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Abstract
Aiming at the problem of luminance uniformity for injection electroluminescent display panels, we present a new sample compensation method based on column-control according to successive scans theory. On the basis of our ideas, a small part of pixels of each column are selected as samples, and the column gating time calculated by sample average luminance value of corresponding column is written in hardware program. We adopt the 64 × 32 LEDs display panel as an example to expound the compensation method and obtain good result that the reduction in amplitude of luminance non-uniformity is 65.42% for the sample area, 58.67% for the non-sample area and 60.21% for the entire display panel.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Electroluminescence (EL) is an optical and electrical phenomenon in which a material emits light in response to an electric current passed through it, or to a strong electric field. Electroluminescence is divided into two categories: one kind is electron and hole recombination emitting light, such as mini-LEDs, micro-LEDs and OLEDs, which is the main research object in the paper; another kind is a material emits light in response to electrons obtaining energy from high electric field and then colliding with luminescence center, that is high field electroluminescence which mainly used for military field and not the discuss object in the paper.
OLEDs have found wide applications in augmented reality/virtual reality (AR/VR) devices, high-resolution, large-area televisions and handheld displays of smartphones and tablets [1–6]. Nowadays, OLEDs have been at the forefront of next-generation flat-panel display for the advantages of excellent contrast ratio, high brightness, high color gamut, wide viewing angle, fast response time, low power consumption, high flexibility, light weight, ultra-thin, self-emitting [7–17], and the possibility to realize displays on skin [18].
Micro-LEDs display is considered to be a new next-generation display technology that may subvert the existing flexible organic light-emitting diode display, and has become a new growth and explosion point in the field of display industry [19–22]. Micro-LEDs have attracted increasing research and commercial attention for applications such as micro-displays [23,24], visible light communications [21–26], micro-lighting [27], wearable devices [22,23], augmented reality and virtual reality [23,26,28,29], and so on. Micro-LEDs have many advantages, such as a high resolution, high brightness, high contrast, fast response, long lifetime, self-luminous, high efficiency, low power consumption, high integration and high stability [19,27,30–32]. They are small in chip size, flexible and easy to disassemble and merge. They can be applied to any display application from small size to large size [19].
Currently the luminance uniformity of mini-LEDs and micro-LEDs displays can’t meet the market requirements [33]. High luminance uniformity is good for white balance that has already become one of the key factors which restrain OLEDs’s development in medium size or large size display market. In order to solve the problem of luminance uniformity, a series of measures in process were adopted by many researchers [34], and other researchers proposed some solutions by designing luminance drive circuit or compensation method [35–44], but the problem has not been solved fundamentally.
A luminance compensation method is presented for display panels in the paper to improve the luminance uniformity and reduce the luminance non-uniformity. The advantage of this method is that it does not require measuring the luminance data for all pixels in the entire display panel, only measuring that for the sample area, thereby significantly reducing the testing workload.
2. Luminance measurement and uniformity calculation for display panels
Figure 1 is the schematic diagram of a 64 × 32 common cathode traditional LEDs rectangular panel. According to this image, the panel is divided into two areas: the sample area and the non-sample area. Rows 1 to 24 belong to the non-sample area, while rows 25 to 32 constitute the sample area.
Fig. 1. The 64 × 32 traditional LEDs rectangular panel schematic. The panel is divided into two areas: the sample area and the non-sample area. Rows 1 to 24 belong to the non-sample area, while rows 25 to 32 constitute the sample area.
The luminance data for pixels in the last 8 rows have been measured by using a luminance meter along the normal direction of the vertical panel, the luminance data of the display panel can be denoted as a matrix. We build a 64 × 32 matrix H by using Matlab program, and record the above-mentioned data into H. Figure 2 is the original luminance distribution image for all pixels in the sample area. All luminance data provided in the paper don’t contain the luminance of background.
Fig. 2. The three-dimensional original luminance distribution image was plotted by using Matlab according to the data of luminance values for all pixels in the sample area.
The total number of pixels in the sample area is 512(8 rows × 64 columns). The pixels average luminance value of the sample area can be described as follows:
In order to prove that the compensation method is effective for the entire display panel, the luminance data of the non-sample area also need to be measured. Figure 3 is the original luminance distribution image for all pixels in the non-sample area. By adopting the same method, we can easily figure out the average luminance value, the luminance total variance and the luminance uniformity for all pixels in the non-sample area. Specifically, LAVE_NON_SAM ≈ 98.10 cd/m2, LVAR_NON_SAM ≈ 126.70 cd2/m4 and LUNI_NON_SAM ≈ 88.53%, respectively.
