A high accurately theoretical analysis for high uniformity illumination distribution on the ceiling of indoor lighting LED system based on the super-positions is proposed and performed. Starting from the non-uniform illumination distribution of two LED sources, the dependences of the uniformity ratio on the height of the LED source and the length of the ceiling are investigated. The low intensity in the region near the side-wall can be avoided by rotating the LED source with a small angle, but the low uniformity illumination distribution at the center of the ceiling still exists. The aim of this work proposes the alternative design and approach for several configurations and demonstrates that can be obtained high uniform illumination distribution. The redistributed factor of the illumination through the lens is also considered and calculated. The investigation of this work will be a general guideline for designing the high uniform LED indoor lighting systems as well as being valuable materials for the LED lighting manufacturing industry.

In Vietnam, physical education is mostly still taught in a traditional model. Volleyball is one of the most practiced sports in Physical Education classes. The study of this article was to explore students' difficulties in the practice of volleyball in school Physical Education (PE) classes, according to the tactical principles of the sport, and to compare them with teachers' understanding of the discipline. This qualitative study was developed in two stages: a) observation of 14 hours/class PE volleyball classes in two different schools and field diary records of students' difficulties during the game according to the tactical principles of the sport, and b) application of a Linkert scale with active PE teachers to classify such problems (Thomás A.P, et al 2020). In the first stage, the following situations were identified as difficulties, in decreasing order of occurrence in the observations: building the attack, occupying spaces on the court, looking for empty spaces, intending to contact the ball, and directing serves. In the teachers' view, however, difficulties in serves were indicated as students' main problem and, on the other hand, occupying court spaces as the action they performed more easily. On this basis, it is concluded that there are differences between the researcher's and the teachers' views, possibly related to teachers' initial coaching and previous experiences based on the traditional model, whose focus is not on the teaching of tactical principles.

Mn55Al45, Mn55Al44C1, Mn52.2Al45.8C2 and Mn54.2Al43.8C2 were synthesized by the mechanical alloying method. It was the first time that a high purity τ phase up to 99% of weight percentage was obtained in Mn54.2Al43.8C2, which gave the highest saturation magnetization Ms = 570 kAm-1 ever reported by mechanical alloying method up to date. The crystallite size of the τ phase of MnAl(C) alloy decreased with increasing carbon doping, varying from 79 to 159 nm. Additionally, the coercivity (Hc) was found to be inversely proportional to the crystallite size of τ phase. Effect of doping carbon and its position in the τ phase of MnAl(C) alloy were also investigated for the first time by first-principle calculations. It was found that by inserting carbon at the interstitial site in the tetragonal structure, a strong stabilization effect and an expansion of unit cell were observed, which are in good agreement with the experimental results. Moreover, our results indicate that carbon doping reduces the magnetic moment of Mn.