ISO/CIE 10916:2024

International
Standard
ISO/CIE 10916
First edition
Light and lighting — Energy
2024-11
performance of lighting in buildings
— Calculation of the impact of
daylight utilization
Lumière et éclairage — Performance énergétique de l'éclairage
des bâtiments — Calcul de l'impact de l'utilisation de la lumière
du jour
Reference number
© ISO/CIE 2024
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Published in Switzerland
© ISO/CIE 2024 – All rights reserved
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols, indices, and abbreviated terms . 4
4.1 Symbols .4
4.2 Indices and abbreviated terms .5
5 Proof calculation method . 6
5.1 Energy demand for lighting as function of daylight .6
5.2 Subdivision of a building into zones .8
5.3 Operating time .8
5.4 Electric lighting.8
5.5 Constant illuminance control .8
5.6 Daylight .8
5.7 Occupancy dependency factor F .9
O,n
6 Daylight Performance Indicator. 9
Annex A (informative) Simple calculation method . 10
Annex B (informative) Comprehensive hourly calculation .54
Annex C (informative) Daylight performance indicator .89
Annex D (informative) Examples .90
Bibliography .97

© ISO/CIE 2024 – All rights reserved
iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
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The procedures used to develop this document and those intended for its further maintenance are described
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This document was prepared by Technical Committee ISO/TC 274, Light and lighting, in collaboration with
the International Commission on Illumination (CIE).
This first edition of ISO/CIE 10916 cancels and replaces ISO 10916:2014, which has been technically revised.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

© ISO/CIE 2024 – All rights reserved
iv
Introduction
This document is part of a set of standards which allows users to rate the overall energetic performance
of buildings. Facades and rooflights have a key impact on the building’s energy balance. This document
supports daylighting and lighting-energy-related analysis and optimization of facade and rooflight systems.
It is 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.

© ISO/CIE 2024 – All rights reserved
v
International Standard ISO/CIE 10916:2024(en)
Light and lighting — Energy performance of lighting in
buildings — Calculation of the impact of daylight utilization
1 Scope
This document 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 document is applicable for existing buildings and the
design of new and renovated buildings.
This document 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 electric 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 affect 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 analysis, an approach to calculate the effect of daylight on the lighting energy demand on an
hourly or sub-hourly basis is provided. Unlike the simple table-based annual calculation approach, which is
regression based, this method relies on an emulation concept. Relevant quantities are modelled explicitly
and are then interacting directly with sensors, actuators and functional elements of the building automation
and control system (BACS) or are triggering user interaction. By this approach, model configuration and
parametrization from the design stage can seamlessly be used in the BACS configuration.
To support overall building performance assessment, additional daylight performance indicators on the
overall building level are provided.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
CIE S 017:2020, ILV: International Lighting Vocabulary

© ISO/CIE 2024 – All rights reserved
3 Terms and definitions
For the purposes of this document, the terms and definitions given in CIE S 017, and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
CIE maintains a terminology database for use in standardization at the following address:
— e-ILV: available at https:// cie .co .at/ e -ilv
3.1
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;
— electric lighting in relation to the currently available daylight;
— systems used to detect and record the presence of occupants
3.2
daylight factor
D
quotient 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 and 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
Note 1 to entry: Glazing, dirt effects, etc. are included.
Note 2 to entry: When calculating the lighting of interiors, the contribution of direct sunlight has to be considered
separately.
Note 3 to entry: CIE S 017:2020 defines the unit as 1. However, daylight factor is in practice, usually presented in
percent values.
Note 4 to entry: The term daylight factor is normally used when considering an overcast sky as sky type 1 or 16 in
ISO 15469.
[SOURCE: CIE S 017:2020, 17-29-121, modified — Notes 3 to 5 deleted, new Notes to entry 3 and 4 added]
3.3
electrical power of electric lighting system
P
total electrical power consumption of the lighting system in the considered space
3.4
illuminance
E
density of incident luminous flux with respect to area at a point on a real or imaginary surface
dΦ
v
E= ,
dA
where
© ISO/CIE 2024 – All rights reserved
Φ is luminous flux;
v
A is the area on which the luminous flux is incident
−2
Note 1 to entry: The illuminance is expressed in lux (lx = lm ⋅ m ).
[SOURCE: CIE S 017:2020, 17-21-060, modified — Notes 1, 2, 4 and 5 deleted.]
3.5
insolation
incidence of solar radiation on a surface or body
3.6
luminaire
apparatus which distributes, filters or transforms the light transmitted from at least one source of optical
radiation and which includes, except the sources themselves, all the parts necessary for fixing and protecting
the sources and, where necessary, circuit auxiliaries together with the means for connecting them to the
power supply
[SOURCE: CIE S 017:2020, 17-30-001, modified — Notes deleted.]
3.7
luminous exposure
H
v
H
density of incident luminous energy with respect to area at a point on a real or imaginary surface
dQ
v
H =
v
dA
where
Q is the luminous energy;
v
A is the area on which the luminous energy is incident.
−2 -2
Note 1 to entry: The luminous exposure is expressed in lux second (lx⋅s = lm·s·m ) or lux hours (lx h = lm·h·m ).
[SOURCE: CIE S 017:2020, 17-21-072, modified — Notes 1, 2, 3, 5 and 6 deleted, added possibility to express
in lux hours.]
3.8
luminous flux
Φ
Φ
v
change in luminous energy with time
dQ
v
Φ =
v
dt
where
Q is the luminous energy emitted;
v
t is time.
Note 1 to entry: The luminous flux is expressed in lumen (lm).
[SOURCE: CIE S 017:2020, 17-21-039, modified— Notes 1, 2, 3, 5 and 6 deleted.]

