# Refined Test and Evaluation Method of Side Viewing Angle of LED Display Module
## 2 Methods
### Relationship between Brightness and Angle of the Display Module  
The brightness after packaging is the real brightness of the LED display. Due to the random and unmeasurable horizontal angle offset caused by the bonding and packaging process, the brightness variation is unknown.  
Therefore, the LED display module packaged by COB (chips on board) is measured by using the equipment in Figure 2. The measurement must meet the following requirements: the axis of the luminance meter passes through the rotation axis of the turntable, luminance meter lens' center height from the ground is the same as the module's center height from the ground, the rotation axis of the turntable and the surface of the display module coincide and the center line of the module coincides with the rotation axis of the turntable. The display module is rotated with a step of 9 degrees, and the brightness at the center of the  
Figure 1: The optical axis of the LED light-emitting chip is offset during bonding (red, blue and green from left to right).  
display module at each angle is measured to explore the change of the luminance with the angle.  
The attenuation curve of the luminance of the three primary colors in the display module along with the angle is shown in Figure 3, which indicates the relationship between viewing angles and normalized luminance.  
Usually, the measurement of the viewing angle uses the device shown in Figure 2, but this device measures the entire light field distribution [15], which takes a long time and is complicated to operate, and cannot select display modules with inconsistent luminous viewing angles in real time among a large number of display modules. In view of the above problems, this paper explores whether it is possible to find a fixed angle to directly select the display modules with inconsistent light emission, which not only saves time and simplifies the operation but also lays a foundation for engineering applications.  
Since the measured data are scattered points, the luminance attenuation curve is fitted in Cartesian coordinate system. The eighth-order Gaussian function fitting in the fitting method is closest to the true value. The fitting curve is shown in Figure 4.  
Figure 3: The attenuation curve of the luminance of the three primary colors of the display module.  
Figure 2: Visual angle measuring equipment.  
The inflection point of the curve was then found. Under the inflection point, the brightness of the display module did not change significantly, so it was impossible to distinguish whether the luminescence angle of the display module was consistent. The luminance of the display module at the inflection point is different. Taking this point as a fixed angle measurement, the module with inconsistent luminance can be picked out without measuring the overall luminance distribution so as to save time and simplify the operation. We call this point the luminance inflection point.  
The luminance inflection point of the above curve is the second-order extreme point of the curve. The quadratic derivation of the luminance attenuation fitting curves of the three primary colors is shown in Figure 5. Taking \(0^{\circ}\) as the center, the minimum point of the second derivative is the first luminance inflection point, and the maximum point of the second derivative is the second luminance inflection point. However, the serious attenuation of the brightness at this time is no longer suitable for viewing angle. Therefore, in this experiment, the first luminance inflection point, namely the second-order derivative minimum of the luminance luminescence fitting curve, was selected as the experimental object to explore the improvement of side view uniformity.  
Figure 4: Luminance attenuation fitting curve of the three primary colors.  
Figure 5: The second derivative of the luminance attenuation fitting curve.