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What is Greyscale of LED display?
Greyscale is the ability to show how many grades the led display has from the darkness to the brightness.
If the led display is able to have 16,484 different shade brightness, then it has 214 grscale and we call it 14bit. (214=16,484)
If the led display is able to have 65,536 different shade brightness, then it has 216 greyscale and we call it 16bit.(216=65,536)
If the LED display is able to have 30000 different shade brightness, then it means this LED display’s greyscale is bigger than 14bit, but smaller than 15bit.
In the specs of the LED display, we can always see 14-22bit, it means the greyscale range that the led display can achieve.
Why led display need high greyscale?
For SDR displaying content ( the displaying content that coming from the computer), normally the color depth is 8bit, which means that the greyscale of the displaying content only has 2^8 = 256 different shade. Theoretically, the led display with 8bit greyscale is enough to present the display content. So why we need high greyscale?
The reason is that the different color of the led
display is determined by the LED lamp’s brightness (greyscale), and the LED lamp’s brightness is controlled by the PMW (Pulse-width modulation)
1 width pulse means it has greyscale 1,
2 width pulse means it has greyscale 2.
Sensitivity of human eye to Brightness
The sensitivity of human eye to brightness is not liner, it is in relation of exponential. For example, if the brightness of an area is 10nit, if we want our eye to feel the brightness of another area is two times brighter, this place needs to be 100nit.
Hence for the led display, if it is only 8bit greyscale, then the the brightness of the led display is divided into 256 different shades, and the responsive relation is liner. Like the greyscale 1 of the displaying content is mappedto the greyscale 1 of the led display. greyscale 2 of the displaying content is mapped to the greyscale 2 of the led display.
While if the led display’s greyscale is 14bit, which means the greyscale 256 of the displaying content would be in the 2^14= 16384 greyscale. Greyscale 128 of the displaying content would be 5000 greyscale of the led display. And the responsive relation would be more similar to our eye’s responsive relation to brightness. Which can give the led display a better visual effect, such as a smoother gradient during color-changing areas.
Hence normally the higher the greyscale, the better the visual effect it would have.
What determines the greyscale of the led display
As we learn from above, the greyscale of the LED display is determined by the PMW. PMW signal is provided by the IC, and all the PMW signal combined in a second we call it GCLK. The GCLK ability is determined by the IC. The relation of the GCLK of Greyscale is as below
GCLK = greyscale * scan rate * frame rate
As the GCLK’s ability is determined by the IC, from the above formula, we would know, that for a particular IC, if you want to increase the greyscale, you can decrease the scan rate of the led display in design. Or you can use a good IC.
What’s Refresh rate of led display
Refresh rate means the frequency of the display content be showed on the led display in a second, the unit is Hz. Normally the higher the refresh rate of the led display, less flickering and more stable visual effect the led display would have.
Why does led display need high refresh rate
When it comes to led display’s refresh rate specs, normally we would find 1920hz, 3840hz, or 7680hz, but when we try to buy LCD display, normally the refresh rate is 60hz and 120hz. So why does led display need such a high refresh rate?
The reason is that in the LCD display, the back light is always on, it only change the R,G,B gate to control the passing light to our eyes, the light is consistent.
while for led display the greyscale of the led display is achieved by PMW, which means that the led lamp would turn on and off constantly.
For example ( led display’s frame rate is 60hz, greyscale is 13bit)
If the led display’s refresh rate is 60Hz, . It then means each frame’s duration is 1000/60=16.67ms. In this one frame the led lamp only turn on and off 1 time. To reach 13bit greyscale, it means that it would have 2^13= 8192 different grade shade. For greyscale 1, and greyscale 2000, the PMW would be as below:
And as you see, when there is only 1 time turn on and of for the led lamp, the duration of the led lamp turn on is not evenly distributed among the whole 1 frame duration. And when the turn on time is too focus, the brightness of the led lamp during the 1 frame is also not even. To make the PMW (Greyscale) evenly distributed during the whole frame. We can make the a certain qty of greyscale one session, then PMW evenly divided into those session, and led lamp would turn on and off once among one session. We would know the refresh rate = the session qty * frame rate.
The greyscale quantities in a session is determined by the IC, and we know that the Greyscale = session qty * greyscale quantities in a session. From above we would know that, The higher the refresh rate, it means the PMW is more evenly distributed in the GCLK, which would mean more stable image quality.
Low brightness high greyscale
As we know from above, in low greyscale, the refresh rate is not high. To be able to have a good visual effect under low greyscale, then the IC’s output current need to be accurate, as little accuracy or flickering of the current, would result the led display’s color uniformity under low greyscale not consistent.
While if you want to achieve low brightness of the led display, we would need the IC to have a lower current output. For example, the IC’s output current is from 4-25mA, normally the IC is working under the current of 15mA, and it is normal to have a 5% current difference between different IC. Hence the output current would be around 14.25-15.75mA, for example, the brightness of the led lamp would be 900-1000nit. While if we want to make a low brightness say 300nit, the current would be around 5mA, and the output current would be around 4.75-5.25mA, while the brightness would be from 280-320nit. As our eye is more sensitive to low brightness, hence the difference of 40nit in low brightness is very easy to detect.
Hence in order to achieve low brightness high greyscale, we need to use high end IC, and the design of the PCB is also different, as we need to make sure the output current from the IC is very precise and accurate.
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