Why Calibration

LED displays are display systems for media playing indoor and outdoor, with image quality as the key parameter of their performance. Because of the limitation on the LED displays themselves and the manufacturing process, LED displays suffer brightness/chroma ununiformity, which is also the most serious problem of this industry. The causes of brightness/chroma ununiformity can be summarized as follow.

1. Difference of LED lights when manufactured

Optical characters of LED lights can not be precisely controlled at present because of the limitation of manufacturing process. There is pretty big difference in brightness, chroma, visual angle and other optical characters between different batches of LED lights. Even when of the same batch, working on the same voltage and current, the brightness difference between different LED lights can be up to 40% - 50%, and the main wavelength range difference up to 15nm – 20nm. In order to improve the uniformity, when produced, spectrums of LED lights are measured and then sorted into different grades according to their brightness and main wavelengths. As sorting is time consuming and expensive, it can not be performed meticulously. Normally, after sorted, the brightness difference of LED lights in the same grade is within 20% - 30%, and the main wavelength range difference within 5nm. The remained difference of LED lights when manufactured results in ununiformity.

2. Difference introduced from manufacturing process of LED displays

A LED display is an optical system with a 2-dimentional matrix of LED lights being one of the main components. Many factors may affect the uniformity of this optical system, such as the difference between LED light driver chips, welding of LED light modules, the flatness of module assembly and axis directions of plunged LED lights. Especially, if the axis directions of plunged LED lights are not well controlled, LED lights will differ greatly in the decay curves of brightness with angle, resulting in serious lose of brightness uniformity.

3. Difference introduced from the usage of LED displays

When being used, lights of a LED display will decay in brightness and shift in the main wavelength. This will worsen the uniformity of the LED display as the brightness decay and main wavelength shift differ between different LED lights. For a LED display bad in cooling design, temperature difference of different area will cause greater difference in brightness decay and wavelength shift. So, in order to reduce brightness/chroma ununiformity, the whole manufacturing process of a LED display need to be well managed and controlled. But at the same time, as we can see from the causes mentioned above, brightness/chroma ununiformity of a LED display is not likely to be avoided completely.

Solution for brightness/chroma ununiformity calibration in pixel level

Facing this fact, pixel level calibration system does not intervene the manufacturing processing of a LED display to reduce its brightness/chroma ununiformity. Instead, it performs brightness/chroma adjustment to the display after it has been completely produced. By adjusting the brightness/chorma of each LED light according to the software analytical results from the measured brightness/chroma values of the LED lights, the pixel level calibration system can help the LED display acquiring perfect uniformity. As a new technology enabling great improvement of LED displays brightness/chroma uniformity, pixel level brightness/chroma ununiformity calibration has been developed quickly at home and abroad in the recent couples of years, and has also become a core technology indispensable in improving image quality for high-end LED displays. Following are two images showing the uniformity improvement of a LED display before and after calibration.

The Nova LED display pixel level calibration system is a professionally designed, strictly tested and well used LED display brightness/chroma ununiformity calibration system, which was developed for the goal of helping LED display manufacturers and operators to get rid of the problem of ununiformity. By performing LED light brightness/chorma adjustment, this calibration system can greatly reduce the ununiformity of a LED display, improve its image quality and extend its service life. In fact, the above two pictures are from a real case of the Nova LED display pixel level calibration system.

. Principle of calibration

At first, the calibration system acquire the brightness/chroma values of each LED light through data acquire instruments. Then the calibration software will analyze the acquired data and generate the corresponding calibration coefficients. These coefficients are input back into the LED display. Finally, the display adjusts the brightness/chroma of each LED light according to the calibration coefficients and thus achieve high brightness/chroma uniformity.

Fig.1.1 LED display before and after calibration

Shown in Fig.1.1 are two pictures from the same LED display before and after calibration. The left one is captured before calibration, in which the brightness/chroma difference between different lights is obvious. The right one is captured after calibration, which presents satisfying brightness/chroma uniformity. As we can see form the principle of calibration, there are two sections within the calibration process, brightness calibration and chroma calibration. Brightness calibration is to adjust the brightness of LED lights to improve the brightness uniformity. In the brightness calibration, brightness of most lights will be properly lowered. Fig.1.2 shows an example of brightness adjustment of green LED lights, in which are two brightness distribution curves before and after calibration (adjustment). Before calibration, the brightness values of green LED lights are scattered between 2400 – 3300 cd/m2, while those after calibration are concentrated almost at 2500 cd/m2, representing high brightness uniformity.

