## 1 Introduction
[PERSON] et al. [11] proposed a quasi-Lambertian approach to real conditions in indoor  
Figure 1: Light distribution of different kinds of luminaires: (**a**) conventional symmetric distribution, fluorescent or LED tubular luminaires, the red line refers to planes C0–C180, the blue line refers to planes C90–C270. Unit: cd/klm; (**b**) LED panel luminaires (right); (**c**) definition of distribution plane. workplaces with a special aim in educative environments. The luminous flux (lumen) and light intensity projected by the projector are important factors related to the visual clarity of the projection screen image. If the luminous flux of the projector is insufficient, the lower the luminance value (cd/m\({}^{2}\)) displayed on the projection screen, the lower the image luminance and the visual clarity. Moreover, due to the luminous influence of the luminaires coming from the desk area, the images and text on the screen will become much more difficult to recognize, since the resolution and clarity has worsened, as shown in Figure 2. The projectors equipped in classrooms generally output luminous flux of about 3000 lumens, which is sufficient for creating an acceptable and clear vision. Although the blackboard luminaires have been turned off to enhance the projection screen clarity when teaching, the luminaires around the classroom desk area are still turned on for students to take notes and writing purpose. Unfortunately, the light distributed by the symmetrical luminaires from the desk area illuminated the projection screen, which blurs the screen, making it very difficult for the students to recognize contexts projected [12; 13]. [PERSON] [14] presented both modeling and measurements of screen brightness and brightness uniformity. However, if all the luminaires above the desk areas are turned off, with only the projectors turned on, the image displayed on the screen will be clear. However, with high luminance contrast it will increase the risk of discomfort glare and is harmful to eyesight when undergoing long hours of teaching [15; 16; 17]. [PERSON] [18] evaluated impact of luminous intensity distribution of the direct lighting luminaires on ceiling and wall illumination, and discomfort glare. It is a consensus agreed upon that glare problem could be predicted and calculated by Unified Glare Rating (UGR), one of the main methodologies, which evaluated the luminance relationship between the watching target and surrounding environment. [PERSON] et al. [19] described UGR measurement with luminance analyzer and discusses overall impact of background luminance on the calculation result. [PERSON] et al. [20] investigated the change of various indicators that reflect people's attention and visual fatigue under different indoor lighting conditions. [PERSON] et al. [21] propose a design strategy for the health light environment in classroom. Therefore, studying the luminance threshold required for the clarity of the projection screen and the light interference of the luminaires from the desk area is the first and foremost task when planning and designing a classroom lighting project. With these results, IoT technology can be applied in the future, and table-side sensing elements can be added to luminaires for intelligent dimming control [22; 23; 24; 25]. In this paper, we propose an effective methodology for evaluating the visual clarity when teaching or presentation while using projectors in the classroom. For practical lighting improving engineering, we propose an innovative asymmetric luminous distribution design of LED light tube and light switch control mechanism of luminaires to improve visual clarity of the projection screen while maintaining the sufficient illuminance on the desk area for taking notes or writing purpose. Figure 2: The lamp light affects the clarity of the projection screen. Reprinted with permission from ref. [26]. Accessed on 22 November 2021. [PERSON].