Illuminance detecting of the headlamp has become an obligatory step to keep the traffic safety. Now the full screen video method is used in the production line of headlamps of most relevant industries. This article describes an improving design proposal based on the full screen video method to solve its large floor area and the alignment of its center reference point "HV" lowering system measuring precision problems. By analyzing the optical character of Fresnel Lenses and globe lens, an optical simulation software named "LightTools" is used to set up the whole system model. Laboratory result finally shows globe lens in the improved system is better than Fresnel Lenses in the aspect of improving system measuring precision. Besides, this system has a little floor area, and the assembling light property of globe lens can also easily regulate the center reference point "HV". In view of automobile market, this system must have a broad prospect.