no need of adjusting the contrast. This has the advantage of eliminating the need of trimming during production. If the display is being used for portable applications such as mobile phones, a battery is used for the supply voltage (input voltage) as shown in this example circuit.

Note that pin 2 (SHDN) serves as an enable pin to turn on or off the bias voltage. This is important for power sequencing which will be discussed shortly.

It is critical that the LCD bias is applied at the appropriate time with reference to another signal (power sequence). The power sequence is the sequence in which the power is applied and shut down, and includes specifications of the time intervals between steps in the sequence.

For LCD modules, the simple rule is Vo (Vee or Vi) must never be present without VDD also being present. Otherwise, the display may be damaged even if it is only for a short period.

Buyers and others who are developing systems that incorporate FocusLCDs products (collectively, “Designers”) understand and agree that Designers remain responsible for using their independent analysis, evaluation and judgment in designing their applications and that Designers have full and exclusive responsibility to assure the safety of Designers" applications and compliance of their applications (and of all FocusLCDs products used in or for Designers’ applications) with all applicable regulations, laws and other applicable requirements.

Designer agrees that prior to using or distributing any applications that include FocusLCDs products, Designer will thoroughly test such applications and the functionality of such FocusLCDs products as used in such applications.

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!

Got a free 27 LED Cinema Display with dark back light (BL) 99% of the time plugging in Display Port. Finally isolated a deterministic sequence that worked for a day. Wife has the same monitor and also not coming on 80% of time when plugging in Display Port. Here is the working sequence.

This sequence got the monitor up and working for a couple of hours but since then seem to have more difficulty and backlight just flickers now on the lowest sitting and shutoff at any higher settings. I guess there are thermal related weak solder points likely in the LED strip in the panel.

Plug in Cinema Macbook Magsafe connection (probably don’t need this but might as well get MB ready to close the clamshell later while Cinema Display remains on)

Use external keyboard (pair it in advance if wireless) F1 key. Click it many times to set external monitor brightness to lowest setting (even though you think its not on)

Click F2 1x (brighter), strobing probably go a little faster, wait 10s and see if it comes on permanently. if not, click F2 1x brighter and repeat until backlight stays on stable.

This is probably the common LED strip solder resistance problem. The backlight circuit compares the different LED channels for voltage. If difference too large, circuit shuts off. Had this in my 27” iMac (2009 so same LCD panel as the 27 LED Cinema Display) It would not go very bright before self shutting off backlight. I pulled apart the LCD panel and reflowed the LED connections and now works perfect. Tearing apart the panel to get to LED strip require handy skills and cautious approach. Very easy to damage and hard to keep dust+threads out of panel+diffuser layers during reassembly. Below is the full link on that.

Fixed 2 of these LCDs (2009 iMac 27”, Cinema LED Display 27”) Only need to apply nice leaded solder to the LED strip connectors. On my Cinema LED Display, one of the connectors just fell off the strip during disassembly. Was barely hanging on. Wasn’t just the 6 pin connection. Also the 2 connector anchor solder points. Must be really bad soldering during manufacturing.

Each connector has 6 lines so each line drives 6 LEDs (36 LED/strip). The connector solder joints is the highest thermal stressed solder joint in the circuit.

On the iMac, I also touched up every LED. I’m guessing this wasn’t necessary and the iMac color temperature is also a lot warmer. I’m guessing the heat from soldering iron affected the LEDs.

No brightness control (after swapping a failed panel with one with early failure symptoms (still lights up but not all the time unless mac brightness is set to low at connection time)

Max brightness is really dim (after swapping a failed panel with one with early failure symptoms (still lights up but not all the time unless mac brightness is set to low at connection time)

The hyper dimming technology applies the new PWM method to expand the dimming range higher than the previous generation of analog methods. The traditional methods for dimming enhancement are by adjusting the lamp current or by regulating the average current feeding the CCFL inverter. This dimming method is referred to as "analog dimming." This provides the highest efficiency circuits, limiting the dimming range or violating lamp specifications to achieve wide dimming ratios.

The "Pulse Width Modulation" (PWM) technology overcomes this problem, expanding the dimming range. We pulse the backlight at the full rated lamp current and modulate intensity by varying the time the lamp operates at the full rated current. PWM dimming can significantly extend the dimming range capabilities. PWM control of multiple lamps eliminates the typical problem of one lamp extinguishing before others at low intensities. Lamp layout and parasitic are no longer the limitations for display dimming capabilities.

With the user-friendly VR knob, it is possible to reach a truly dim-to-black state which allows for night vision operation. Combining the high-quality TFT LCD panel with a smoothly dimmable backlight offers crisp and excellent contrast images that fulfill most of the requirements of marine applications.

Ever had your TV showing nothing but a black screen even if the audio was working? Unfortunately, that’s a common issue with low/middle-end LCD/LED TVs these days… Even more frustrating, this issue often comes from a rather tiny and cheap component that can be easily replaced. Most common issues are:

One of my relatives had this exact symptom happening all of a sudden. This problem on low-end TVs often occurs within the first couple years. As the repair costs for that kind of TV is pretty low, considering repairing it yourself might be a good idea!

The first step into repair is to find the root cause of the issue. As backlight failure is a very common issue, this is the first thing to test. To do so, the easiest way is to power on your screen, put a flashlight very close to it and check if you can see the image through. The image would be very dark, like turning the brightness of the screen very very low.

That implies disassembling the TV to access the backlight which is between the LCD screen in the front and the boards in the rear. In my case, with a Samsung F5000, I had to process as follows:

First we have to remove the back housing to reveal the boards (from left to right: main board, T-CON, power supply) and disconnect the LCD panel from the T-CON board.

Note: Older TVs have neon tubes for backlight, which is thicker and less exposed to this kind of failure. LED backlight is the most common thing these days, but do not mistake an LED TV with an OLED TV. The first one is a classic LCD panel with a LED backlight, whereas the second is an OLED panel that doesn’t need any backlight as it is integrated in each pixels (making the spare parts much more expensive by the way).

As we can see, the backlight system is made of 5 LED strips. First thing to do is look for burnt LEDs. Most LED backlight systems have strips set in series, meaning that if one of the them fails, all the system goes dark…

Using a multimeter, we can confirm that the strips are indeed set in series, so now we have to test each strip individually. Professionals use LED testers such as this one (about 40$ on amazon) but as I didn’t had one at the time, I decided to make one, McGyver style!