Fig.1.2 brightness values distribution before and after calibration

Chroma calibration is base on the theory of RGB color match. It adjusts the coordinates of LED lights in the RGB color coordinate system to reduce the chroma difference. As shown in Fig.1.3, the large triangle is the gamut of a LED display before calibration while the small one is the gamut of same LED display after calibration. The R, G and B color coordinates of LED lights scatter in relative large areas when before calibrated while those after calibration concentrated, representing high chroma uniformity.

Fig.1.3 gamut of a LED display before and after calibration Note that when performing chroma calibration, coordinates for R, G and B should be properly chosen in order to avoid color distortion. 2. Calibration System Composition

A calibration system normally consists of three main parts, ● Instruments for LED lights brightness/chroma values measurement. These instruments are usually industrial cameras, professional digital cameras and colorimeters. ● Calibration software. Calibration software is used for calibration process management and data analysis, and it usually runs in the calibration computer. ● Devices for brightness/chroma adjustment according to calibration coefficients. These devices can be the LED display control system, LED driver chips or DVI video sources. Connection of these three parts and the LED display is shown in the following figure. Here the three parts are the professional digital camera (lower left), the calibration software run in the calibration computer (upper left) and the control system (upper right).

Fig.2.1 sketch map of LED display calibration system

Data and instruction interaction between these three parts and a LED display is as follow. Fig.2.2 Scheme of LED display calibration

Calibration:

1. Camera calibration enabling precise brightness/chroma values measurement

It is a new measurement technology of using area array cameras for brightness/chroma values measurement. Area array cameras are good in measurement efficiency, but their accuracy is far bellowing professional instruments like Colorimeters and Spectral color measurement instruments. To precisely measure brightness/chroma values with cameras, camera calibration is the core technology must be mastered.

Normally, cameras without strict calibration can not acquire the real CIE-XYZ (the real brigtness/chroma values), which will result in color and brightness distribution distortion in LED display calibration. LED display manufacturers who have ever used calibration systems developed by other companies may find that blue and greed LED lights are difficult to calibrated. Especially the blue lights, snow-like noise is likely to appear after calibration. The cause is that the LED lights differ in wavelength pretty much, and the measurement instruments did not acquire the brightness/chroma values with sufficient accuracy.

Nova calibration system can do these well. Shown in the following is a real case of blue LED lights calibration with Nova calibration system. The left picture is captured before calibration while the right one is after calibration. It can be obviously seen that the LED display is even and exquisite after calibration.

2. Eliminating the difference between sub-areas or cabinets As the area of a LED display is usually larger than that a calibration system can process at one time, the LED display will be divided into sub-areas for calibration. Although each sub-area can be well calibrated, difference usually still exists between sub-areas. If the display is calibrated cabinet by cabinet, there will be difference between cabinets. This is thought to be acceptable. But in fact, these differences can be eliminated by specially designed calibration systems. And after that, the LED display will be able to show amazing fine images.

Following are three pictures from the same LED display. The first is captured before calibration. The second is captured after calibration with the normal calibration technology, in which the differences between sub-areas can be obviously seen. The third is captured after calibration with Nova calibration technology, from which we can see that the display is free from sub- area difference after calibration.

Before calibration Sub-area effects of normal calibration technology Perfect calibration with Nova calibration technology

3. Arc shape and irregular shape LED display calibration

Arc shape or irregular shape LED display calibration is always not supported by normal calibration systems as its image is distorted or the image shape is irregular. With a correction model specially developed for the calibration algorithm, Nova calibration system is eligible for Arc shape and irregular shape LED display calibration

A arc shape LED display before calibration The same arc shape LED display after calibration

4. Oblique cabinet calibration on production lines When being calibrated in workshop, cabinets of a LED display are always placed on the same horizontal surface as the camera and also with their LED lights matrix surfaces perpendicular to the optical axis of the camera. But the actual view angle will not be like this after the cabinets are assembled into a whole as a LED display. This will cause great loss in the calibration result. Nova calibration system equips specially designed algorithm, which enables calibrating a cabinet in the angle the same as in which it will be viewed actually. And the LED display assembled with these calibrated cabinets will also free from boundary effects.