Which LED Display?
Which LED Display?
You’ve decided you need an LED display screen but how do you decide which one? There are hundreds, perhaps thousands of manufacturers all with their own variations and literally thousands of models to choose from.
XLdisplays is in regular contact with over 25 LED display manufacturers in Shenzhen alone, and can certainly help find the display you want based on your target specification and budget but it pays to understand a little about the product you are considering before the process starts and I hope the following will prove a useful guide.
Indoor or Outdoor?
At first glance this seems a very simple starting point and for the vast majority of applications the starting point really is a matter of whether your screen will be located indoors or outdoors but be careful when making your choice. As an example if you are considering installing a screen in an area of high humidity such as at an indoor swimming pool you may consider an outdoor screen which is better protected against moisture laden atmosphere.
Unfortunately too often indoor screens have been installed in covered “semi-outdoor” locations with the assumption they are fine since the shelter prevents rain and direct sunlight, but even in sheltered outdoor environments humidity and outdoor roadside pollution can quickly damage an indoor rated screen.
In general indoor screens have LED pixels on smaller pitch allowing shorter viewing distances than the larger pitch outdoor screens, while outdoor screens have improved weather proofing and are capable of higher brightness. There is now however a trend towards higher definition outdoor screens employing new weather proofing techniques and in the events industry a weather proof high definition screen allows the same stock to be used both outdoors and indoors.
When choosing an LED display it pays to take note of its IP rating, but what is the IP rating?
What is an IP Rating?
The Ingress Protection (IP) rating of a screen is an indication of how well the screen will perform against ingress of solid particles and liquid. In broad terms it is regarded as an indication of how well as screen will survive outdoors. The IP rating consists of two numbers, the first indicating protection against solid particles and the second liquids.
In general an outdoor screen should be IP65 on the front face and at least IP54 on the rear. Screens with enclosures having lower rated rears such as IP43 are best installed with additional enclosures or protection. Generally a display has lower rating at the rear since most LED displays have fan cooling however there is now a trend towards fully IP65 front and back but note that this comes with different issues on cooling which will be discussed in a later point.
For a more detailed explanation I recommend the following link.
http://en.wikipedia.org/wiki/IP_Code
What Type of Display Screen?
In broad terms display screens fall into two categories, fixed installation and events/rental.
If you are in the events business then you are most likely looking for a display screen which can be quickly installed before and event and taken down after an event. This will be a modular system delivered in flight cases where the modules or modular panels have fast-rig locking systems allowing columns of modules to be hung from hangers, or mounted on stacking systems, and individual panels to be locked horizontally to form the finished display screen. Connections between modules are made with cables or harnesses at the rear of the screen. Such display screens when used outdoors will generally have a minimum IP54 rating at the rear for protection when the weather turns to rain.
An exception in the events industry may be mobile truck mount screens where the screen is actually mounted onto a trailer or truck for transport from event to event. These screens may be fixed installation screens but would still have higher IP rating at the rear.
Fixed installation screens are used where the screen is installed onto a structure once and is expected not to be removed for the life of the screen. Typical examples would be billboard advertising, architectural screens, and stadium screens.
Apart from the basic fixed installation or events categories, a range of screen applications include the original video screens, scoreboards, perimeter advertising, rooftop LED-strip type screens, mesh or curtain screens often used as stage backdrops, architectural screens in forms such as pixel, mesh or strip, curved and even flexible screens.
What Type of LED?
There are two types of LEDs used in LED displays, DIP and SMD, with new hybrid versions combining the benefits of both in experimentation.
The most established LED, and that most commonly used for outdoor screens, is the discrete LED commonly referred to as DIP. A discrete package LED will either be red, green or blue and consists of a bullet shaped colored epoxy enclosure in which the LED die or chip is protected. The LED die or chip is mounted on a metal lead frame which protrudes from the base of the epoxy bullet as two legs. The lead frame conducts electricity to the LED die and also helps conduct heat away from the LED die. In better LEDs this lead frame will be brass, referred to as copper in China, but lower cost LEDs will employ a steel lead frame with less efficient thermal conduction. Discrete LED’s are brighter and better protected against outdoor environments however it is important that LEDs are from reputable sources since poor quality epoxy will not protect the die from moisture or UV and may discolor or crack in time.
