ISO 10916:2014

INTERNATIONAL ISO
STANDARD 10916
First edition
2014-06-15
Calculation of the impact of daylight
utilization on the net and final energy
demand for lighting
Calcul de l’effet d’utiliser la lumière du jour à la demande énergétique
net et finale pour l’éclairage
Reference number
©
ISO 2014
© ISO 2014
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ii © ISO 2014 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols, indices, and abbreviated terms . 3
4.1 Symbols . 4
4.2 Indices . 5
5 Proof calculation method . 5
5.1 Energy demand for lighting as function of daylight . 5
5.2 Subdivision of a building into zones . 7
5.3 Operating time. 8
5.4 Artificial lighting . 8
5.5 Constant illuminance control . 8
5.6 Daylight. 8
5.7 Occupancy dependency factor F .
O,n 9
6 Daylight Performance Indicator . 9
Annex A (informative) Simple calculation method.10
Annex B (normative) Comprehensive calculation .53
Annex C (informative) Daylight performance indicator .54
Annex D (informative) Examples .55
Bibliography .63
Foreword
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to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 163, Thermal performance and energy use in the
built environment, Subcommittee SC 2, Calculation methods.
iv © ISO 2014 – All rights reserved

Introduction
This International Standard is part of a set of standards allowing to rate the overall energetic performance
of buildings. Facades and rooflights have a key impact on the building’s energy balance. This International
Standard supports the daylighting and lighting-energy-related analysis and optimization of facade and
rooflight systems. It was therefore specifically devised to establish conventions and procedures for the
estimation of daylight penetrating buildings through vertical facades and rooflights, as well as on the
energy consumption for electric lighting as a function of daylight provided in indoor spaces.
INTERNATIONAL STANDARD ISO 10916:2014(E)
Calculation of the impact of daylight utilization on the net
and final energy demand for lighting
1 Scope
This International Standard defines the calculation methodology for determining the monthly and
annual amount of usable daylight penetrating non-residential buildings through vertical facades and
rooflights and the impact thereof on the energy demand for electric lighting. This International Standard
can be used for existing buildings and the design of new and renovated buildings.
This International Standard provides the overall lighting energy balance equation relating the installed
power density of the electric lighting system with daylight supply and lighting controls (proof calculation
method).
The determination of the installed power density is not in the scope of this method, neither are controls
relating, for instance, to occupancy detection. Provided the determination of the installed power density
and control parameters using external sources, the internal loads by lighting and the lighting energy
demand itself can be calculated. The energy demand for lighting and internal loads by lighting can then
be taken into account in the overall building energy balance calculations:
— heating;
— ventilation;
— climate regulation and control (including cooling and humidification);
— heating the domestic hot-water supply of buildings.
For estimating the daylight supply and rating daylight-dependent artificial lighting control systems,
a simple table-based calculation approach is provided. The simple method describes the division of a
building into zones as required for daylight illumination-engineering purposes, as well as considerations
on the way in which daylight supplied by vertical facade systems and rooflights is utilized and how
daylight-dependent lighting control systems effect energy demand. Dynamic vertical facades with
optional shading and light redirection properties are considered, i.e. allowing a separate optimization
of facade solutions under direct insolation and under diffuse skies. For rooflighting systems standard,
static solutions like shed rooflights and continuous rooflights are considered. The method is applicable
for different latitudes and climates. For standard building zones (utilizations), operation times are
provided.
For detailed computer-based analysis (comprehensive calculation), minimum requirements are
specified.
To support overall building performance assessment, additional daylight performance indicators on the
overall building level are provided.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
CIE S 017/E:2011, ILV: International Lighting Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in CIE S 017/E:2011 ILV apply.
3.1
ballast
unit inserted between the supply and one or more discharge lamps, which by means of inductance,
capacitance, or a combination of inductance and capacitance, serves mainly to limit the current of the
lamp(s) to the required value
3.2
control system
various types of electrical and electronic systems including the following:
— systems used to control and regulate;
— systems to protect against solar radiation and/or glare;
— artificial lighting in relation to the currently available daylight;
— systems used to detect and record the presence of occupants
3.3
daylight factor
D
ratio of the illuminance at a point on a given plane due to the light received directly and indirectly from
a sky of assumed or known luminance distribution to the illuminance on a horizontal plane due to an
unobstructed hemisphere of this sky, where the contribution of direct sunlight to both illuminances is
excluded
[SOURCE: CIE S 017/E:2011 ILV, modified]
Note 1 to entry: CIE S 017/E:2011 defines the unit as 1. However, daylight factor is in practice, usually presented
in percent values.
3.4
electrical power of artificial lighting system
P
the total electrical power consumption of the lighting system in the considered space
3.5
illuminance
E
quotient of the luminous flux dΦ incident on an element of the surface containing the point, by the area
dA of that element
[SOURCE: CIE S 017/E:2011 ILV, modified]
–2
Note 1 to entry: Unit: lx = lm × m .
3.6
lamp
source made to produce optical radiation, usually visible
3.7
light reflectance
ratio of the reflected luminous flux to the incident luminous flux in the given conditions
Note 1 to entry: Unit: 1.
2 © ISO 2014 – All rights reserved

