I. The Concept of Grayscale
Generally speaking, grayscale means the brightness levels varying from the darkest to the brightest, that is, showing the quantity of brightness or the quantity of grayscale. The higher the quantity of grayscale, the more delicate the grayscale expression. Taking a LED screen with brightness 1000nit and bit depth 14bit as an example, theoretically speaking, the brightness of Grayscale 1 = 1000÷16384=0.061nit. Without considering the coupling and other issues, the minimum brightness difference between grayscales is 0.061nit.
If bit depth is changed into 16bit, the brightness of Grayscale 1=1000÷65536=0.015nit, then it means the brightness of the low grayscale is lower, and the brightness difference between grayscales is smaller, that is, the brightness change or gradient is more delicate.
II.The Definition of Grayscale on the Driver IC
The quantity of grayscale defined on the driver IC is generally calculated by a formula: Grayscale = GCLK / Number of Scans/ Frame Rate. That means the grayscale is determined by the number of GCLK, the number of scans, the frame rate, and the black field time. In general, if the grayscale is linear, that is, brightness, from the darkest to the brightest, is very evenly divided into several brightness levels, then the Grayscale = the brightness at the brightest grayscale / the brightness at grayscale 1. In fact, these grayscales are not equally divided, and in most cases, the brightness at grayscale 1 is slightly higher, which is why we have to achieve [Fine Grayscale] and [22bit+] algorithms.
III. Grayscale Performance and Grayscale Expression of LED Display
Considering of the LED display, the human eye is more concerned about the fine level of low grayscale, even if the grayscale is relatively high, but if the brightness at grayscale 1 is relatively high, it is not acceptable. Therefore, the ratio of brightness at grayscale 1 to that of at the maximum grayscale can also be used to characterize the actual grayscale performance of the LED screen and the number of grayscale, to a certain extent, with a formula [Grayscale = Brightness at the Brightest Grayscale / Brightness at Grayscale 1]. Then if the input source is up to 12bit and the grayscale reaches 12bit, then does it mean the input source equals the corresponding grayscales? No, the input source 12bit refers to the color depth, and the grayscale is the number of brightness changes between the brightest and darkest that the driver IC can achieve, thus, these two are not equal. Reaching 12bit is the basis, otherwise image quality will be distorted. Therefore, under the premise of not affecting the display effect, the grayscale should be as more as possible, turning out that the brightness gradient of the LED screen will be more delicate.
IV. How to Increase the Grayscale
Like the visual refresh rate, the grayscale also depends on the design of the LED module and the ability of the driver IC, so improving the grayscale of the LED screen requires comprehensive optimization from many aspects such as the driver IC and control system, grayscale control technology, refresh rate, heat dissipation and luminous chips selection.
1.Optimize Driver IC and Control System
a) Select a high-performance driver IC: The response speed of the driver IC directly affects the grayscale performance of the LED screen, thus, choosing a driver IC with a narrower OE pulse width allows the control system to achieve a finer grayscale.
b) Upgrade the control system: The control system is responsible for converting the image data into signals that the LED screen can understand. By upgrading the control system and adopting a more advanced coding solution, the fineness of the grayscale can be further improved on a given driver IC.
2.Improve Grayscale Control Technology
a) Adopt pulse width modulation (PWM) technology: PWM technology achieves grayscale by controlling the luminous time of the LED. Making advantage of the visual inertia of the human eye, as long as the cycle of repeated lighting is short enough, the human eye will not feel the luminous pixels shaking. By precisely controlling the width pulse of the light, high precision grayscale control can be achieved.
b) Optimize the nonlinear response of the grayscale: Because the relationship between the brightness of the LED and the current through it is linear, but the human eye’s perception of the brightness is non-linear, it is necessary to optimize the nonlinear corresponding problem. Through optimization, this problem that LED screen in the high grayscale can not distinguish the difference of color shift can be improved, making the grayscale effect more delicate.
3.Improve the Refresh Rate of the LED Screen
a) Improve refresh rate: Refresh rate refers to the number the LED Screen image is presented in full grayscale within 1 second. When the refresh rate is insufficient, the human eye will perceive the breathing effect and rolling effect of the screen. By improving the refresh rate, the impact of these effects can be reduced, making the grayscale performance more stable.
b) Optimize the scanning mode: The scanning mode will also affect the grayscale of the LED screen, thus, the visual refresh rate can be improved by optimizing the scanning mode.
4.Other Notes
a) Ensure good heat dissipation: The LED screen will generate heat when working, if the heat dissipation is poor, it will affect the performance and lifespan of the LED screen, and then affect the grayscale performance. Therefore, it is necessary to ensure that the LED screen has a good cooling system.
b) Choose the suitable LEDs: The quality of LEDs will also affect the grayscale performance, that means the grayscale performance can be improved by choosing LEDs with high brightness, high stability and long lifespan.