Fig. 3. The three-dimensional original luminance distribution image was plotted by Matlab according to the data of luminance values for all pixels in the non-sample area.
3. Principle of the driver circuit
The range of positive voltage drop variation for different LEDs and OLEDs is large, thereby using a constant voltage source to drive them has a significant impact on the luminance uniformity of LEDs and OLEDs. For LEDs, when the current exceeds 10mA, the relationship between luminance and current is approximately linear, thereby a constant current source should be used for driving them. For OLEDs, the relationship between luminance and current is approximately linear, thereby a constant current source also should be used for driving them.
Figure 4 is the driving circuit schematic diagram of a single LED. The row driver uses a Darlington transistor TIP127 with a base resistance RROW, which is 200 Ω; while the column driver uses a transistor 2N3904 with a base resistance RCOL, which is 30KΩ. When the row signal (i.e., the suspended end of RROW) is at a low level and the column signal (i.e., the suspended end of RCOL) is at a high level, the UBE of transistor 2N3904 is approximately equal to 0.7V, and the column signal VCOL is at a high level (3.3V).
Fig. 4. The driving circuit schematic diagram of a single LED. The row driver: a Darlington transistor TIP127 is used, and RROW = 200 Ω; The column driver: a transistor 2N3904 is used, and RCOL = 30KΩ. For the FPGA chip, the RROW is connected to one of 32 output pins for the row driver, and the RCOL is connected to one of 64 output pins for the column driver. The emitter of TIP127 is connected to a voltage of 3.3V, providing power support for the entire driving circuit.
The voltage of the base resistance RCOL can be calculated using the following formula:
Because the LED is connected to the collector of the transistor 2N3904, the current passing through it can be calculated using the following formula:
Generally, pulse width modulation(PWM) is used to control the luminance of LEDs and OLEDs, which can reduce the requirement for accuracy of constant current source. Taking LEDs as an example, the principle of PWM will be elaborated in detail below. The PWM method utilizes the visual inertia of human eyes by repeatedly power on and off the LEDs to make them light up and down in turn. If the frequency of light up and down exceeds 100 Hz, what the human eyes see is the average luminance, not the flicker of the LEDs. From the perspective of the luminous principle, the luminance of LEDs is basically proportional to the current passing through them. This indicates that under the same conditions of average pulse current and DC current, the LEDs have the same luminance. By adjusting the duty cycle of the pulse, an appropriate average current can be obtained. To obtain a luminance value of DC driving, the average current value of the pulse driving should be equivalent to the current value of DC driving.
For instance, if the frequency of light on and off exceeds 100 Hz and the LED1’s instantaneous luminance value exceeds that of the LED2, we can adjust the column selection time for both LEDs(i.e., their conduction time) during a row scanning time to achieve the goal that the average luminance of the LED1 can be approximately equal to that of the LED2. The adjustment method is as follows:
In summary, we adopt constant current source, PWM and row-by-row addressing technologies for driving the LED display panel in this paper. What is more important, the paper presents a new sample compensation method based on column-control which will be elaborated in detail in sections 4 and 5.
4. Calculation of compensation data for the column-control method
One of the major issues lies in the uniformity of transistors and circuit parameters over the entire panel and over the lifetime of the device [45,46]. LEDs and OLEDs are current-controlled devices, the luminance uniformity is a function of the current density and degraded with increasing current density [47]. In order to expound the compensation method, we take the single screen driving as an example, where all pixels of each column have an identical bipolar junction transistor(BJT), whose electrical parameters have a certain extent dispersity. Because of the dispersity, different BJTs can have different effect to corresponding column pixels.
Figure 5 shows that different columns have different luminance values for the entire display panel. By adopting the same method, we can easily figure out the average luminance value, the luminance total variance and the luminance uniformity of all pixels in the entire display panel. Specifically, LAVE_ENT ≈ 98.07 cd/m2, LVAR_ENT ≈ 125.23 cd2/m4 and LUNI_ENT ≈ 88.59%, respectively. (ENT stands for “entire”, that is applicable for the entire paper.)
Fig. 5. The three-dimensional original luminance distribution image was plotted by Matlab according to the data of luminance values for all pixels in the entire display panel.
