Robust license plate detection in nighttime scenes using multiple intensity IR-illuminator

The functionality of video surveillance is significantly degraded by the low illumination and poor visibility under the nighttime environment. However, the demand for nighttime surveillance is no less than the daytime one because of the high incidence of accidents during night. The Infrared (IR) light source with fixed intensity works for only certain distance, resulting in the defect of underexposure/overexposure due to the object being too far from/close to the light source. In this paper an innovative idea is brought up that we use a multiple intensity IR-illuminator to enhance the effective distance of license plate detection. Based on the stroke width of the license ID, license plates are detected in the images under different illuminations and then the results are integrated into a synthesized high dynamic range image, in which the license plate regions and the background scene can be better visualized. Experimental results show that the proposed approach can effectively enlarge the monitored area in both depth and width, as well as enhance the security level of nighttime video surveillance.