Discrete LED’s are grouped into pixels usually consisting of two red, one green and one blue LED (2R1G1B), sometimes 1R2G1B and also 1R1G1B depending on the application.
The second type is the SMD LED which is a single ‘surface mount’ package in which each of the red, green and blue LED die are mounted. SMD LEDs tend not to be as bright as DIP LEDs but offer better color mixing and allow for smaller pixel pitch. SMD LEDs are most commonly used in indoor screens but increasingly the larger 5050 package SMD LEDs are being employed in outdoor screens. Recently smaller 4040 package SMD LEDs have been introduced into outdoor applications and even smaller high bright versions of indoor 3mm package SMD LED’s are being offered protected by a waterproof coating. This coating will also be discussed later.
Care needs to be taken when choosing to use SMD LEDs outdoors because those with silicon based potting over the LED die may suffer from moisture ingress and shortened life as a result of failure of the bond between the silicon potting and substrate material. The failure results from differences in the coefficient of thermal expansion between silicon and the substrate material. Better LEDs use epoxy and not silicon.
What Brand of LED?
The most common brands of LEDs used in LED display screens come from Nichia and Cree. Nichia is long regarded as the best brand for LED displays but is also by far the most expensive. Cree is generally cheaper but not always and increasingly manufacturers are turning to alternatives such as LEDs based on die from Silan which can be 50% or more below Nichia price.
When considering a lower cost screen using LED’s based on Silan die it is important to understand who is packaging the LED. Generally a display screen manufacturer will order LEDs from a third party packaging company which in turn orders the LED die from Silan. The quality of the image on your display screen will therefore depend largely on the quality of that packaging company and this may be something completely out of your control.
Better screen manufacturers will be packaging their own LEDs and therefore have complete control over both the packaging process and the LED die supplied by Silan.
What Pixel Pitch?
When selecting a pixel pitch two factors should be considered initially. The target screen height and the expected minimum viewing distance.
Ideally for good video a screen should have a minimum of 192 pixels high although reasonable video can be achieved at 144 pixels. For banner or perimeter advertising with logos and graphics perhaps 40-48 pixels is acceptable.
Assuming you are looking for good quality video and have a target screen height of 3 meters then 192 pixels equates to a pixel pitch of around 16mm.
1mm of pixel pitch equates to 1m of minimum viewing distance, therefore a 16mm pixel pitch equates to a minimum viewing distance of 16 meters. That is the distance at which the individual pixels/LEDs blend into an image. If the expected minimum viewing distance is less that that calculated from the pitch a smaller pitch should be considered.
What Display Size?
If the minimum viewing distance is known but there is no target limit on display size then the minimum viewing distance to pitch calculation can be used to calculate the minimum screen height using the 192 pixel rule however this is a minimum screen height and the overall impact of the display should also be considered by looking at the maximum viewing distance.
In general the screen height should be 3% to 5% of the maximum viewing distance. So, assuming the maximum viewing distance to be 100m and calculating 3% and 5% of maximum viewing distance the screen height should be a minimum of 3m and better 5m. Using the previous example of a 16mm pitch screen 3m high, the height of the screen would be acceptable at 100m maximum viewing distance.
What about the screen width? Typically screens use “landscape” format but “portrait” format is also popular for advertising or architectural screens. Assuming “landscape” typically screens will follow either the traditional 4:3 configuration with minimum pixel matrix of 256×192 pixels width x height, or increasingly 16:9 wide screen format. However, it should be pointed out that based on the same screen height a 16:9 format has 20% more area than 4:3 and would therefore be 20% more expensive.
What Viewing Angle?
LED display screens are calibrated perpendicular to the screen and at this location the screen should be at its brightest and most uniform. Viewed from wider angles the brightness will become lower and the uniformity may worsen depending on the quality of the LEDs and calibration system.