For any column, we can calculate the average luminance value if we choose some pixels in the column as samples to measure, and average luminance value of all pixels in the column is approximately equal to the average luminance value of the sample pixels in the column. In this paper, we select pixels from 25 to 32 rows of each column as samples. According to the above method, it is very easy to calculate LCOL_AVE_SAM that is the average luminance value of the sample pixels in each column. Specifically, the minimum value of LCOL_AVE_SAM (LCOL_AVE_SAM_MIN) is 85.25 cd/m2, and the maximum value of LCOL_AVE_SAM (LCOL_AVE_SAM_MAX) is 117.125 cd/m2. Therefore, the minimum average luminance value of all pixels in each column is approximately equal to 85.25 cd/m2 and the maximum average luminance value of all pixels in each column is approximately equal to 117.125 cd/m2.
The duration of each row scanning is TROW_SCAN which can be described as follows:
where N = 128, CLK is the clock signal provided by the external oscillator. Set LAVE_SAM(j) as an average luminance value of sample pixels in the column j. Common cathode driving mode is used in the design. In order to achieve luminance uniformity, when the any row is scanned (its signal is low level), if the corresponding pixels of column j are required for emitting, its high level duration is clearly expressed as:5. Luminance uniformity compensation for display panels based on FPGA
The PWM and Row-by-row addressing technologies are adopted in the paper. Figure 6 shows the design schematic diagram of VHDL program for luminance compensation of the LED panel. The input signals “reset” and “clk” are controlled by a reset switch and a clock crystal oscillator which is 10MHz on the circuit board, respectively. The output signal “div_out” of the “div_cnt” module, generated by clock division, provides the clock for the input signal “clk_row” of the “row_column_scan” module. The output signal “cnt_out[31..0]” of the “div_cnt” module provides the data that which row being scanned for the input signal “cnt_in[31..0]” of the “column_control” module. The input signal “rom_data[63..0]” of the “row_column_scan” module is provided by the output signal “rom_out[63..0]” of the “rom_data” module which includes the original column data. The output signal “row_out[31..0]” of the “row_column_scan” module is the row output signal of FPGA driving circuit for the entire display panel. The output signal “column[63..0]” of the “row_column_scan” module provides the data which columns need to be conducted for the input signal “column_in[63..0]” of the “and_gate_64” module. The output signal “row_number[4..0]” of the “row_column_scan” module provides a requirement that which row needs to be selected for the input signal “row_number[4..0]” of the “rom_data” module. The output signal “con_out[63..0]” of the “column_control” module provides tCLK_HIGH for the input signal “col_con_in[63..0]” of the “and_gate_64” module, additionally, the luminance compensation value of the entire display panel, denoted as tCLK_HIGH, can be calculated using formula (10). The output signal “column_out[63..0]” of the “and_gate_64” module is the column output signal of the FPGA driving circuit for the entire display panel.
Fig. 6. The design schematic diagram of VHDL program for luminance compensation of the 64 × 32 common cathode traditional LED panel made with Quartus II.
6. Result and discussion
For hardware verification, we select a FPGA chip EP1C6Q240C8 belong to Altera company Cyclone series to design a VHDL program, and the 64 × 32 common cathode traditional LED panel is stitched by 32 “8 rows × 8 columns” LED lattice screens using the display stitching technique. In the end, we design a display device including “power driving”, “FPGA driving” and “display panel” modules.
Figure 7 is the luminance distribution image after compensation for all pixels in the sample area. The image contrast between Fig. 2 and Fig. 7 indicates that luminance uniformity of the sample area has improved after compensation.
Fig. 7. The three-dimensional luminance distribution image after compensation was plotted by Matlab according to the data of luminance values for all pixels in the sample area.
The pixels average luminance value after compensation of the sample area can be described as follows:
The pixels luminance total variance after compensation of the sample area can be described as follows:
The pixels luminance uniformity after compensation of the sample area can be described as follows:
On the basis of the above data, we can calculate the luminance uniformity after compensation of the sample area, that is LAFTER_UNI_SAM ≈ 96.12%.
After compensation, we use a luminance meter to measure the luminance data of all pixels in the non-sample area, and the luminance distribution image of the area is shown in Fig. 8. The image contrast of Fig. 3 and Fig. 8 indicates that luminance uniformity of the non-sample area has improved after compensation. Similarly, we can calculate luminance uniformity after compensation of the non-sample area, which is LAFTER_UNI_NON_SAM ≈ 95.26%.
