Brightness, Window Sizes and Visual Links

The position, size and type of windows essentially determine the pattern of daylight in an internal area – (35). The appropriate window sizes for living and work rooms of various dimensions are defined in – (38). The following conditions provide the basis for these calculations for living rooms:

- D% = 0.9 at the centre of a living room and at the lowest point in a workroom,
- width of window = 0.55 x room width,
- clouded sky,
- reflection from the wall = 0.6,
- reflection from the ceiling = 0.7,
- reflection from the floor = 0.2,
- light losses from the glass = 0.75,
- light losses from window-frame scatter k1 = 0.75,
- light losses from contamination k2 = 0.95,
- reflected light from neighbouring buildings Dv = 0.2,
- angle of light reflected from neighbouring buildings a = 0-50° (see (36) + (37)).

Note: This applies by analogy to workrooms when their dimensions correspond to those of living rooms:
- room height (h) < 3.50m,
- room depth (t) < 6 m,
- room area (A) < 50m²

Visual links with the outside also demand the requisite window dimensions for living rooms and workrooms. Minimum recommended requirements are summarised in –(38) and – (39). These recommendations contain the following points:

- limiting clearances and clearance areas for the relevant building heights must be maintained,
- visual link with the outside is a requirement for all accommodation;
- as a rule, a window size of approx. 1/8 – 1/10 of the usable room area must be provided for living rooms.

Among other factors in the town planning interpretation of building instructions and standards, incident light, building separation, the external aspects of neighbouring buildings and window design all have to be taken into account – (40). For example, a building separation of В = 2H (> 27°) is the desired value. This results in an aperture angle of > 4° (limited by building geometry and neighbouring buildings) to achieve the minimum level of daylight in rooms.

Newly developed town planning schemes should be carefully checked for the quality of light in internal areas since, in general, the building regulations and standards only set minimum requirements.

It is advisable to carry out a visual inspection of the designs to check the expected appearance of internal and external areas, either in model form, under an artificial sun and artificial sky, or using an endoscope device.

Illuminance, level of reflection, colour rendering and glare. The interplay of these characteristics of daylight has a great influence on the brightness in internal areas. To fulfil specific visual tasks, specific daylight illuminance levels are required, depending on the type of activity – (41).

Therefore, the choice of reflection levels for the walls has to be coordinated with the requirements of the visual tasks which are to be performed. The varied structuring of the brightness in a room is dependent on the reflection levels of the surfaces and the choice of arrangement of the windows in the fagade – (42) (and see also (35)).

The uniformity G of the daylight illumination (defined as Dmin/Dmax) should be > 1:6 in the case of light from the side – (43). In the case of light from above, G > Dmin/Dmax 1:2 – (44). This, in principle, characterises the variation of daylight in internal areas. The uniformity is better in the case of overhead illumination, since the zenith luminance is three times greater than the luminance on the horizon.

Measures used to vary the uniformity can be influenced by:
- the level of reflection (if very high),
- the direction of any glare,
- the arrangement of the windows.

Glare is caused by direct and indirect reflection from the surfaces and by unfavourable luminance contrasts – (45) – (46), Measures for the avoidance of glare include:
- solar shading outside,
- glare protection, inside and outside, in association with solar shading,
- matt surfaces,
- correct positioning of daylight-enhancing illumination.

Shadow is desirable to a certain degree, in order to be able to distinguish objects or other aspects of the room (- (47), schematic). Measures required for a more threedimensional shadow effect in the case of side lighting include:
- solar shading,
- glare protection (even in the north),
- balanced distribution of daylight,
- no direct glare,
- multi-layered or staggered facade.

Measures for appropriate shading with light from above include:
- incident daylight on the lower edge of the light opening, through translucent materials, light gratings or similar filters (- (48), schematic),
- daylight-enhancing illumination,
- bright matt surfaces combined with coloured differentiation (e.g. a supporting structure).

Summary: Quality criteria, daylight coming from the side. In essence, the named quality criteria for daylight must be interpreted in such a way that spatial identity results. The variation of daylight in the internal area, combined with a good external view, are largely the result of the design of the fagade, that is, the transition from inside to outside. A staggered, multi-layered and simultaneously transparent transition from inside to outside can satisfy the various requirements relating to daylight throughout the seasons of the year – (49).