3.8
light transmittance
ratio of the transmitted luminous flux to the incident luminous flux in the given conditions
Note 1 to entry: Unit: 1.
3.9
luminaire
apparatus which distributes, filters, or transforms the light transmitted from one or more lamps and
which includes, except the lamps themselves, all the parts necessary for fixing and protecting the lamps
and, where necessary, circuit auxiliaries together with the means for connecting them to the electric
supply
[SOURCE: CIE S 017/E:2011 ILV]
3.10
luminous exposure
quotient of quantity of light dQ incident on an element of the surface containing the point over the given
v
duration, by the area dA of that element
–2
Note 1 to entry: Unit: lx × s = lm × s × m .
3.11
luminous flux
Φ
quantity derived from the radiant flux, Φ , by evaluating the radiation according to its action upon the
e
CIE standard photometric observer
Note 1 to entry: Unit: lm.
3.12
maintained illuminance
E
m
value below which the average illuminance over the specified surface is not allowed to fall
-2
Note 1 to entry: Unit: lx = lm × m .
3.13
obstruction
anything outside the window which prevents the direct view of part of the sky
3.14
rooflight
daylight opening on the roof or on a horizontal surface of a building
3.15
task area
partial area in the work plane in which the visual task is carried out
[SOURCE: CIE S 017/E:2011 ILV]
3.16
visual task
visual elements of the work being done
[SOURCE: CIE S 017/E:2011 ILV]
4 Symbols, indices, and abbreviated terms
For the purposes of this document, the following symbols and units apply.
4.1 Symbols
Quantity Unit
τ light transmittance —
ρ light reflectance —
Φ luminous flux lm
η efficiency —
Q energy kWh
γ angle, geographical latitude °
δ declination of the sun °
a depth M
A area m
b width M
bf occupancy factor —
C correction factor —
D daylight factor —
mean daylight factor —
D
E illuminance lx
maintained illuminance lx
E
m
f, F factors —
g g-value —
H luminous exposure lxh
h height m
I index —
k space index —
k correction factor —
J counter for number of areas being evaluated —
N counter for number of zones —
p area-specific power W/m
t time H
U U-value of glazing system W/m K
v distribution key —
wi light-well index —
4 © ISO 2014 – All rights reserved

4.2 Indices
A absence ND no daylight
At atrium Night night-time
c control O occupancy
Ca carcass opening R room
D daylight rel relative
Day day-time Rd room depth, space depth
dir direct s transparent or translucent surface of the
daylight aperture
D65 standard lightsource D65 s supply
e energic quantity SA sun-shading activated
eff effective, root-mean-square Sh shading, obstruction
ext external, outdoors SNA sun-shading not activated
GDF glazed curtain wall, glazed double facade start start
glob global sunrise sunrise
hf horizontal fin or projection t building use (operating) time
i,j,n serial counter indices Ta task area
In internal courtyard Tr transparency
Li lintel u lower
lsh linear shading usage usage
max maximum v visual quantity
mth monthly vf vertical fin or projection
5 Proof calculation method
5.1 Energy demand for lighting as function of daylight
The final energy demand for lighting purposes is Q to be determined for a total of N building zones
l,f
which can be subdivided into J evaluation areas:
N J
QQ= (1)
l,fl∑∑ ,,fn,j
n=1 j=1
The energy demand of any one evaluation area j is calculated by applying Formulae (2) and (3).
 
