Studies in northern Europe show that shading is primarily used to control glare, both from direct sun and high diffuse sky luminance. Glare from the sun is particularly uncomfortable and disabling when the angle between the line of sight and the sun is less than 45°. Diffuse sky luminance can cause glare above 3000 cd/m˛. However glare can be tolerated up to 9000 cd/m˛. Individual responses vary widely and glare is age related. Illuminating the window wall reduces glare.
In southern Europe, the sun is associated with overheating and shading devices are not simply used to control glare but also the threat of overheating (this does not imply that shading should not be used to control overheating in northern Europe)
Shading devices may therefore be additionally controlled by absolute and rate of change of temperature. In winter solar radiation is useful as a direct gain passive solar system. Although when rooms are occupied the shades may be used to control glare, it is recommended that an occupancy detector be incorporated so passive solar benefits may be achieved.
The system being developed is autonomous and therefore should not have any extensive wiring system. Prototype sky type sensors have been developed in principle using a photocell on five sides of a cube and placed on the roof of the building. They are expensive and require considerable wiring and thus while permitting sophisticated control are not appropriate for the system being developed here.
These should be placed on the ceiling and away from the window and thus require wiring. Placed on the frame facing inwards they are in practice often obscured by internal blinds, curtains, net curtains etc. However these practical problems are dwarfed by the problems of the electric lights. Without knowledge of the state of the internal electric lighting which means some connection to the artificial lighting and the loss of autonomy, the system could only work with an additional external sensor or sensors . The problems are exacerbated in domestic buildings where the lighting could be described as 'random'.
This is the favoured approach. It must be placed on the exterior of any blind system but the blind holding structure itself could provide an appropriate fixing point. The motor itself is clearly incorporated in the structure. It should be located such that the view of the sky is not obstructed by the reveals of the window openings. The vertical luminance levels corresponding to a 90 degree sky view ( incl obstructions) with a mean luminance of 3000- 9000 cd/m˛ are approx 4500-13500 lux. As glare tolerance varies widely and we cannot predict the reflection of the internal walls etc it is suggested that an internal manual rheostat ( simple sliding) is used to manually set the glare tolerance level. Note that refected sun glare should be picked up by the photocell if it is likely to prove annoying.
Where internal venetian blinds are used it is suggested that the motor is purely used to control the blade angles. A manual control should be available for the up and down movement. The blade angle needs to be adjusted on a solar clock. A 'learning' algorithm should be used to predict sun position according to the window orientation.