A variety of shading devices are
employed to deliver daylight to where it is
needed and to reduce glare by limiting
excessive luminance in the field of view.
Shading devices are divided into two catego-
ries: movable and stationary.
Movable controls adjusted in response
to varying sky conditions are the most effi-
cient, but they require a human operator or
an automatic device. Stationary (static) con-
trols are less expensive but are also less effi-
cient, because they are unresponsive to
daily and seasonal changes.
Movable Controls
Draperies, shades, and screens are avail-
able with a wide range of materials that vary
in their openness of weave and surface
reflectivity. They provide almost any desired
degree of light transmission or a complete
blackout. Greater flexibility is achieved with
two separately tracked draperies over a
window area—one used to reduce light and
the second to block it completely.
Light-colored venetian blinds (interior
horizontal louvers) can be adjusted to
exclude direct sunshine but reflect light to
the ceiling, increasing its penetration into
the space while still allowing a view of the
outdoors (figure 5.11). Or they can be closed
completely, blocking both light and view.
But for venetian blinds to function
appropriately under changing sky conditions,
they must be operated with an understand-
ing of their potential by someone who has
the opportunity and the incentive to perform
the task. Venetian blinds collect dirt easily
and are tedious to clean; they are subject to
mechanical failure of support straps and
control strings.
Double-glazed windows are available
with a narrow venetian blind positioned
between the inner and outer glass, which
eliminates the dirt-collection problem. These
blinds are fully operable and reject heat
more efficiently than interior blinds or other
shading devices such as roller shades or
draperies.
Motorized controls allow convenient
adjustment of sun-filtering and light-blocking
fabrics on windows and skylights. They
permit one-touch adjustment of blinds and
draperies for changing activities or changing
sun, or they can be programmed to move
fabrics with the sun automatically.
Motorized controls may be linked to
home or building automation systems, so
that shades move at preset times or at spe-
cific illuminance values. They can also com-
bine control of daylight with electric light so
that sun-filtering or light-blocking fabrics and
electric lights are adjusted at the same time.DAYLIGHTFigure 5.11Properly adjusted venetian blinds reflect
daylight to the ceiling and do not prevent a view outside.