Qp=+FA tt +At +t (2)
() ()
l,n,jj c,jjD, eff,Day,D,jjeff,Night, ND,ej ff,Day,,ND,ejjff,Night,
 
where
AA=+ A (3)
jjD, ND,J
applies to the total area of the respective evaluation area,
and where
Q is the final energy demand for lighting;
l,f
N is the number of zones;
J is the number of areas;
F factor relating to the usage of the total installed power when constant illuminance
c,j
control is in operation in the room or zone;
p is the specific electrical evaluation power of area j;
j
A is the floor area of area j;
j
A is that part of area j which is lit by daylight;
D,j
A is that part of area j which is not lit by daylight;
ND,j
t is the effective operating time of the lighting system, during day-time, in area j which
eff,Day,D,j
is lit by daylight;
t is the effective operating time of the lighting system, during day-time, in area j which
eff,Day,ND,j
is not lit by daylight;
t is the effective operating time of the lighting system, during night-time, in area j.
eff,Night,j
The effective operating time, during day-time, in an area which is lit by daylight is calculated using
Formula (4).
tt= FF (4)
eff,Day,D,jnDay, D,,jO j
The effective operating time, during day-time, in an area which is not lit by daylight is calculated using
Formula (5).
tt= F (5)
eff,Day,ND,Djnay,,Oj
where
t is the operating time of zone n during day-time, as defined in 5.3;
Day,n
F is the part-utilization factor to account for the illumination by daylight in the evaluation
D,j
area j as defined in 5.6;
F is the part-utilization factor to account for the presence of persons (occupancy) in the
O,j
evaluation area j as defined in 5.7.
Formula (6) is used to calculate the effective operating time during night-time.
tt= F (6)
eff,Night,j,Night,njO
where
t is the operating time of zone n during night-time, as defined in 5.3.
Night,n
Figure 1 illustrates the order in which the individual steps of the calculations are carried out.
6 © ISO 2014 – All rights reserved

