This issue is sponsored by the Philips Romania, Lighting Division

Figure 6
It could be argued that the majority of
installations are over-lit in their working
state and that the daylight systems are
contributing to this. Only 10% of the
measured daylight levels would on their
own satisfy SLL illuminance requirements
for offices and for these the daylight
contribution represents between 10% and
40% of total illuminance. A major reason
for this is that the number of luminaires
exceeded the number of daylight output
devices in all installations, in some cases
greatly so. Also there is a big difference
between the light outputs of the luminaires
and the daylight output devices. The most
common type of luminaire used – 600 mm
x 600 mm downlight with four PL lamps
has a typical light output of approximately
4800 lumens. The daylight output of the
devices typically ranged from
approximately 1000 lumens in prevailing
temperate overcast conditions to 8500
lumens under a midsummer clear sky with
sunlight. Examination of likely daylight
illuminance levels during working hours in
Recent developments in daylight guidance systems
the UK suggests that, for most of the year,
each daylight device provides less than half
the light output of the luminaires used.
Analysis of user views showed
differences in response to installations with
and without windows and this gave some
clues as to the impact of the guides on the
perception of daylight. The total workstation
illuminance was generally assessed as too
low but only considered satisfactory in
installations with windows. Also much
higher levels of dissatisfaction with daylight
than for total illuminance were evident
particularly in windowless installations.
Although there was a strong correlation
between window/guide area and DPF there
was only a weak correlation between
perception of daylight and guide area.
Installations with windows also elicited
more favourable responses to perceptions of
shadows and distribution of light. The DPF
values were generally below 1%, some way
below the 2% value considered necessary to
give a ‘well day lit’ space. A pattern thus
emerged that although daylight guidance
devices were regarded as providers of
daylight they were inferior to windows in
delivery of both quantity and quality (mainly
view and illuminance variation). User
response to questions on the appearance of
the systems suggested that although they are
not unduly visually intrusive they had little
aesthetic appeal either.
The lack of daylight linked control
meant that diurnal variation increased
illuminance above the electric lighting
design values. Although this permitted
temporal illuminance variation it prevented
energy saving being realised even in
summer conditions when daylight was
capable of providing more than SLL
INGINERIA ILUMINATULUI 18-2006 23