Brightness Of Full Color Led Display

Comparing with LCD display, LED display has a lot of advantages: higher brightness, bigger and customized size, wider viewing angle, longer viewing distance, longer life span and so on.
The high brightness is one of the most important features of LED screen. It makes the LED screen can be used in outdoor environments. When the brightness is more than 6500nits, even under direct strong sunshine, the screens can still show us colorful videos.
The unit of brightness of LED screen is Nit. It is defined as below:
Nit (unit) : a unit of luminance equivalent to one candela per square metre(1 cd/m2).
1 nit = 1 cd/m2
For most of LCD screens, the brightness is among 300 nits to 400 nits. For indoor LED screen, the brightness is among 1800 nits to 2000 nits when a outdoor LED screen has the brightness more than 6500 nits. When we calculate the brightness of a LED screen, we usually use a formula as below:
Brightness = Brightness of one pixel * Quantity of pixels per SQM * Scan method

1. Brightness of one pixel.As we know, if we want a pixel to shows white color, the brightness of R:G:B should be 3:6:1. So when we design the PCB layout, we adjust the circuit to let the brightness of RGB to be 3:6:1. Take NICHIA (Japanese brand)SMD 3in1 LED for example, I will show you how to get the actual biggest brightness according to the brightness in the LED specification sheet which the vendor give us.

NESM180A: it is obvious the brightness of blue color LED is always enough. If we use red color as standard and according to the ratio 3:6:1, the brightness of green color should be : 280/3*6= 560mcd. but the brightness of green color is 440mcd, not enough, so we use green color as standard, then the brightness of red color should be 440/6*3=220mcd, less than announced value, so this way can be realized. So blue color will be defined as : 440/6*1=73.3 mcd.
NESM227A: the same principle, we use red color 850 mcd as standard, the green color should be 850/3*6=1700, less than 2000, it can be realized, so blue color is 850/3*1=283.3.
Now in above method, we can work out the biggest target brightness in design:

The biggest brightness of one pixel when use NESM180A is 733.3 mcd.
The biggest brightness of one pixel when use NESM227A is 2833.3 mcd.

2. Quantity of pixels per square meter.Quantity of pixels = (1000 mm / pixel pitch) * (1000 mm / pixel pitch)
For example of a P3 screen, there are total: 1000*1000/3/3 =111111 pixels in each squre meter

3. Scan method.There are general two scan methods: static scanning and dynamic scanning .
Static scanning: each output (lead pin) of the driver chip only control one led. In one lighting clock cycle , the LED is bright in the whole cycle time. So static scanning can support a high brightness especially for outdoor screen. But this scan method uses more drive chips, so it can only be used for big pixel pitch screens, such as P16 screens, P20 screens. Or else there wouldn't be enough space for placing the drive chips on PCB.
Dynamic scanning: each output (lead pin) of the driver chip control 2, 4, 8, or 16 LEDs, and they are also combined controlled by another circuit. In one lighting clock cycle, the LEDs are enabled one by one. So each LED is bright only for 1/2, 1/4, 1/8, or 1/16 cycle time(just human eyes couldn't sense it well). So these are named as 1/2 duty scan, 1/4 duty scan ….and so on. Now we can easily understand when we use the same LEDs, the same driver chips, the same pixel pitch, using 1/4 duty scan method will get a higher brightness than using 1/8 scan method , and 1/8 scanning need more driver chips than 1/16 scanning. So when we produce P3 screens, we have to use 1/16 scanning , because we don’t have enough space to layout so much driver chips in the PCB board if we use 1/4 or 1/8 driving. But when we produce P10 screens, we choose 1/4 scan method but not 1/16 scan for a higher brightness.

Now , we can try to calculate the biggest brightness of LED screens. Suppose we are going to produce a taxi top sign.
a. When we use NESM180A SMD LED to produce P4 screen,
   The biggest brightness of each LED is 733.3 mcd
   The quantity pixels of each square meter : 1000*1000/4/4 =62500
   The scan method is 1/16 duty
   So the brightness = 733.3*62500/16 =2864453 mcd = 2864 cd/m2 = 2864 nits
A led screen with 2864 nits brightness is not suitable for outdoor usage, because the brightness is not enough - when there is sunshine, we can’t see it clear.
So we don’t recommend to choose P4 solution for the taxi top signs.
b. When we use NESM227A SMD led to produce P6 screen,    The biggest brightness of each LED is 2833.3 mcd
   The quantity pixels of each square meter: 1000*1000/6/6 =27778
   The scan method is 1/8
   So the brightness = 2833.3 *27778/8 = 9837 nits