Under the uniform sky, the direct illuminance and incident luminous flux of each rectangular surface in a cuboid space with rectangle window can be easily calculated using the average direct daylight factor which is the form factor of the illuminated surface relative to the aperture surface. This paper presents a method for deriving the average direct daylight factor under the overcast sky between two basic rectangular surfaces and the result of derivation. This allows us to calculate the average direct daylight factor of the rectangular surface in a cuboid space, using a method like that under the uniform sky.