Fig. 8. The three-dimensional luminance distribution image after compensation was plotted by Matlab according to the data of luminance values for all pixels in the non-sample area.
Figure 9 is the luminance distribution image after compensation for all pixels in the entire display panel. The image contrast of Fig. 5 and Fig. 9 indicates that luminance uniformity of the entire display panel has improved after compensation. Similarly, we can calculate luminance uniformity after compensation of entire display panel, which is LAFTER_UNI_ENT ≈ 95.46%.
Fig. 9. The three-dimensional luminance distribution image after compensation was plotted by Matlab according to the data of luminance values for all pixels in the entire display panel.
The increase in amplitude of luminance uniformity can be described as follows:
We can easily figure out LINC_AMP_UNI_SAM ≈ 8.27%, LINC_AMP_UNI_NON_SAM ≈ 7.60%, LINC_AMP_UNI_ENT ≈ 7.75%, LRED_AMP_NON_UNI_SAM ≈ 65.42%, LRED_AMP_NON_UNI_NON_SAM ≈ 58.67%, LRED_AMP_NON_UNI_ENT ≈ 60.21%, thereby the increase in amplitude of luminance uniformity is 8.27% for the sample area, 7.60% for the non-sample area and 7.75% for the entire display panel, and the reduction in amplitude of luminance non-uniformity is 65.42% for the sample area, 58.67% for the non-sample area, and 60.21% for the entire display panel. The luminance uniformity of the LED display panel is very well, so LINC_AMP_UNI has a low percentage and doesn’t have a good effect; but as another point of view, LRED_AMP_NON_UNI has a high percentage which indicates a very good adjustment effect.
On account of traditional LED display panels usually have very high luminance uniformity; it is very good to have the above compensation effect in the small improvement space. OLEDs, traditional LEDs, mini-LEDs and micro-LEDs are current injection-type electroluminescent and have similar operational principle, thereby the compensation method is also applicable to OLED, mini-LED and micro-LED display panels. For the luminance uniformity of OLED, mini-LED and micro-LED display panels is not very good, and has already become one of the key factors which restrain their development in display market. By applying the proposed method to OLED, mini-LED and micro-LED display panels, their luminance uniformity can be greatly improved.
Figure 10 shows the image when six Chinese characters (University of Electronic Science and Technology) are displaying in the traditional LED display panel which uses all the technologies introduce in the paper.
Fig. 10. The image when six Chinese characters (University of Electronic Science and Technology) are displaying in the traditional LED display panel which uses all the technologies introduce in the paper.
7. Conclusions
We adopt constant current source, PWM, row-by-row addressing and display stitching technologies, and design a display device including “power driving”, “FPGA driving” and “display panel” modules in this paper. What is more important, the paper presents a novel method for luminance compensation about traditional LED display panels, based on column-control. Traditional LEDs, OLEDs, mini-LEDs and micro-LEDs are current injection-type electroluminescent devices, and they have similar operational principle, thereby the compensation method for traditional LED display panels is also applicable to OLED, mini-LED and micro-LED display panels. For simplicity, we adopt the 64 × 32 rectangular traditional LED display panel based on FPGA driving as an example to expound the compensation method. By applying the proposed method to the display panel, the increase in amplitude of luminance uniformity is 8.27% for the sample area, 7.60% for the non-sample area and 7.75% for the entire display panel, and the reduction in amplitude of luminance non-uniformity is 65.42% for the sample area, 58.67% for the non-sample area, and 60.21% for the entire display panel. The hardware validation result indicates that the method is feasible. The luminance uniformity of OLED, mini-LED and micro-LED display panels is not very good, especially after 10000 hours of work. If we apply the proposed method to improve the luminance uniformity of OLED, mini-LED and micro-LED display panels, we will obtain better compensation effect than that of traditional LED display panels.
Funding
National Natural Science Foundation of China (No. 62275009); Scientific Research Foundation for Yangtze Delta Region Institute of University of Electronic Science and Technology of China, Huzhou (U03220150).
Acknowledgments
This work is supported by the National Natural Science Foundation of China, and the Scientific Research Foundation for Yangtze Delta Region Institute of University of Electronic Science and Technology of China, Huzhou.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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