© ISO/CIE 2024 – All rights reserved
3.9
maintained average illuminance
E
m
value below which the average illuminance over the specified surface is not allowed to fall
Note 1 to entry: In specific contexts of this document the maintained average illuminance can in limit case be the
maintained point illuminance
−2
Note 2 to entry: Unit: lx = lm ⋅ m .
3.10
shading
anything inside, in between or outside the window which prevents the direct view of part of the sky
Note 1 to entry: Shading can be manually operated or automatic and can as well be moveable or fixed.
Note 2 to entry: For example shutters, external or internal blinds.
3.11
daylight opening
any area in the building envelope that is capable of admitting daylight to an interior
3.12
rooflight
daylight opening (3.11) on the roof or on a horizontal surface of a building
3.13
task area
partial area in the work place in which the visual task is carried out
[SOURCE: CIE S 017:2020, 17-29-171, modified — Notes deleted.]
3.14
visual task
visual elements of the work being done
[SOURCE: CIE S 017:2020, 17-22-084, modified — Notes deleted.]
4 Symbols, indices, and abbreviated terms
4.1 Symbols
Quantity Unit
Φ luminous flux lm
Q energy kWh
γ angle, geographical latitude °
δ declination of the sun °
a depth m
A area m
b width m
D daylight factor —
mean daylight factor —
D
E illuminance lx
-
E irradiance
W·m
e
E maintained illuminance lx
m
f, F factors —
© ISO/CIE 2024 – All rights reserved
Quantity Unit
H luminous exposure lx h
h height m
I index —
I Transparency index —
Tr,j
I Space depth index —
RD,j
I Shading index —
Sh,j
I Linear shading, correction factor —
Sh,lsh
I Horizontal projections, correction factor —
Sh,hf
I Vertical projections, correction factor —
Sh,vf
k space index —
si
k correction factor —
cf
t time s, h
v distribution key —
4.2 Indices and abbreviated terms
A absence ND no daylight
At atrium Night night-time
c control O occupancy
Ca carcass opening R, Room room
D daylight rel relative
Day day-time Rd room depth, space depth
transparent or translucent surface of
dir direct s
t
the daylight aperture
D65 standard lightsource D65 s supply
s
e energy quantity SA sun-shading activated
Eff effective, root-mean-square
Sh shading, obstruction
eq equivalent
ext external, outdoors SNA sun-shading not activated
glazed curtain wall, glazed double
GDF start start
facade
sunrise sunrise
glob global
sunset sunset
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
Month Month
mth monthly vf vertical fin or projection

© ISO/CIE 2024 – All rights reserved
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 which
l,f
can be subdivided into J evaluation areas:
J
N
QQ= (1)
lf,,lf ,,nj
∑∑
n==11j
The energy demand of any one evaluation area j is calculated by applying Formula (2) and Formula (3).
Qp=+ FA tt +At +t  (2)
() ()
lf,,nj,cjj,,Dejjff ,,DayD,,effNight ,,jjND eff ,,DayN,,Dejjff ,,Night
 
where
AA=+ A (3)
jjDN,,D 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 control
c,j
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 is
eff,Day,D,j
lit by daylight;
t is the effective operating time of the lighting system, during day-time, in area j which is
eff,Day,ND,j
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,jnDayD,,jjO,
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,O,j
© ISO/CIE 2024 – All rights reserved
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 area j as
D,j
defined in 5.6;
F is the part-utilization factor to account for the presence of persons (occupancy) in the evaluation
O,j
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,jnNightO,,j
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.
Figure 1 — Flowchart showing calculation of the energy demand for lighting

