Welcome to Part 2 of our look at choosing the right LED screen for your application. Last month, we discussed basic information about LED technology. Just to summarize, here's a brief recap:

  • LED = a semi-conductor that emits light
  • Pixel = the basic luminous dot, which can contain one or more LEDs
  • Pitch = the distance between pixels, expressed in millimeters
  • Pure white = the perfect balance of red, green, and blue
  • Minimum Viewing Distance (MVD) = the closest you can get before image appears pixelated. The type of screen, as well as the type of content displayed, plays a role here.
  • LED screen = smaller modules (from 4×4cm to over 1×1m) assembled together
  • Colors available = up to 280 trillion (65,000 × 65,000 × 65,000).
  • Gamma correction = the exponential correction needed to adapt colors to the logarithmic vision of the human eyes

Here, we'll take a deeper look into further features that define the quality of an LED panel. Part 1 mentioned the importance of having a pure white (the perfect balance of red, green, and blue) in order to have the highest image quality. Now I'll discuss the importance of having pure black.

How do you obtain a pure black on an LED panel?

By turning off all the LEDs.

Pretty easy, huh?

Actually, no. The biggest problem in obtaining pure black with an LED giant screen is the fact that you do not use LEDs to get the black. In fact, if all the LEDs are switched off, you can't rely on your advanced electronic control system.

So how do you do that?

You ask the mechanical engineer — incredible, but true. Obtaining pure black is a question of mechanical structure of the LED panels. Indeed, pure black entirely depends on how dark the surface can get when the screen is turned off.

Trust me, it's not easy. The main complication is that such a wide surface will always have a reflection factor, meaning that, if you are not careful, you might end up with a huge black mirror when you turn off the screen. That's why the frontal design of the LED panels is so important, because it must be dark without being flat and reflective as a mirror.

Why, exactly, is pure black so important?

It defines the contrast level of the screen. The contrast level is the difference between the light emitted by the display and the environmental light reflected by the screen's surface. Minor is the light reflected, higher is the contrast and, therefore, the color depth and the naturalness of the images. To increase the contrast level, professional LED screen manufacturers provide particular anti-reflex flyers that split the light and don't reflect it, increasing the visibility of the screen even in direct sunlight.

The contrast level of a LED giant screen is a fundamental factor to take into consideration, especially during huge events broadcast via television.

What else should I be aware of when broadcasting an event?

First of all, the LED screen refresh rate. The refresh rate indicates the number of times that the image refreshes every second. For example, a PC monitor has a refresh rate of 72Hz and, therefore, refreshes all the pixels of the screen, from top to bottom, 72 times every second. To provide a comparison, most TV monitors have a refresh rate of 50Hz and only few products of the last generation have a 100Hz refresh rate.

Professional LED screens have a minimum refresh rate of 240Hz, which is almost triple of that of most advanced TV monitors. The refresh rate is fundamental, because it determines the image's quality: if it is too low, the LED screen would display a flickering image when recorded by a professional camera. Some manufacturers offer a refresh rate as high as 2000Hz to meet the demands of the broadcast industry, especially important for events with rapid camera movements and HD format.

Secondly, be aware of brightness levels. Depending on the location, you might have different needs for brightness. Outdoor events will obviously require an incredibly high brightness to contrast the sunlight. However, if the event is indoors, you must have a lower brightness in order to avoid solarization. To provide you with a practical example, an average frontal brightness is 2,000 to 3,000 NITs for indoor use and up to 5,000 to 6,000 NITs for outdoor use.

So are LED screens suitable for outdoor use?

Yes. As with all electronic devices, their waterproof level is defined by the IP rating — the Ingress Protection, an international standard used to measure the resistance of any electronic product against humidity and dust. The IP level is described by two figures: the first one refers to the protection level against dust, the second against water. Devices with a higher IP level keep the internal components safe and clean. Most LED giant screens guarantee an IP65 rating.

LED giant screens were developed at first to be used outdoors, where higher brightness and lower resolution is required. With the development of the technology that now allows higher definition, they started to be used more for indoor applications, as well.

What is the maximum side-view?

That depends on the visibility angle. The visibility angle of an LED screen is, by convention, the angle within which the brightness of a display is equal to 50% of the frontal brightness. For example, a maxi-screen with 5,000 NITs frontal brightness (NIT = candela/sq. m) has a visibility angle equal to the angle by which the brightness is reduced to 2,500 NIT. This visibility angle can vary depending on the LED and the technical features of the display. An average visibility angle can be 120°. Some companies achieve a higher visibility angle by amplifying the LED light with special mechanical design and lens (up to 160°).

What is the virtual pixel?

It's a fundamental video management feature. The virtual pixel is a technology of image management that allows visualizing pictures of a definition higher than the physical one. This is possible because the screen can visualize a pixel that does not exist but is perceived by the human eyes. Thanks to this feature, LED giant screens with virtual pixel technology can visualize images with a definition four times higher than the physical definition of the display; therefore, images are more detailed and realistic.

To obtain this virtual pixel, each manufacturer adopts its own method, and some are better than others. In general though, there are two main virtual pixel technologies: the classic “geometric method” and the newer “logarithmic interpolation”. Further explanation of both techniques would require an entire article, but for now, let's just say that they both provide remarkable results, dramatically improving the image definition and quality.

What video sources can I visualize on the display?

Professional LED maxi-screens function as a huge monitor, and they are able to visualize in real time anything visualized on video camera, satellite, television, DVDs, CD-ROM, PC monitor (DVI, S-video, video composite, video component, YUV, RGB, SDI, HDSDI, SXGA, NTSC, PAL).

What about the maintenance costs?

It varies, depending on the manufacturer. Some manufacturers offer a 24-month warranty on their giant screens. This means that your costs for maintenance are close to zero for the first two years. Others don't offer such coverage and prefer to stipulate a specific contract with each customer.

Usually, you have the opportunity to subscribe to an annual maintenance contract that includes any intervention that might be needed. Otherwise, you can use spot assistance, the cost of which varies, depending on the manufacturer.

In general, just to provide you with an idea, the cost of maintenance is about the 15% of the total value of the screen.

As you probably realize, the LED giant screen world is pretty complicated, at least, more than many people would expect. We have tried to provide you with some useful information when purchasing or renting one. Stay tuned for future Tech installments on LEDs and related technology for visual designers.

Fabio Aversa is in corporate sales and marketing at Italian-based LED panel manufacturer Euro Display Srl (www.EuroDisplay.com). He can be reached at fabio.aversa@eurodisplay.com.