# The Necessity of LED to Ambient Light Ratio Optimization for Vehicular Optical Camera Communication
## 3 LED to Ambient Light Ratio Optimization  
### The Necessity of LED Dimming and the Proper Exposure Mechanism of Camera  
The two most important differences between the VOCC system in this paper compared to that in existing works are the dimming capability of LED and the exposure setting of cameras. Regarding the LED dimming capability, this paper assumes that LEDs are dimmable. In existing papers related to VOCC, the dimming aspect of LED has not been concerned. However, in practice, for the sake of driving safety, LEDs must not be much brighter than ambient light. The iris control in human eyes works in the principle of closing in bright ambient light to allow less light through the pupil and opening in dim ambient light to allow more light to enter the pupil. Because of this reason, vehicle headlights which appear dim in daytime are still much brighter than the ambient light in nighttime and thus can cause serious flare for  
Figure 2: LED bit detection algorithm.  
humans. Similarly, daytime running lights, which are very suitable for using as transmitters in VOCC systems thanks to their brightness that make them clearly visible even in daytime must not be used at night. Consequently, in the VOCC system, the luminance of LED must be changed corresponding the ambient light to maintain the ratio between their brightness within a safe range.  
Regarding the exposure setting of cameras, this aspect has also not been concerned in existing papers on VOCC. The experiment and simulation in those papers considered only the communication between two vehicles. Therefore, the exposure of the camera was simply adjusted corresponding to the LED luminance. This mechanism helps prevent the LED blooming caused by bright LEDs to the captured images. However, this mechanism is not practical. The reason is because the vehicle communicates with multiple other vehicles, of which the LEDs might have different levels of luminance. Moreover, in practice, while the exposure of the camera must be set before capturing the image, the luminance of LEDs can only be estimated after capturing the image. Furthermore, in our vision for intelligent transport system, the video output from the camera should be used for not only communication but also detection of lanes, pedestrians, and traffic signs, and other kinds of object detection required for autonomous vehicles. Changing the camera exposure to the LED luminance would not ensure sufficient greyscale levels for all objects in the image to be detectable. Consequently, in this paper, the camera exposure is assumed to adjust automatically to ambient light.  
As the ambient light is the common factor between vehicles, the luminance of LEDs and the exposure of cameras in all vehicles in the road can all be adjusted to the current ambient light to ensure the traffic safety and the ease for camera exposure setting in all vehicles. These mechanisms also bring about the harmony between the VOCC system and other object detection components in autonomous vehicle as the output images from the camera can be used for both communication and object detection. It is the mechanisms of LED dimming and camera auto exposure adjusted to the ambient light that lead to the optimization problem concerned in this paper.