Figure 1 — Flowchart showing calculation of the energy demand for lighting
5.2 Subdivision of a building into zones
The final energy demand for lighting is calculated for all building zones N. The building zones are to be
defined in accordance with the zoning boundary conditions as requested by other criteria like utilization
of spaces and technical requirements.
It can be necessary to subdivide a building zone n into J evaluation areas to determine the final energy
demand for lighting. This subdivision can be necessary due to differences in the boundary conditions
(e.g. technical design of the artificial lighting system, lighting control systems, characteristics of the
facades).
From practical experience, a simplification rule can be recommended: One and the same boundary
condition can be assumed to apply for an entire building zone or an evaluation area if the corresponding
input parameter applies to at least 75 % of the area being evaluated. Input parameters of the remaining
parts (e. g. window areas) assigned to the dominating areas are not taken into account in the calculations.
The specific energy demand is calculated for that part of the evaluation area which occupies at least
75 % of the total area and is then assumed to apply to the total area.
5.3 Operating time
The times during which the areas of a zone being evaluated are used are subdivided into intervals t
Day,n
during which daylight is available, and intervals t without daylight. The operating time t is equal
Night,n n
to t + t . Day-time is thus the time span between sunrise and sunset. Annual daylight hours and
Day,n Night,n
night hours are defined in relation to the different utilization profiles given in Annex A. For operating
times which do not match the cases listed in the tables, the values shall be determined separately.
5.4 Artificial lighting
The specific electrical power of the artificial light installation p can be obtained by, for instance, using
j
standard lighting design software, as provided by luminaire manufacturers. Simplified methods as
[2]
defined in DIN V 18599-4 can as well be employed.
5.5 Constant illuminance control
When constant illuminance control is in operation in the zone or evaluation area, the installed power
will be lowered by a factor F .
c
5.6 Daylight
In zones which have windows or rooflights, daylight can contribute to the amount of the luminous
exposure required. Therefore, this proportion of the required light does not need to be provided by the
artificial lighting system.
The daylight available in the outdoor environment depends on the geographical location, the climatic
boundary conditions, the time of day, and the season. Furthermore, the daylight availability in a building
also depends on the external building structure and surrounding buildings, spatial orientation, and the
technical specifications of the facades and internal spaces (rooms). Since the available daylight varies
with the time of day and the season, the lighting energy substitution potential is dynamic and therefore
has a dynamic effect on the overall energy balance (for heating, cooling, and air-conditioning) of the
building.
The daylight dependency factor F used to account for lighting of an area j by daylight is defined as
D,j
FF=−1 F (7)
D,jD,s,,jD c,j
where
F is the daylight supply factor;
D,s,j
F is the factor representing the effect of the daylight-responsive control system.
D,c,j
The daylight supply factor F accounts for the amount of lighting of the evaluation area j by daylight.
D,s,j
This factor describes the relative proportion of the light needed for the visual task provided by daylight
within the reference time interval at the point where the illuminance is measured (control point). When
determining this factor, the type of lighting control system shall be taken into consideration. The factor
[4]
corresponds to the relative luminous exposure as, for instance, defined in DIN 5034-3, also referred
to as “daylight autonomy”. The factor F additionally accounts for the efficiency of the lighting control
D,c,j
system in using the available daylight to achieve the required luminous exposure level in the area j.
The daylight dependency factor F which takes the daylight illumination into consideration can be
D,j
determined for any given time interval (e. g. year, month, hour).
8 © ISO 2014 – All rights reserved

Annex A comprises simplified approaches to calcutate F for vertical facades (A.3) and rooflights
D,S,j
(A.4) and to obtain tabulated values for F . Annex B contains specifications for using comprehensive,
D,c,j
detailed computer-based tools to calculate F .
D,j
5.7 Occupancy dependency factor F
O,n
The occupancy dependency factor F for a room or zone correlates the time when a space is occupied with
O,n
the efficiency to benefit from this potential by either manual or automatic switching. Parametrizations
[2] [6]
of F might, for instance, be found in DIN V 18599-4 and EN 15193.
O,n
6 Daylight Performance Indicator
To judge the overall daylight performance of a building or a building design and to compare different
buildings or building designs, integral daylight performance indicators are helpful. Annex C gives
definitions and explains their application.
Annex A
(informative)
Simple calculation method
A.1 General
This Annex specifies a simplified approach to calculate the effect of daylight on the lighting energy
demand on monthly and annual bases. The method involves the following stages to obtain, according
to Clause 5, the daylight dependent quantities F , t and as a function thereof t , as also
D,n,j Day,n,j eff,Day,n,j
depicted in Figure A.1:
— A.2 contains a scheme of how to subdivide the zone to be evaluated into area sections which receive
daylight and those which do not;
— A.3 specifies a procedure on how to determine the daylight supply factor F for spaces lit by
D,S,n,j
vertical facades;
— A.4 specifies a procedure on how to determine the daylight supply factor F for spaces lit by
D,S,n,j
rooflights;
— A.5 specifies a procedure on how to rate daylight responsive control systems described by the
parameter F ;
D,C,n,j
— A.6 describes how to convert annual values into monthly values of F ;
D,n,j
— A.7 provides a procedure to determine day- and night-time hours;
— A.8 provides a list of precalculated day- and night-time hours for 41 different utilization types of
building spaces.
10 © ISO 2014 – All rights reserved

