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Mcd Blue Led

A Guide to Outdoor Video Screens

Big outdoor screens have been used in stadiums, towns centre advertising and are now popular at concerts and outdoor events. Early designs of big screens utilised power hungry halogen lamps. Simpler types used electromechanical pixels. Sony launched their Jumbotron in 1985 using CRT (cathode ray tube) technology.

During the 1990s LED's (light emitting diodes) started becoming brighter so they could be seen in sunshine, because of this they became the technology of choice. They offered numerous other advantages. LED's were considerably cheaper, had a much longer lifespan (10 x CRT), offered a much sharper picture and considerably lower power consumption.

First, let me explain some of the basic terminology:

Pixel - an addressable graphic dot. RGB (red, green, blue) - from these colours it is possible to make all the colours and shades.

Pixel composition - the number and colour of LED's in the pixel. E.g. 2 x red + 1 x green + 1 blue.

Resolution - matrix of pixels, the number of rows x number of columns of pixels. Pitch - the distance between the pixels.

Sharpness - determined by the density of pixels, higher the density the sharper the picture quality.

Brightness - measured in MCD (mill candela) or Nit (unit of luminance equivalent to one candela per square metre (1 cd/m²).

Viewing angle - Typically 120 degree horizontally and 60 degrees vertically.

Compound distance - the minimum distance at which you can view the picture without it appearing too pixilated. For small pitch e.g. 10mm the distance can be 10m, larger pitches e.g. 20mm it can go up to 30m.

Viewing distance - the maximum viewing distance at which text can be read. This is related to character height.

Colours - the millions of combinations generated by lighting the LED's at various brightness levels and combinations. There are usually 10 bits per colour for grey scale and 256 levels of brightness.

LED life - nowadays 100,000 hours (11+ years) is typical. The brightness reduces to 50% over its life.

Control Software - The software will operate on a Windows platform and allow you to compose various zones on the screen. A zone can play video, show text, images, time, etc. The control computer will allow importing of live video via video camera, DVD player, and TV receiver. The following formats are standard: VGA, DVI, RGBHV, PbPr (HDTV), Composite, S-Video and TV.

Screen Construction - Big screens are made up of modules. A typical module can be 1 x 1m, The module would have for example a resolution of 96 x 96 pixels. In this case the pitch is 10mm. So to achieve a 4 x 3m big screen you would need 12 modules. The total resolution in this example would be 384 x 288 pixels.

For portable screens - the modules would be made with snap on type connections to allow for easy set-up and breakdown at events. The modules would be made of steel with an ingress protection level of IP65 (complete protection from the weather) at the front and slightly less at the rear access for maintenance. Quite running fans are incorporated for cooling and ventilation.

Communication - between control computer and screen is via unshielded twisted pair net cable allowing maximum transmission distance of 130m. Using multi-mode fibre optics transmission distance can be increased to 500m.

Maintenance - There will be rear access comprises doors on each module. Replaceable service parts are the internal CPU, LED boards and switching power supplies. Unless safe access exists at the location, to comply with Health and Safety requirements the mounting structure will need to include walkways for service personnel.

In 1997, the first LED video screen was installed in a European stadium (Reebok Stadium of Bolton Wanderers Football Club). It was a multinational effort between MT Hungary, an Italian LED board manufacturer and Jayex. At the time the 7 x 5 square metre screen was installed at around £280,000 when the Jumbotron was around £1m. That revolution has continued with the LED video screen market having expanded and matured. Nowadays, the majority of LED boards are made in China helping to reduce prices dramatically. US and European suppliers add value with their control software and project management.

The buyer should particularly investigate the financial standing, project management skills and experience of the supplier. The customer would no doubt want to satisfy himself that spares and support would be available several years down the line. After having selected the supplier, he will need the supplier to survey the site and design the supporting structure. The customer has to make provision for power supply and arrange laying of data cable from location of control room to the screen. After installation the supplier would train the customer's operating staff in the control software and simple maintenance.

About the Author

The author, Agam Jain has many years of experience in LED displays gained in his capacity as Managing Director of Jayex Technology Limited.