Analysis of the influence of viewing angle on the brightness uniformity of LED display (Figure)

1. Introduction The electronic industry standard SJ/T11281-2003 "

LED display screen test method" promulgated in 2003 , in which the measurement of brightness uniformity is based on colorimetry theory. The measurement data has nothing to do with human vision and does not involve factors such as the observer's spatial position and changes in observation angle. Since LED display screens are large-screen display products, the visual effects they give people can be imagined. Therefore, it is necessary to observe LED display screens from different angles and qualitatively analyze the impact of different viewing angles on the brightness uniformity of LED display screens, so as to give a more objective evaluation of LED display screens.

2. LED viewing angle

LED light-emitting tubes are generally classified into round lamps, square lamps, rectangles, surface-emitting tubes, lateral tubes, and micro tubes for surface mounting, etc. Round lamps are classified into φ2mm, φ4.4mm, φ5mm, φ8mm, φ10mm, and φ20mm, etc., according to their diameters.

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, for the same LED, the larger the viewing angle, the smaller the maximum luminous intensity , but the accumulated luminous flux on the entire three-dimensional hemisphere remains unchanged.

The half-value angle (or half -power angle) of an LED refers to the angle between the direction where the luminous intensity is half of the intensity along the optical axis and the direction of the optical axis. Twice 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 the LED light tube from different angles, its luminous intensity is very different. The size of the luminous intensity is determined by the luminous intensity angle distribution curve of the LED light tube itself, and its distribution curve is shown in Figure 1. The relative luminous intensity in the direction of the LED optical axis (that is, the ratio of the luminous intensity to the maximum luminous intensity) is 1, that is, the luminous intensity is the maximum. The greater the angle away from the optical axis, the smaller the relative luminous intensity. From Figure 1, the half-value angle of the LED can be estimated, and the viewing angle value is.

In order to meet the requirements of LED display screens with wide viewing angles, LED light-emitting tubes with larger viewing angles are generally selected. This article analyzes LED light-emitting tubes with circular light-emitting surfaces.

3. Measurement of LED display brightness uniformity

The measurement of brightness uniformity specified in the "LED Display Screen Test Method" is only for the brightness measurement in the optical axis direction of the LED light-emitting tube. Figure 2 is a front view of the brightness measurement of the LED display screen. For convenience, only three areas of the LED display screen are analyzed. The positions of display area 1, display area 2, and display area 3 are shown in Figure 2. The luminous intensity angular distribution curves of display area 1, display area 2, and display area 3 and detection position 1, detection position 2, and detection position 3 are shown in Figure 3. According to the "LED Display Screen Test Method", the brightness of the three display areas is measured, and 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 article assumes that the luminous intensity angular distribution curves of the three display areas are the same, so the luminous intensity of the three display areas of the LED display screen is the same, and the brightness uniformity of the LED display screen reaches the national Class C standard.

4. The influence of LED viewing angle on the brightness uniformity of the display

Due to the viewing angle of the LED itself, the brightness of the LED display will change with the change of the viewing angle, affecting the display effect of the display. When the observer views the display from different angles, the change in brightness will have a negative impact on the brightness uniformity of the LED display.

In actual use, the observer will observe the LED display screen within a certain range. Taking Figure 4 as an example, assuming that the observer observes the LED display screen at observation position 1, observation position 2, and observation position 3, the observer's visual axis at observation position 1, observation position 2, and observation position 3 coincides with the optical axis of display area 1, display area 2, and display area 3 shown in Figure 2.

At observation position 1, the brightness of display area 1 is the highest, the brightness of display area 3 is the lowest, and the brightness of display area 2 is between the two; at observation position 2, the brightness of display area 2 is the highest, the brightness of display area 1 and display area 3 is basically the same, but lower than the brightness of display area 2; at observation position 3, the brightness of display area 3 is the highest, the brightness of display area 1 is the lowest, and the brightness of display area 2 is between the two. Although the LED luminous intensity angular distribution curves of the three display areas are the same, due to the influence of the LED viewing angle, the brightness values ​​of the three areas are different at the same observation position.

Display area 1 has the highest brightness at observation position 1, the lowest brightness at observation position 3, and the brightness is between the two at observation position 2; display area 2 has the highest brightness at observation position 2, and the brightness is basically the same at observation position 1 and observation position 3, but lower than the brightness at observation position 2; display area 3 has the highest brightness at observation position 3, the lowest brightness at observation position 1, and the brightness is between the two at observation position 2. Therefore, the brightness of the same area of ​​the LED display will change with different observation angles.

According to the variation law of the LED luminous intensity angular distribution curve, the brightness distribution of the entire LED display screen is simulated at observation position 1, observation position 2, and observation position 3. The brightness distribution of the LED display screen is shown in Figure 5. Figure 5(a) is the simulation of the brightness distribution of the LED display screen observed at observation position 1. Figure 5(b) is the simulation of the brightness distribution of the LED display screen observed at observation position 2. Figure 5(c) is the simulation of the brightness distribution of the LED display screen observed at observation position 3.

Within a certain observation range, the brightness change of adjacent areas of the LED display is determined by the slope of the luminous intensity angular distribution curve. The greater the curvature of the luminous intensity angular distribution curve, the greater the LED brightness change in adjacent areas; the smaller the curvature of the luminous intensity angular distribution curve, the smaller the LED brightness change in adjacent areas. The brightness is distributed in concentric circles with the projection of the observation position on the display as the center. The center is the brightest, and 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 maximum brightness and minimum brightness of different display areas of the LED display screen at observation position 1 and observation position 3 are quite different, so the brightness uniformity of the LED display screen observed at observation position 1 and observation position 3 is poor. The maximum brightness and minimum brightness of different display areas of the LED display screen observed at observation position 2 are slightly different, so the brightness uniformity of the LED display screen observed at observation position 2 is better. However, the brightness uniformity of the LED display screen observed at observation position 2 is much lower than the brightness uniformity of the LED display screen measured by the "LED Display Screen Test Method".

Under ideal conditions, the observer observes the LED display screen at different positions, and obtains the brightness uniformity data of the display screen observed at different positions, which constitutes the three-dimensional distribution diagram shown in Figure 6. Through the analysis of Figure 6, when the observer observes the LED display screen in the center area, 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 the brightness uniformity of the LED display screen measured by the "LED Display Screen Test Method". When observing the LED display screen at the edge position, the brightness uniformity is poor. The farther the observation position is from the central observation area, the worse the brightness uniformity of the LED display screen.

5. Discussion

In the actual application of LED display screens, the relationship between the brightness uniformity of LED display screens and the observation position can be simulated through the LED luminous intensity angular distribution curve. The optimal observation range can be estimated by using objective evaluation criteria to complete the evaluation that could only be done by human eyes before. Its advantage is that the final display effect can be predicted in the research and development stage to reduce risks, save time and money. Since the evaluation of display screens is a relatively cumbersome process with many uncertain aspects, this article only briefly introduces the visual impact of LED viewing angle on the brightness uniformity of the display screen, and does not take into account other LED packaging forms and the color arrangement of full-color LED display screens, the uneven distribution of module light emission, and the color mixing effect. More complex situations, due to space limitations, will be described in detail later.