Figure A.1 — Flowchart illustrating the simplified approach
As for the determination of the daylight supply factor F , as well for vertical facade (A.3) as for
D,S,n,j
rooflights (A.4), Figure A.2 shows the applied three-stage approach:
— Stage 1: Use of a simple criterion approximating the daylight factor to classify the type of daylight
availability on the basis of the geometrical parameters of the building zone being evaluated. This
assumes a combination of standard reflectances, ρ = 0,2 for the floor, ρ = 0,5 for the walls, and
F W
ρ = 0,7 for the ceiling. The reflectance of the external surroundings is assumed to be 0,2. Instead of
C
using these approximations, a more detailed determination of the daylight factor can be carried out
for more complicated space geometries and other reflectance values using for instance computer
tools.
— Stage 2: Describe the facade characteristics.
— Stage 3: Determine the annual amount of daylight available on the basis of the daylight supply
classification of the building zone and the facade characteristics as a function of location and climate.
Figure A.2 — Three-stage approach to determining the daylight supply factor F
D,s,j
A.2 Building segmentation: Spaces benefiting from daylight
Evaluation zones which are illuminated by daylight entering via facades or rooflights shall be subdivided
into a daylight-lit area A and an area A which is not illuminated by daylight. For simplified estimate
D,j ND,j
calculations, the more favourable respective lighting conditions can be assumed to apply in cases
where one area is illuminated by daylight entering via several facades or via a facade and rooflights.
Alternatively, it is also possible in these areas to determine the daylight factor according to A.3 and A.4
by superposition. This can nevertheless only be applied for areas being lit by only one type of daylight
aperture (either vertical facade or rooflight).
Depth and width of the area daylight by vertical facades
12 © ISO 2014 – All rights reserved

The maximum possible depth a of the area A lit by daylight entering via a facade is calculated
D,max D,j
using Formula (A.1).
ah=×25, −h (A.1)
()
D,max LTi a
where
a is the maximum depth of the daylight area;
D,max
h is the height of the window lintel above the floor;
Li
h is the height of the task area above the floor.
Ta
In this case, the maximum depth a of the daylight area is calculated from the inner surface of the
D,max
external wall and at right angles to the reference facade. If the real depth of the area being evaluated
is less than the calculated maximum depth of the daylight area, then the total area depth is considered
to be the depth of the daylight area a . a can also be assumed to be equal to the real depth of the area
D D
being evaluated if the real area depth is less than 1,25 times the calculated maximum daylight area
depth.
The partial area A which is lit by daylight within the area j is thus calculated as follows:
D,j
Aa= b (A.2)
D,j DD
where
a is the depth of the daylight area;
D
b is the width of the daylight area.
D
The width b of the daylight area normally corresponds to the facade width on the inner surface of
D
the building zone or the area being evaluated. Internal walls can be overmeasured (i. e. their thickness
ignored) to keep the equations simple. If windows only constitute a part of the facade, then the width
of the daylight area associated with this facade is equal to the width of the section which has windows,
plus half the depth of the daylight area.
Depth of the daylight area lit by rooflights
Areas to be evaluated having rooflights evenly distributed all over the roof area are always deemed to
be lit by daylight. In the case of individual or single rooflights and at the boundaries of areas which have
evenly distributed skylights, those parts of the area which are within a distance of
ah≤×2 −h (A.3)
()
D,max R,jTa,j
from the edge of the nearest skylight are deemed to be lit by daylight,
where
h is the clear ceiling height of the area (room) which has a skylight.
R,j
For all parts of the area under evaluation which are not lit by daylight, the factor F is equal to 1.
D,j
Distinction between vertical facades and rooflights
In case of doubt as to whether a specific opening or aperture is to be evaluated as being a window or
a rooflight, all such openings of which the entire glazed areas are above the ceiling of the space under
consideration are deemed to be rooflights.
Key
A
D
A
ND
Figure A.3 — Impact of facade opening on daylight area for vertical facades
Key
A
D
A
ND
Figure A.4 — Impact of roof opening on daylight area for rooflights
14 © ISO 2014 – All rights reserved

