According to the working principle of LCD, the LCD module is a device that controls the light projection intensity when they are emitted from the backlight and passing through the polarizers and LCD panel for imaging.
In another word, the brightness of the LCD is determined by the amount of light that passing through the LCD panel (the transmittance rate of the LCD), the color filter (the transmittance rate of the CF), and the brightness of the backlight.
3 common ways to improve the brightness of LCD:
1. Improve the brightness of the light source.
2. Improve the aperture ratio of TFT pixels.
3. Improve the light utilization of all materials.
As the below figure and table show, when the light source (CCFL/LED) on side, assuming that the CCFL is 100%, it will lose 40% when passing through the light guide plate, lose 36% through the bottom polarizer, 18% through the LCD panel, and 1% on the surface reflection. Eventually, the utilization rate of CCFL is less than 5%.
|Layer||Light Loss Portion|
from Each Layer
the Light Transmittance
|Light Guide Plate||40%||60%|
From here we can know, how to improve the utilization rate of light is significant to enhance the brightness and color performance of LCD modules, even to the industry.
As an important accessory and light source of LCD, the backlight is the most direct breakthrough for brightness improvement.
Effective and common ways to adjust the brightness from Backlight
Backlight unit consists of the light source and layers of optical sheets which are used to guide the light direction as well as to enhance their utilization.
Hence, we can start from the source to each layer to find the solution to increase the brightness:
1. From the light source, we can increase the brightness, quantity, and power of the lamp, but the solution needs to consume more power and increase the size of BL.
2. From the light guide plate, it has high requirements for the material and design of the light guide plate. The solution is expensive but the increment of brightness is small.
3. From the reflective film/layer, to improve the reflection efficiency and brightness.
4. Add prism film and BEF(brightness enhancement film) between the light guide plate and the bottom polarizer.
Except for the light source, the above method third and fourth is more effective and common to adjust the brightness. Therefore, we will discuss them in the following article.
High-efficiency reflective film
The reflective film is a component of LCD.
As shown below, it lays under the lamp tube and light guide plate.
It can be made in new technologies to achieve high reflective efficiency, close to 100%.
In addition to the high-reflective film at the lamp tube, the reflective film under the light guide plate is also essential. It requires a special particle structure to match the printed points of the light guide plate, which can reflect light, and make the reflected light more uniform.
With these particular reflective films, the brightness of the axis center of the LCD can be improved by nearly 30% without changing the design and mold.
As we know, LCD has a certain visual angle. When viewing the computer from the vertical direction, the brightness is relatively high; when viewing from a direction beyond the normal line, the brightness becomes low.
This meets the personal habit of laptop users. The function of prism film is to make the originally dispersed light concentrate to a certain angle around central observation so that the brightness at the axis center can be greatly increased.
As shown below, the surface of the prism film consists of serial prismatic structures.
In another word, the prism film is to make the dispersed light output concentrate within the range of 70 º from the normal line.
By the principle of refraction and total reflection, the prism film makes the emitted light with more than 70 º reflect back and be reused, which can increase the brightness by 110% at the center of the axis.
Brightness Enhancement Film (BEF)
The BEF is also one of the optical elements of the backlight.
As known, when the light passes through the bottom polarizer, there are 50% of the light is absorbed and wasted. If we use a series of BEF, we can reuse 50% of the polarized light that is originally absorbed.
As shown in the figure, the BEF allows P1 polarized light to pass through, and reflects P2 polarized light back for reuse to become P1 and P2, in the recycles, the brightness can be increased by 60%.
All in all, with the high-efficiency reflective film, prism film, and BEF together, the overall brightness can be increased by 230%. This method will need no change in design and tooling, but the cost increment of the backlight.
In different scenarios, terminals need particular brightness in order to provide a clear display to the end users. Hence, brightness is one of the common custom requirements in LCD projects.
Tell us your requirement or terminal usage, we can provide a suitable solution to your application.