Very few members of any audience actually view a screen perpendicular to its face so wide angle performance is an important factor often overlooked and good image quality out to a horizontal viewing angle of 140deg arc is expected. Vertically for outdoor screens good quality image should be expected over a 70deg arc with perhaps 15deg above horizontal and 55deg below horizontal. On indoor screens with SMD LED’s 140deg both horizontal and vertical is expected.
Manufacturers will typically quote horizontal and vertical viewing angles which refer to the angle at which the display screen brightness drops to 50% of its perpendicular maximum value. Some will also offer a maximum viewing angle which in the case of horizontal angle may be a subjective figure at which it is thought an observer can still see an image, or in the case of vertical may actually be the angle at which the shader over the LEDs block the image completely. In the case of vertical viewing angles it is also important to understand the angle above and below horizontal which are figures most manufacturers do not state.
Unfortunately neither a large 50% brightness angle nor or large maximum angle are good indicators of uniformity image quality which is a factor influenced by both the original LED quality and calibration system used and to be sure the buyer is receiving a good quality screen it is essential that the screen is inspected before shipment. Better still that inspection happens at key stages in manufacture. This service is available from XLdisplays.
What Cabinet Material?
LED display screen cabinets are generally available in either steel or aluminum, while some smaller plastic based assemblies are available. Aluminum cabinets are used where displays are intended for the events industry to reduce weight, while aluminum cabinets may also be preferred for fixed installation display screens where life of the screen structure it expected to be long.
What is Ageing?
Manufacturers will talk about ageing or burn in. This is the process of taking the LED display screen and running it to kill off any weak or defective LEDs. Generally an LED display screen will have between 75hours and 100hours of ageing including at LED tile level, LED module level and finished LED screen.
Reviewing the ageing data should show a reduced level of defective or failed LEDs as the ageing proceeds. Warning signs include steady failure rates or increasing failure rates over the 100 hours, or sudden failures towards the end of the ageing period. A discussion over such unusual failures may end with extended burn in being agreed.
Ageing is not only intended to test LEDs but the control and power electronics also.
What Type Of Weather Proofing?
Most outdoor screens are rated IP65 on the front and either IP43 or IP54 at the rear.
The front of the screen will usually employ black silicon potting around the LEDs to prevent moisture entering the LED tiles, while the rear of the LED tile is sealed to the metal cabinet with a silicon rubber gasket.
Modern high definition outdoor screens are appearing with a thin, clear weather proof coating over the LEDs, PCBs and entire face to protect from moisture ingress but although this technology has been around for many years it is only now being introduced to LED display screens and there caution is needed to ensure a reputable coating material and supplier. The coating must be UV resistant, must withstand the temperatures experienced on the face of an LED display screen, must not crack, fade or peel and it must be possible to replace failed LED’s without compromising weather proofing.
A display screen with an IP43 rating at the rear will require additional protection from severe weather while an IP54 rating is an improvement. Increasingly IP65 rear protection level is being demanded but at this level it is not possible to exhaust hot air from the cabinet using the normal method of employing fans because vents cannot be used. Instead some form of heat exchanger is needed and thermal management needs to be carefully designed.
What Warranty?
An LED display screen is a major investment and it is important that not only is a quality screen assured and shipped, but that it performs reliably over its expected life. Manufacturers will provide spare parts but it is important to select a reliable manufacturer who will both honor a screen warranty but also be around long enough to support you during the life of the display screen.
Warranties offered can be anywhere between 1 year and 5 years typically. If a manufacturer is only willing to offer a 1 year warranty then think very carefully about whether that manufacturer is worth dealing with. 2 year warranties are typical but better manufacturers will offer 3 year to 5 year warranty depending on the product and LED used.
Selecting a good and reliable manufacturer is essential. XLdisplays regularly deals with over 25 LED display manufacturers in Shenzhen alone and provides a factory inspection service to its clients.
In conclusion therefore there are many factors to consider when specifying and selecting an LED display screen manufacturer and product, and it pays to have experienced people to support you able to meet with the manufactures and conduct essential discussions and inspections.
Feel free to contact us to discuss your LED display screen needs.