Daylight areas of the different vertical Approach 1: Superposition of Approach 2: The more favourable
facades (1 – 5) and the rooflight (6) daylight factors in the overlap- lighting condition is taken for the
ping daylight areas respective daylight area: F > F
D,S,1 D,S,
> F = F > F > F
6 D,S,5 D,S,3 D,S,2 D,S,4
Key
A
D
A
ND
a
rooflight
Figure A.5 — Superposition and penetration of daylight areas for vertical facades and rooflights
A.3 Daylight supply factor for vertical facades
A.3.1 Daylight factor classification
The daylight factor for vertical facades can be obtained by several means, e. g. graphical, analytical, or
computer-based approaches. Here a simplified analytical approach allowing to account for the major
parameters classifying the daylight availability in vertical lit rooms is provided.
The amount of daylight available in an area j being evaluated depends on the transparency index I , the
Tr,j
space depth index I , and the shading index I . These index values are determined as follows.
RD,j Sh,j
Transparency index I
Tr,j
Formula (A.4) is used to calculate the transparency index.
A
Ca
I = (A.4)
Tr,j
A
D
where
A is the area of the raw building carcass opening of the area under evaluation;
Ca
A is the partial area which is lit by daylight as calculated by Formula (A.1).
D
All areas below the work plane (e. g. 0,8 m above floor level in office spaces) are ignored. The height of
[3]
the work plane is given for individual utilization profiles in DIN V 18599-10.
Space depth index I
RD,j
Formula (A.5) is used to calculate the space depth index I .
RD,j
a
D
I = (A.5)
RD,j
hh−
Li Ta
Obstruction index I
Sh,j
The shading index I accounts for all effects which restrict the amount of daylight striking the facade.
Sh,j
This includes shading by parts of the building itself, such as might occur due to horizontal and vertical
projections, light wells, courtyards, and atrium arrangements. It also takes into consideration any
reduction of incident light by the glazed double facades (GDF — also glazed curtain walls). The shading
index I is calculated using Formula (A.6).
Sh,j
I = I I I I I (A.6)
Sh,j Sh,lsh Sh,hf Sh,vf Sh,In,At Sh,GDF
where
I is the obstruction index of the area j under evaluation;
Sh,j
I is the correction factor for linear obstruction of the area under evaluation as calculated
Sh,lsh
using Formula (A.7);
I is the correction factor for an overhang shading of the area being evaluated, calculated
Sh,hA
using Formula (A.8);
I is the correction factor for a side shading of the area under evaluation as calculated using
Sh,vA
Formula (A.9);
I is the correction factor for internal courtyard and atrium shading of the area under
Sh,In,At
evaluation as calculated using Formula (A.11);
I is the correction factor for glazed double facades of the area being evaluated, calculated
Sh,GDF
using Formula (A.12).
To facilitate the calculations, a window located at the centre of the facade area being evaluated can be
used as the reference point for which the shading is calculated. If different forms and degrees of shading
affect the area being evaluated, the mean value of the respective factors shall be calculated.
I , I , I , I and I can be determined using the following methods:
Sh,lsh Sh,hf Sh,vf Sh,In,At Sh,GDF
Linear shading, correction factor I
Sh,lsh
Figure A.6 — Cross-section diagram to illustrate the effect of the linear shading altitude
angle γ
Sh,lsh
16 © ISO 2014 – All rights reserved

The linear shading altitude angle γ is measured from the centre of the facade section being evaluated
Sh,lsh
for lighting aspects (on the plane of the external wall surface) as shown in Figure A.5. Formula (A.7) is
used to calculate the correction factor which accounts for linear obstruction.
I = cos(1,5 × γ )   for γ < 60°
Sh,lsh Sh,lsh Sh,lsh
(A.7)
I = 0                  for γ ≥ 60°
Sh,lsh Sh,lsh
where
γ is the obstruction altitude angle as shown in Figure A.5.
Sh,lsh
Horizontal projections, correction factor I
Sh,hf
Figure A.7 — Cross-section diagram to illustrate the effect of the horizontal shading angle γ
Sh,hf
The horizontal shading angle γ due to a horizontal projection is measured from the centre of the
Sh,hf
facade section being evaluated for lighting aspects (on the plane of the external wall surface) as shown
in Figure A.6. Formula (A.8) is used to calculate the correction factor which accounts for shading by a
horizontal projection.
I = cos(1,33 · γ )   for γ < 67,5°
Sh,hf Sh,hf,j Sh,hf
(A.8)
I = 0                   for γ ≥ 67,5°
Sh,hf Sh,hf
where
γ is the horizontal shading angle due to a horizontal projection as shown in Figure A.6.
Sh,hf
Vertical projections, correction factor I
Sh,vf
Figure A.8 — Cross-section diagram to illustrate the effect of the vertical shading angle γ
Sh,vf
The vertical shading angle γ due to a vertical projection is measured from the centre of the facade
Sh,vf
section being evaluated for lighting aspects (on the plane of the external wall surface) as shown in
Figure A.7. Formula (A.9) is used to calculate the correction factor which accounts for shading by a
vertical projection.
γ
Sh,vf,j
I =−1 (A.9)
Sh,vf

