Lighting engineering in construction is a complex scientific field, which requires the amalgamation of knowledge in the field of daylighting, construction and architecture, as well as other areas, including the humanities. Many studies have proven the benefits of natural light on health, activity, visual well-being and human productivity. Global environmental issues and the sustainable development movement require architectural design to achieve maximum energy efficiency. For all intents, daylighting calculations depend on the luminance distributions of the sky types. Currently, standard documents are used for the luminance distribution and daylighting calculations under overcast sky conditions in Vietnam, where the overcast sky and clear sky are not typically considered. For these reasons, an update of theoretical studies in the daylighting calculations and design the daylighting systems must be completed. Accordingly, this study offers the modern methods of analyzing the firmament luminance distributions when calculating Daylight Factor on the more realistic sky conditions. For this, the sky type have to be defined according to the location. Fifteen international standard types of the firmament with their descriptors are provided by Kittler et al. and a technique using a relation of diffuse and total solar illuminance levels named the cloudiness coefficient Ko are considered to define the sky condition in Hanoi and Ho Chi Minh City. As the results, the typical sky type for Hanoi is the partly cloudy sky no gradation towards zenith, slight brightening towards the Sun; when the sky type for Ho Chi Minh City is the partly cloudy, with brighter circumsolar region. From these results, the sky luminance distributions for daylighting calculations were proposed. A comparison shows the difference between these calculations can be allowed at the altitude angles γ of a point in the sky above 50o with the relative errors below 10 %. The method offered and verified in this study showed that, it has potential to be used for difference climate areas.