Local Dimming – improving LCD displays

Backlighting, or backlighting in LCD displays, affects a number of parameters in the final application. There are two types of backlights on the market: edge-lit and direct-lit. Their performance can be further influenced by the use of dynamic brightness adjustment in individual screen zones, i.e. local dimming technology (and, in some cases, full-array options). From the article you will learn the differences, advantages and disadvantages offered by today’s backlighting technologies.

What is a backlight?

Trying to classify LCD displays, we can distinguish three types depending on how the content presented on the screen is illuminated:

• solutions like reflective – Displays in which content is illuminated by ambient light, and the effect is achieved using, for example, a mirror that reflects it; they are used, among other things. In calculators;
• solutions like transmissive – displays, in which content is illuminated by light emitted from a backlight;
• solutions like transflective – displays that combine both technologies; they use both mirrors and backlights, for example.

From the perspective of the subject matter addressed in the article, we are interested in solutions of the type transmissive i transflective, as backlights are used in them. This is an additional “layer” of the LCD display, whose task is to emit light. It is necessary for the image to appear on the screen. Originally, the light source in backlights was CCFL-type technologies, but they have been quite displaced by more efficient LEDs.

Types of backlights in LCD-TFT displays

LCD-TFT displays use a variety of liquid crystal matrix backlighting technologies to ensure adequate image brightness and contrast. The most popular are edge-lit backlighting (ELED), where LEDs are placed at the edge of the screen, direct-lit backlighting (DLED) with LEDs evenly distributed behind the matrix, and full-array local dimming (FALD), which is a development of DLED with individual brightness control of individual LEDs. Each of these technologies has its own advantages and disadvantages, which determine their use in specific display models.

What is an edge-lit backlight (ELED)?

In edge-lit (ELED) solutions, the LEDs are arranged in a line near the edge of the LCD display. For industrial applications, the backlight is usually placed at the bottom edge, sometimes being reinforced with an additional strip of LEDs, such as at the top edge. A rare solution is to fill all four edges.

Read also: Displays and solutions for industry from Unisystem

What is direct-lit backlighting (DLED)?

In direct-lit (DLED) solutions, the LEDs are evenly distributed on a plane behind the LCD panel. It is worth noting that even one LED placed directly behind the panel instead of at the edge makes the solution considered type. Of course, manufacturers use more than one LED in backlights, as the number of LEDs significantly affects the readability of the information presented.

Edge-lit vs direct-lit

Here it must be honestly said, “it depends”. – first of all, on the type of application for which the LCD is selected. The two technologies can be compared in terms of differences, which are due, for example, to the number of LEDs used in the backlight design. These include:

• Energy consumption – the fewer LEDs, the lower the energy consumption, here edge-lit solutions have the advantage;
• production costs – the more LEDs, the higher the production costs, which again gives edge-lit solutions an advantage;
• Image quality – the more LEDs, the better the uniformity of backlighting, which translates into better image quality, including higher brightness and contrast, at the same time the phenomenon of glare around bright elements presented against a dark background is eliminated; here direct-lit solutions lead the way.

For some applications, the “compactness” of the LCD display, and therefore its dimensions, including thickness, may be a determining factor. Here, therefore, it may be a necessity to use an edge-lit variant, which – due to the design – are thinner.

We have tabulated the key differences:

What is a lightguide?

In the context of backlights, it is also worth mentioning lightguides, which – depending on the arrangement of LEDs – allow to achieve a satisfactory level of backlight uniformity by dispersing the light coming from individual LEDs. This uses a specially designed lightguide (LGP – light guide plate), which is a layer of semi-transparent plastic diffuser (usually made of PMMA) that diffuses light through a series of unevenly spaced tabs. The idea is to achieve the most even distribution of brightness over the entire surface of the screen.

Luminance uniformity, or uniformity/linearity of backlighting

Backlightuniformity/linearity (luminance uniformity) is one of the parameters in the technical specifications of an LCD display. It is measured by determining the brightness of several randomly selected spots on the screen. Based on the difference between the brightest and darkest spots, backlight uniformity, expressed as a percentage, is calculated. The closer to 100%, the greater the uniformity of backlighting, i.e. smaller differences in brightness across the screen.

How does local dimming work?

Local dimming is a zonal (defined as rectangles, columns or rows) screen backlight adjustment technology that involves dynamically controlling the amount of light in a particular part of the display. Darker portions of the image will be less illuminated and lighter portions will be brightened accordingly, which not only improves contrast, but also produces true depth of black.