where
γ is the vertical shading angle due to a vertical projection as shown in Figure A.7.
Sh,vf
Courtyards and atria (glazed forecourts), correction factor I
Sh,In,At
There are very many different design variants for courtyards and atria or glazed forecourts. The
calculations are based on an internal courtyard surrounded on all four sides by the building. Better
daylight availability can be expected if only three or two sides (linear courtyards) of the courtyard
are bordered by the building. This can be calculated and proved using separate, detailed calculation
methods.
The geometry of an internal courtyard is characterized by a geometrical index value, the so-called well
index wi:
ha()+b
In,AtIn,At In,At
wi= (A.10)
2×ab
In,AtIn,At
When determining the well index wi for an area being evaluated, the height measured from the floor of
the respective area is considered to be the height of the courtyard or atrium.
where
a is the depth of the courtyard or atrium;
In,At
b is the width of the courtyard or atrium;
In,At
h is the height of the courtyard or atrium, measured from the floor of the storey being evalu-
In,At
ated;
wi is the well index used to account for the geometry of the courtyard or atrium.
Figure A.9 — Illustration of the geometrical parameters used to define the well index wi
18 © ISO 2014 – All rights reserved

The correction factor for taking into consideration building shading in internal courtyards, light wells,
or atria is
I = 1 – 0,85 wi                                             for internal courtyards
Sh,In,At
(A.11)
I = τ k k k (1 – 0,85 wi) for atria
Sh,In,At Sh,In,At,D65 Sh,In,At,1 Sh,In,At,2 Sh,In,At,3
I = 0                                                     for wi > 1,18
Sh,In,At
where
τ is the transmittance of the atrium glazing for vertical light incidence;
Sh,In,At,D65
k is the framing factor for frames in the atrium facade;
Sh,In,At,1
k is the dirt on glazing factor of the atrium glazing;
Sh,In,At,2
k is the reduction factor for diffuse light incidence on the atrium glazing (usually, 0,85
Sh,In,At,3
is considered to be adequate).
Glazed double facade (glazed curtain wall), correction factor I
Sh,GDF
The correction factor for glazed double facades or curtain walls bounding on the space being evaluated
is directly deduced from the parameters of the additional glazing layer:
I = τ k k k (A.12)
Sh,GDF Sh,GDF,D65 Sh,GDF,1 Sh,GDF,2 Sh,GDF,3
where
τ is the transmittance of the external layer of glazing of the facade, for vertical light
Sh,GDF,D65
incidence;
k is the reduction factor for frames in the double-glazed facade;
Sh,GDF,1
k is the reduction factor for pollution of the glazing of the double-glazed facade;
Sh,GDF,2
k is the reduction factor for non-vertical light incidence on the facade glazing (usually,
Sh,GDF,3
0,85 is considered to be adequate).
The effects of vertical and horizontal subdivisions in the space between the two facade layers can be
approximated by treating these as vertical and horizontal projections with the index values I and
Sh,vA
I . In glazed double facades, it is assumed that pollution of the space between the outer glazing and
Sh,hA
the space wall is negligible, so that it is usually adequate to take only the dirt deposited on the actual
facade surface into consideration [also refer to Formula (A.21)]. In this case, k = 1. The correction
Sh,GDF,2
factor for frames and subdivisions is calculated as follows:
area of structural components transparenta rea
k =−1 = (A.13)
Sh,GDF,,1 j
area of rawb uilldingc arcassopening area of rawb uildinggc arcassopening
Only that part of the external facade glazing which is projected onto the transparent portions of the
inner facade is taken into consideration when calculating the factor k .
Sh,GDF,1
Daylight factor of the raw building carcass opening
The index values I , I , and I can be used to calculate an approximate value for the daylight factor
Tr,j RD,j Sh,j
