1. Introduction
The electronic industry standard SJ / t11281-2003 LED display screen test method issued in 2003, in which the measurement of brightness uniformity is based on the colorimetric theory. The measurement data has nothing to do with human vision and does not involve the changes of observer's spatial position and observation angle. As a large screen display product, the visual effect of LED display screen can be imagined. Therefore, it is necessary to observe the LED display screen from different angles and qualitatively analyze the impact of different angles on the brightness uniformity of LED display screen, so as to give a more objective evaluation of LED display screen.
2. Led viewing angleThe light emitting surface characteristics of LED light-emitting tubes are generally divided into circular lamps, square lamps, rectangular, surface light-emitting tubes, lateral tubes, micro tubes for surface installation, etc. Circular lamps are divided into 2mm 4.4mm 5mm 8mm 10mm and 20mm, etc.The shape of the light emitting surface used in LED packaging and the position of the LED chip from the top lens determine the LED viewing angle and light intensity distribution. Generally speaking, the larger the viewing angle of the same led, the smaller the maximum luminous intensity, but the cumulative luminous flux on the whole three-dimensional hemisphere remains unchanged.
The half value angle (or half power angle) of LED refers to the angle between the direction where the luminous intensity value is half of the intensity value in the optical axis direction and the optical axis direction. Two times the half value angle is called the viewing angle of the LED.
The luminous intensity of LED changes with the change of observation angle. When observing LED light-emitting tubes from different angles, its luminous intensity is very different. The luminous intensity is determined by the luminous intensity angular distribution curve of the LED luminous tube itself, and its distribution curve is shown in Figure 1. The relative luminous intensity in the optical axis direction of LED (i.e. the ratio of luminous intensity to maximum luminous intensity) is 1, i.e. the luminous intensity is the largest. The greater the angle away from the optical axis, the smaller the relative luminous intensity. The half value angle of the LED can be estimated from Figure 1, and the viewing angle value is.
In order to meet the requirements of large viewing angle display effect of LED display screen, led with large viewing angle is generally selected. In this paper, the LED light emitting tube with circular light emitting surface is analyzed.3. Brightness uniformity measurement of LED display screen
The measurement of brightness uniformity specified in the test method for LED display screen is only for the brightness measurement in the optical axis direction of LED light-emitting tube. Figure 2 is the front view of brightness measurement of LED display screen. For convenience, only three areas of LED display screen are analyzed. See Fig. 2 for the positions of display area 1, display area 2 and display area 3. The luminous intensity angular distribution curve of display area 1, display area 2 and display area 3 and the detection position 1, detection position 2 and detection position 3 are shown in Fig. 3. The brightness of the three display areas is measured according to the test method for LED display screen. The detection position 1, detection position 2 and detection position 3 are located in the optical axis direction of display area 1, display area 2 and display area 3 respectively. The measured brightness value is the maximum brightness value of each area. This paper assumes that the luminous intensity angular distribution curves of the three display areas are the same, so it is obtained that the luminous intensity of the three display areas of LED display screen is the same, and the brightness uniformity of LED display screen reaches the national C-level standard.
4. Influence of LED viewing angle on brightness uniformity of display screenDue to the problem of the viewing angle of the LED itself, the brightness of the LED display screen will change with the change of the viewing angle, which affects the display effect of the display screen. When the observer looks at the display screen from different angles, the change of brightness will have a negative impact on the brightness uniformity of LED display screen.In the actual use of LED display screen, the observer will observe the display screen within a certain range. Taking Fig. 4 as an example, suppose the observer observes the LED display screen at observation position 1, observation position 2 and observation position 3, and the visual axes of the observer at observation position 1, observation position 2 and observation position 3 coincide with the optical axes of display area 1, display area 2 and display area 3 shown in Fig. 2.
When viewing position 1, the brightness of display area 1 is the largest, the brightness of display area 3 is the smallest, and the brightness of display area 2 is between the two; When viewing position 2, the brightness of display area 2 is the largest, and the brightness of display area 1 and display area 3 are basically the same, but less than that of display area 2; When viewing position 3, the brightness of display area 3 is the largest, the brightness of display area 1 is the smallest, and the brightness of display area 2 is between the two. Although the angular distribution curves of LED luminous intensity in the three display areas are the same, the brightness values of the three areas are different in the same observation position due to the influence of LED viewing angle.
In the display area 1, the brightness is the maximum in the observation position 1, the brightness is the minimum in the observation position 3, and the brightness is between the two in the observation position 2; The display area 2 has the maximum brightness at the observation position 2, and the brightness is basically the same at the observation position 1 and the observation position 3, but less than that at the observation position 2; In the display area 3, the brightness is the maximum in the observation position 3, the brightness is the minimum in the observation position 1, and the brightness is between the two in the observation position 2. Therefore, the brightness of the same area of the LED display screen will change with different observation angles.
According to the change law of LED luminous intensity angular distribution curve, the whole screen brightness distribution of LED display screen is simulated at observation position 1, observation position 2 and observation position 3. The brightness distribution of LED display screen is shown in Figure 5. Fig. 5 (a) is a simulation of the brightness distribution of the LED display observed at the observation position 1. Fig. 5 (b) is a simulation of the brightness distribution of the LED display observed at the observation position 2. Fig. 5 (c) is a simulation of the brightness distribution of the LED display observed at the observation position 3.
In a certain observation range, the brightness change of adjacent areas of LED display screen is determined by the slope of luminous intensity angular distribution curve. The larger the curvature of the angular distribution curve of luminous intensity, the greater the change of LED brightness in adjacent areas; The smaller the curvature of the luminous intensity angular distribution curve, the smaller the change of LED brightness in adjacent areas. The brightness takes the projection of the observation position on the display screen as the center of the circle and is distributed in a concentric circle. The center of the circle is the brightest. The brightness decreases with the increase of the radius. The brightness change is related to the curvature of the luminous intensity angular distribution curve.
By analyzing the brightness distribution of the three cases, it can be seen that the difference between the maximum brightness and the minimum brightness in different display areas of the LED display screen in observation position 1 and observation position 3 is relatively large. Therefore, the brightness uniformity of the LED display screen observed in observation position 1 and observation position 3 is poor. The difference between the maximum brightness and the minimum brightness in different display areas of the LED display screen observed at observation position 2 is small, so the brightness uniformity of the LED display screen observed at observation position 2 is good. However, the brightness uniformity of LED display screen observed at observation position 2 is much lower than that measured by LED display screen test method.
In an ideal state, the observer observes the LED display screen at different positions to obtain the brightness uniformity data of the display screen observed at different positions, which constitutes the three-dimensional distribution diagram shown in Fig. 6. Through the analysis of Figure 6, when the observer observes in the central area of the LED display screen, the brightness uniformity of the LED display screen is the best, but the brightness uniformity of the LED display screen observed in the central observation area is much lower than that measured by the LED display screen test method. Observe the LED display screen at the edge, and the brightness uniformity is poor. The farther the observation position is from the central observation area, the worse the brightness uniformity of LED display screen.
5. Discuss
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