Civil Engineering Reference
In-Depth Information
inside it. The most energy efficient way of obtaining these optimal illumination
levels inside a certain environment is through the use of daylight. Therefore, the
illumination level provided by daylight will depend on some physical properties of the
environment such as the number, design and orientation of windows, light reflection
coefficients, etc. (Koçlar et al.
2004
). Nevertheless, sometimes it is not possible to
reach the optimal illumination level using only daylight, thus, it is necessary to use
some artificial lighting mechanism (Kim and Kim
2010
).
To evaluate the illumination level inside a certain environment, in Castilla et al.
(
2010a
) an index called
D
i
, which represents the difference between the optimal and
the real illumination levels, was proposed, see Eq.
3.17
.
D
i
=
E
vd
−
E
vr
(3.17)
where
E
vd
and
E
vr
are the optimal and real illumination levels in
(
lux
)
.
3.4.2 Glare
According to (CIBSE
2002
), glare can be defined as that
condition of vision in which
there is discomfort or a reduction in the ability to see details or objects, caused by
an unsuitable distribution or range luminance, or to extreme contrasts
. Although
there are many indices for visual discomfort due to glare in the literature, the most
extended ones are the Unified Glare Rating (UGR) index and the Daylight Glare
Index (DGI).
3.4.2.1 Unified Glare Rating Index
The UGR index expresses the discomfort glare occasioned by the presence of bright
light sources, luminaries or windows (CIBSE
2009
). It can be estimated according to
Eq.
3.18
. UGR index values are within the range of 13-30, and moreover, the lower
UGR values, the less visual discomfort condition.
N
L
s
i
ˉ
i
p
Guth
i
0
25
L
b
.
UGR
=
8log
10
(3.18)
i
=
1
where
L
b
is the background luminance in (cd/m
2
),
N
represents the number of lumi-
naries inside the room, and
L
s
i
,
ˉ
i
and
p
Guth
i
are the luminance in (cd/m
2
), the solid
angle subtended at the observer's eye and the Guth position index provided by the
i
th luminary, respectively.
The Guth position index,
p
Guth
, was proposed by Luckiest and Guth (
1949
)in
1949 and reflects the relative Borderline between Comfort and Discomfort (BCD)
brightness of a luminary displaced from the line of vision in terms of the BCD