of the area being evaluated on the basis of the raw opening dimensions:
DI=+(,413200,,×−136i×II), n% (A.14)
Ca,Tjjr, RD,Sjjh,
For combinations of a larger space depth index value I and low transparency index values I ,
RD,j Tr,j
Formula (A.14) might produce negative D values. In such cases, D shall be assumed to be zero or
Ca,j Ca,j
shall be calculated by a more detailed method. For simple estimates, daylight availability can be grouped
into four classes as shown in Table A.1.
Table A.1 — Daylight availability classification as a function of the daylight factor D of the
Ca,j
raw building carcass opening
Daylight factor Classification of daylight
D availability
Ca,j
D ≥ 6 % Strong
Ca,j
6 % > D ≥ 4 % Medium
Ca,j
4 % > D ≥ 2 % Low
Ca,j
D < 2 % None
Ca,j
If a daylight factor which has been calculated using another validated method is known, then this can
be used instead of the value calculated by Formula (A.14) when classifying the daylight availability
according to Table A.1. In this case, the daylight factor shall have been determined on the basis of the
mean value of the daylight measured on the axis running parallel to the respective facade section and at
a distance of half the space depth from the facade.
A.3.2 Daylight supply factor
The following section first explains how the facade characteristics are to be described and then how the
daylight availability is determined on the basis of the correlation of the daylight availability (daylight
factor) of the building area as defined in A.3.1 with the facade characteristics. The light passing through
facade systems and the associated illumination of the adjacent space depends on the spatial and temporal
distribution of the external illuminance conditions in relation to the facade element and the spatial
distribution of the light by the facade system (i. e. its optical and control-technological characteristics).
From the lighting-engineering aspect, two facade states shall be distinguished for facades with variable
solar light shading systems and/or glare-protection systems.
— solar and/or glare protection system not activated, i. e. the sun is not shining on the facade;
— solar and/or glare protection system is activated, i. e. the sun is shining on the facade.
The daylight supply factor F shall be determined using Formula (A.15) to achieve temporal weighting
D,s,j
of the orientation-dependent occurrence of two different facade states, i. e. either with activated solar
and/or glare protection or with de-activated solar and/or glare protection. The protection against solar
radiation and/or glare is activated as soon as direct sunlight shines on the facade.
20 © ISO 2014 – All rights reserved

Formula (A.15) is used to calculate the daylight availability factor F .
D,s,j
Ft=+Ft F (A.15)
D,s,jjrel,D,SNA, D,s,SNA,jjrel,D,SA,D,s,SA,j
where
t is the relative portion of the total operating time during which the solar and/or glare
rel,D,SNA,j
protection system is not activated as given in Table A.3. It is a function of the latitude γ
of the considered site, H /H representing the climate and facade orientation;
dir glob
t is the relative portion of the total operating time during which the solar and/or glare
rel,D,SA,j
protection system is activated. t can be obtained by t = 1 – t ;
rel,D,SA rel,D,SA rel,D,SNA
F is the daylight availability factor of the area j being evaluated at times when the solar
D,s,SNA,j
and/or glare protection system is not activated, as given in Table A.5. It is a function
of the latitude γ of the considered site, H /H representing the climate, the facade
dir glob
orientation, daylight availability (daylight factor), and the maintained illuminance;
F is the daylight availability factor of the area j being evaluated at times when the solar
D,s,SA,j
and/or glare protection system is activated, as given in Table A.8.
A set of ratios H /H for representative locations worldw
...