Many model studies using artificial skies have been made in this country and overseas for the prediction of daylight conditions of interiors. Using artificial skies is preferable for model studies to working under the natural sky. The conditions of the natural sky with the specific luminance and luminance pattern can be obtained with the artificial model which has the advantage of remaining stable for long periods of time.
R. Kittler has reviewed the development of artificial skies from 1914 to 1957, but most of these are not satisfactory for the advanced studies of the day. J. Longmore and D. C. Croghan have been discussing artificial skies recently constructed in England such as a rectilinear box type by the Building Research Station (1963), a reflecting dome type by Pilkington Brothers Limited (1962), and a transilluminated dome type by the Cambridge University (1962). These are representative of the three types of artificial skies which are currently being used in any country.
An artificial sky was constructed at the Building Research Institute in Tokyo in 1966. In designing the new sky the following conditions were satisfied as a as possible.
1. The sky surface is adjustable for the luminance distribution of the C. I. E. standard overcast sky.
2. It has the average sky luminance of more than 2000 asb (640cd/m²).
3. Visually, it gives to observers the appearance of the natural sky.
4. A ground surface of known reflection factor can be placed.
5. A model room about 0.8m at is width can be used, minimizing the horizon error.
6. The working space is large enough to conduct an experiment.
7. High temperatures in the working space because of the radiation from lamps should be avoidecd
8. The conditions of an experiment can be easily understood by laymen. The new sky belongs to a transilluminated dome type, but the dome is set in a four-cornered room.
The size of the room is 7.9m wide and 5.5m deep with a 3.3m ceiling. The sky is represented with the hemispherical F. R. P.(fibre reinforced polyester) plastic dome. The dome consists of six pieces which are joined so as not to reveal the joint. The diameter of the dome is limited to 4.1m because of the limitation of the space, and the dome is fitted with 40 cm tall steel legs. As it is considered that the proper width of a model should be less than 1/5 of the dome diameter, a model which is 0.8 m at is width can be used.
All lamps are installed on the ceiling surface. They are one hundred and twenty-two 1.2m, 40W‘white’ fluorescent lamps (4500°K), ten 150W spot type PAR lamps, ten 150W flood type PAR lamps, ten 150W spot type reflector lamps. The electric source is 100V50c/s. The total bare lamp output is 441, 400lm. All these lamps are divided into seven blocks, each being adjusted separately. Fluorescent lamps give direct light mainly to the upper part of the dome while incandescent lamps give indirect light mainly to the lower part by reflection from the wall. The window surface and the exposed side of the dome are coverd with white curtains. In this way the dome can provide the luminance surface which is equivalent to the C. I. E. standard overcast sky, and the average sky luminance reaches more than 2000 asb (640 cd/m²).
This sky has the following special advantages.
1. As the dome is uniform and the graded luminance distribution and the slight changes of color are seen, this sky bears a close resemblance to the real sky. Therefore using this sky is effective for the subjective appraisal of watching the conditions of the sky through the model window.
2. This is made in an ordinary-size square room and consists of a simple set such as the light sources on the ceiling, the reflection walls, and the plastic dome put on the floor. These matters have the advantages of simple design and low cost.

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