LED Displays



Pixel and LEDs

An LED display is a display that uses light-emitting diodes (LED) to create an image. A pixel in a LED screen can be one LED or it can be a cluster of LEDs acting as a unit. LED screens with one LED per pixel are single-color screens mostly used for text display. Any color LED display needs more than one LED per pixel, normally three or four (red doubled to get similar luminance for the three colors).

color spectrum of the three colors and added spectrum for white light

Pixels in color LED displays are comprised of three LEDs: red, green and blue. The three colors are mixed together to get white light (the proper amount/level of each of the three components must be ensured). The resulting white light is not a continuous white light through the entire spectrum as from a light bulb but consists of three individual peaks that in the sum are received by the human eye as 'white' light. The same effect is happening with every regular fluorescent light tube. Each pixel comprised of individual LEDs is called a 'luminous dot'.
By gradually dimming the individual LEDs a very high number of possible colors can be achieved for each pixel.

typical arrangement of red, green and blue LEDs for a LED Display

LEDs can last 100.000 hours and more. The high investment costs are partly substituted by the long life time of a LED display compared to the relatively short live span of projection lamps.

LED displays are always built by individual modules LED area (detail)

Because the displays is built of individual modules a gamma and color correction is always needed and must be repeated in regular intervals. Only with a proper gamma correction is it possible to visualize perfectly natural pictures and videos. Otherwise the image quality could easily diminish. The gamma correction is the exponential correction of color levels and is fundamental to adapt the colors to the logarithmic vision of the human eye.



Pixel Pitch or Display Pitch

The more LEDs are built in the display area the higher the resolution and the better the visual image. The distance between the center of one pixel to the center of an adjacent pixel is called the pixel pitch.

pixel pitch (measured in mm)

Display pitch is the distance between the individual LED units. Typical display pitches are: 3mm, 4mm, 6mm, 7.62mm, 8mm, 10mm, 12mm, 14mm, 16mm, 20mm, 22mm, 25mm, 31.25mm, 41.7mm.
As 'matrix' is the number of pixels called that are combined vertically and horizontally. A screen with 240 pixels from top to bottom and 180 pixels from left to right has a 'matrix' of 240 x 180.



Virtual Pixels

Virtual pixels are arrangements appropriate for LED displays running animation or video where the eye canít focus on static detail and the pixel size seems smaller than it actually is. The virtual pixel technology allows visualization of a higher resolution than the physical one, pixels are 'shared'.

regular visible pixels'virtual pixels' between, using the same LEDs

It is based on the so-called 'pixel interpolation'. It is actually overlying every single pixel to the next one. With moving content (video) this creates the virtual pixel between the two pixels.



Physical Pixels Per Square Foot

The physical pixel count per square foot is the ultimate and true measurement of resolution for a LED screen.



Brightness

The LED screen brightness is measured as the luminance of the individual LED and as the overall brightness of the display. The individual LED brightness is measured in millicandelas (MCD) and the brightness of the LED display as a whole unit is measured in NITs (as brightness per square meter).
Usual LED screens have a brightness of 2.000 to 14.000 NITs



Viewing Angle

The viewing angle is the maximum angle from which the display can be seen clearly. Viewing angles can be as small as 20 degrees for long range displays with a narrow viewing area or as wide as 70 to 80 degrees (half angle) for wide areas.



Viewing Distance and Rule of Thumb

The smaller the pixel pitch (the closer the individual pixels to each other), the closer is the minimum distance that the display can deliver a homogeneous image. The further apart the pixels are, the further away the minimum viewing distance is. The pixel pitch determines the image definition and the optimum viewing distance. Low pixel pitch equals high definition, high cost and closer viewing distance, high pixel pitch equals low definition, low cost and higher viewing distance.

For a needed display the pixel pitch must be defined by the MVD (minimum viewing distance). MVD is the closest distance possible before the pixels start appearing as single dots not combining to a homogeneous image anymore.

The Rule of Thumb for LED displays to calculate MVD is to convert the pixel pitch (in mm) into distance (in m). For instance a 10 mm pixel pitch defines a MVD of 10 m, a 6 mm pixel pitch a MVD of 6 m.

With a higher pixel pitch the LED density increases exponentially and with it the costs. Reducing the pixel pitch by 50% increases the number of pixels by 400%!
If the pixel pitch is smaller than can be perceived by the human eye the higher investment for higher pixel pitch is mostly not necessary.