However, there are situations in which local dimming can distort the image. This happens, for example, when dealing with small objects on a uniform background. A good example is stars in the night sky – they can be less visible on an LCD with local dimming, because the zone will receive information that most of the image is dark and will reduce the amount of backlighting necessary to illuminate the stars.

Local dimming can be used in various ways in both edge-lit and direct-lit solutions. Note that the use of local dimming technology involves, among other things, the need to adjust the control that manages the switching on and off of individual LEDs (PWM – Pulse-Width Modulation – power pulse width modulation for selected LEDs).

What is full-array local dimming (FALD)?

Full-array local dimming is an advanced development of the capabilities of direct-lit backlighting, involving individual control of the brightness of each LED forming this type of backlight. It makes it possible – thanks to the contrast achieved, as well as excellent reproduction of blacks and grays – to achieve even better image quality. However, it is an expensive solution in production and implementation. Its use is recommended in applications that require the highest precision and dynamic range of image presentation.How to settle the difference in the comparison of full-array LED vs direct LED local dimming? Full-array has the ability to control individual LEDs, while direct-lit applies to the entire screen surface, without zoning. If the precision of the resulting image on the screen is paramount in your project, the choice of direct LED vs full-array will fall on the latter. If, on the other hand, you want to optimize your project in terms of cost, but still have the ability to fully illuminate the entire screen surface (e.g., in an intensely lit environment), then you can opt for direct-lit backlighting.

Backlights vs. life span of LCD displays

The lifetime of an LCD display is, in simple terms, the time it takes to present content on an LCD screen without failure. Manufacturers specify a parameter for each LCD model, which is referred to as LED lifetime, or the average period of trouble-free operation of the LEDs. It is expressed as the number of hours after which the LEDs gradually degrade (in which case the luminance of the LEDs is determined to be 50% over the original value). With dynamic backlighting (and it doesn’t have to be a full-array variant), we reduce LED consumption. Learn more about the lifespan of LCD displays.

Full-array local dimming vs OLED

The selection of the optimal type of backlighting does not need to be pondered in the case of OLED displays. Can OLED be an alternative to full-array?The main difference between these technologies is that in OLED displays each pixel emits light, while in LCD displays with full-array local dimming technology you control the backlight zones. This means that OLED solutions can achieve higher contrast, deeper colors and blacks, as well as reduced power consumption. Even so, LCD solutions will still be more attractive in terms of production costs.So what is the verdict of the full-array local dimming vs OLED comparison? If image quality is the only criterion, we recommend OLED in this case. However, if the project’s budget has limitations, then it is worth considering a properly selected edge backlight or the more expensive and energy-intensive full-array option.

Final choice: edge-lit vs direct-lit, or perhaps a full-array variant?

Which backlight technology is the best solution for your project? Direct LED vs Edge LED vs full-array backlighting – which type of LED local dimming is the most popular? When choosing the right backlighting technology, you should be guided by the desired effect, i.e. the key parameters of the final device.Therefore, there is no one-size-fits-all answer for comparing full-array local dimming vs edge lit local dimming, as the backlighting achieved by these two solutions may or may not fit perfectly with the designed end device – depending on how the user of the device will use it.

In summary, the type of backlighting affects, among other things:

• optical parameters, such as brightness, contrast, color gamut, in effect exemplifying the quality of images obtained (and the readability of the content presented),
• energy consumption/energy efficiency,
• LCD display life,
• LCD display price.

Knowing these dependencies, the designer is able to accurately select a solution to best suit the intended use of the application.So, if you are wondering what to choose: edge LED local dimming vs full array dimming – it would be best if you contact us to discuss the target application in which the designed device will be placed.

As an example of the full array dimming vs local edge dimming dilemma, some of the most demanding applications for LCD displays are digital signage systems. They should provide users not only with continuous access to information, but also with the best reception of content, even in extreme conditions, such as in intensely sunny locations. It is in such applications that it will be possible to see the real potential that lies in advanced LCD displays with, for example, implemented full-array local dimming technology – despite unfavorable conditions, it should still be no problem to read the content that is exposed on the screen.

Check out the LCD-TFT displays available in our offer.

If you have doubts whether a particular LCD will work in the device you are designing, contact us – we will be happy to help you choose the right technology 🤓