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A general calculation method is described for determining of illuminance from a vertical louver blind system, which slats are of negligible thickness, uniformly diffusing surfaces and uniform reflectances. Available data in practice are also prepared for this method.First, configuration factors of arbitrary position on the working plane are analytically introduted for the sky to be viewed between louver slats and for these slats themselves. Next, a direct illuminance of this position is obtained by using these configuration factors and average luminances of the sky and slats. The luminance of slat is calculated in two cases, that the window faces to the uniform sky and gronnd, and the slats are uniformly illuminated by the direct sunlight on the critical condition of precisely shading it with slats.For an indirect illuminance a simple formula by author is effective in practice. Necessary transmittances of louver system in this formula are presented as useful data in above mentioned two cases.An overlapping of results calculated in the both cases is approximately applied to almost all weather conditions with or without the direct sunlight.