If you've done design or programming work using light emitting diode (LED) screen technology and still aren't sure exactly how to choose the best one to suit your purposes, then keep reading.

You probably already know that LED technology makes these large screens possible. LED is a semi-conductor that emits visible light when electrons pass through it. From the exterior, it may look like a small lamp, but it functions in a totally different way. In fact, the emission of light depends on electrons flowing between anode and cathode within the LED chip, and the color of the visual emission depends on the materials utilized.

Seeing Dots

Putting many LEDs together forms several pixels. By definition, a pixel is the “luminous dot” present on every LED full-color giant screen. This luminous dot can be formed by one or more LEDs, depending on dimensions and features of the screen. On full-color screens, the pixel is formed by the three basic LED colors, which are red, green, and blue that, together, can form white.

The human eye has three types of receptors for daylight vision, and each one corresponds to a primary color. Any other color is the result of the simultaneous stimulation of these three types of receptors, after a phenomenon called addition synthesis that mixes the chromatic components captured by the eye. The mix of the three primary colors, in different percentages, allows the pixel to generate any possible color.

Each square meter of the LED giant screen can contain from 1,024 pixels (32×32 pixels) up to 9,216 pixels (96×96 pixels) depending on the models. The number of pixels can vary on the same surface because of pixel pitch. The pixel pitch of an LED display defines the distance between the pixels, expressed in millimeters. This is a defining factor of a giant screen's viewing distance, for the following reasons: the closer the pixels are, the closer the minimum distance but higher the screen cost per area; and the further apart the pixels are, the further away the minimum viewing distance is and the lower the screen cost per area. Therefore, the pitch determines the image definition and cost of the screen. Low pixel pitch equals higher definition and cost; high pixel pitch equals lower definition and cost.

You would determine what pixel pitch you need by defining the minimum viewing distance (MVD). The MVD is the closest you can get to the screen before the pixels start appearing as dots. Each LED giant screen manufacturer has its own method on how to calculate the MVD of a screen. There is not an absolute answer, because each one of us has a different eye's perception. A fairly precise and easy way to calculate the MVD would be converting the pixel pitch into distance. For example, a maxi-screen with 10mm pitch has a MVD of 10m, while a 20mm pitch screen has a MVD of 20m.

Why would I ever go for a higher pixel pitch and, therefore, a lower image quality? For two main reasons: one is financial, and the other is biological. The financial reason is pretty obvious: the higher the number of pixels on the LED screen, the higher the cost. It's important to note that, since the visible area of an LED screen has two dimensions, when reducing the pixel pitch, the LED density increases exponentially. In fact, reducing the pitch by the 50% increases the number of pixels by 400%, and cost follows. Let's take, for example, 1sq.m: with a 20mm pixel pitch, you have 2,500 pixels (1,000:20=50; 50×50=2,500 pixels), but with a 10mm pixel pitch, you have 10,000 pixels.

The biological reason is just as important: the human eye is not able to recognize small details from long distances. Therefore, from longer viewing distances, the human eyes won't notice the details that a high-definition LED screen can show. This brings us back to the financial reason. If you watch an LED giant screen from a huge distance, why use a high-definition screen when your eyes won't appreciate the superior performances?

Some Assembly Required

Putting together a 100sq.m screen is simply a result of assembling together smaller modules. Each LED giant screen manufacturer has modules of different sizes and shapes (square or rectangular, from 25×25cm up to 1m×1m). It's impossible to define the best shape/size combination as each one has pros and cons. But, for an easy example, let's consider a 1sq.m module (1m×1m). In this case, if you want to build a 35sq.m giant screen, you will need 35 modules that you will assemble in five lines of seven modules each. This modular system makes LED giant screens suitable for any kind of installation (on truss, walls, roofs, onstage, and on temporary structures for specific events). It enables easy maintenance and does not set any limits in shape, size, and resolution.

So any LED giant screen of the same size and the same pixel pitch has the same image quality, right? Unfortunately not. Many factors can make the difference, including gamma correction. The gamma correction is a video management feature fundamental when visualizing images on every type of screen, from the television to the PC monitor, and to the LED maxi-screen. The gamma correction controls and manages the brightness and color levels of the whole image. If it is not performed correctly, figures will appear either solarized or too dark. But why?

To answer, it is necessary to make another biological consideration: the reaction of the human eye to the luminous stimulations is logarithmic. This means that the eye is able to see both the flash of a thunderbolt and the quivering light of a star. If the reaction to luminous stimulations was linear, as with machines, the human would be totally blind below a certain level of illumination and constantly dazzled by lights that are too bright.

The gamma correction is, therefore, the exponential correction of color levels on which the PC graphic is based, and it is fundamental to adapt the colors to the logarithmic vision of the human eye. Only with a proper gamma correction is it possible to visualize perfectly natural pictures and videos. A perfect correction is often the result of years of research and experience, resulting in the higher quality of the images and in greater depth of the colors.

Obviously, the number of colors available also affects the image quality. On the market, there are several LED giant screen manufacturers, each one with its own technology and products. Displays utilizing an 8-bit technology are able to visualize 256 color levels for each of the three primary colors. This means that they are able to visualize 256×256×256 = 16.8 million colors in total. Displays utilizing a 16-bit technology for each color are able to visualize 65,000 color levels for each of the three primary colors. This means that it is possible to visualize 65,000×65,000×65,000 = 275 billion colors in total.

Now, if the screen works at the highest luminosity, for the human eye there is no big difference between a display with 16 million colors and a 275 billion-color display. Differences begin to be visible when the display works at reduced brightness, for example to slow down the natural decay of the LEDs and to reduce power consumption.

To reduce a screen's brightness, it is necessary to reduce the number of color levels visualized by the display; to provide a proportion, reducing the luminosity by 50% reduces the color levels available by 50% as well.

Having a palette of 65,000 color levels allows an incredibly superior quality and naturalness, even at the lowest brightness levels. Such a high number of colors available guarantees the visualization of all the colors perceptible by the human eye, even when luminosity is reduced to the minimum.

There are many other factors influencing the image quality of LED screens. Here, we just summarized the basics. In future articles, we will describe other fundamental features of an LED screen, including the contrast levels, the refresh rate, the visibility angle, the IP rate, and more.

Fabio Aversa is in corporate sales and marketing at Italian-based LED panel manufacturer Euro Display Srl (www.EuroDisplay.com).