© ISO/CIE 2024 – All rights reserved
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 electric lighting system, lighting control systems, characteristics of the facades).
From practical experience, a simplification rule can be recommended: 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 to
Night,n n
t + t . Day-time is thus the time span between sunrise and sunset. Annual daylight hours and night
Day,n Night,n
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 may be determined separately. This may follow the
scheme of Table A.21, i.e. specifying the specific data in columns 3, 4, 5 and 6. Data for columns 8 and 9 are
separated according to the above described split if the operating time into t and t .
Day Night
5.4 Electric lighting
The specific electrical power of the electric light installation p can be obtained by, for instance, using
j
standard lighting design software, as provided by luminaire manufacturers. Simplified methods, as defined
in ISO/CIE 20086, 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 electric
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,jjD,sD,,,c j
© ISO/CIE 2024 – All rights reserved
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. This
D,s,j
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 corresponds
to the relative luminous exposure as the ratio of luminous flux by daylight up to the required maintained
illuminance to the total luminous flux for the required maintained illuminance over operation time.
NOTE The relative luminous exposure is also referred to as “daylight autonomy”.
The factor F additionally accounts for the efficiency of the lighting control system in using the available
D,c,j
daylight to achieve the required luminous exposure level in the area j. The daylight dependency factor F
D,j
which takes the daylight illumination into consideration can be determined for any given time interval (e.g.
year, month, hour).
Annex A comprises simplified approaches to calculate F for vertical facades (A.3) and rooflights (A.4)
D,S,j
and to obtain tabulated values for F . Annex B contains a methodology for detailed generally computer-
D,c,j
based comprehensive calculations to calculate the effect of daylight on the lighting energy demand on an
hourly or sub-hourly basis.
A calculation example each for daylight penetration through a vertical facade and a rooflight is given in
Annex D.
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
O,n
with the efficiency to benefit from potential energy savings by either manual or automatic switching.
Parametrizations of F can, for instance, be found in ISO/CIE 20086.
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.

© ISO/CIE 2024 – All rights reserved
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 depicted in Figure A.1:
D,n,j Day,n,j eff,Day,n,j
— 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 vertical
D,S,n,j
facades;
— A.4 specifies a procedure on how to determine the daylight supply factor F for spaces lit by rooflights;
D,S,n,j
— 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.
© ISO/CIE 2024 – All rights reserved
Figure A.1 — Flowchart illustrating the simplified approach

© ISO/CIE 2024 – All rights reserved
For the determination of the daylight supply factor F , as well for a vertical facade (A.3) as for rooflights
D,S,n,j
(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 ρ = 0,7
F W C
for the ceiling. The reflectance of the external surroundings is assumed to be 0,2. Instead of 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 a computer program.
— 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). Figure A.3 shows the impact of a facade opening on the daylight area for vertical facades.
— Depth and width of the daylight area lit by vertical facades
The maximum possible depth a of the area A lit by daylight entering via a facade is calculated using
D,max,j D,j
Formula (A.1).
ah=×25, −h (A.1)
()
D,maxL,j iTa
where
a is the maximum depth of the daylight area;
D,max.j
h is the height of the window lintel above the floor;
Li
h is the height of the task area above the floor.
Ta
© ISO/CIE 2024 – All rights reserved
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 . The a value 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 the
D
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,maxR,,jjTa
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. Figure A.4
D,j
shows the impact of a roof opening on the daylight area for rooflights.
— 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. Figure A.5 shows the superposition and penetration of daylight
areas for vertical facades and rooflights.

© ISO/CIE 2024 – All rights reserved
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

© ISO/CIE 2024 – All rights reserved
c) Approach 2: The more favoura-
a) Daylight areas of the dif- b) Approach 1: Superposition of ble lighting condition is taken for
ferent vertical facades (1 – 5) daylight factors in the overlapping the respective daylight
and the rooflight (6) daylight areas area: F > F > F = F > F
D,S,1 D,S,6 D,S,5 D,S,3
> F
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 the
[3]
work plane is given for individual utilization profiles in DIN V 18599-10 .

© ISO/CIE 2024 – All rights reserved
— 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
— Shading 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 those that 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
Sh,j
calculated using Formula (A.6).
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 shading index of the area j under evaluation;
Sh,j
I is the correction factor for linear shading of the area under evaluation as calculated using
Sh,lsh
Formula (A.7);
I is the correction factor for an overhang shading of the area being evaluated, calculated using
Sh,hA
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 evaluation
Sh,In,At
as calculated using Formula (A.11);
I is the correction factor for glazed double facades of the area being evaluated, calculated using
Sh,GDF
Formula (A.12).
NOTE A reduction by thick walls in relation to the carcass opening can be approximately described using the
factors I and I .
Sh,hA Sh,vA
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
© ISO/CIE 2024 – 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.6. Formula (A.7) is used
to calculate the correction factor which accounts for linear shading.
I = cos(1,5 × γ )       for γ < 60°
Sh,lsh Sh,lsh Sh,lsh
I = 0                     for γ ≥ 60° (A.7)
Sh,lsh Sh,lsh
where γ is the shading altitude angle as shown in Figure A.6.
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 facade
Sh,hf
section being evaluated for lighting aspects (on the plane of the external wall surface) as shown in Figure A.7.
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
I = 0                       for γ ≥ 67,5° (A.8)
Sh,hf Sh,hf
where γ is the horizontal shading angle due to a horizontal projection as shown in Figure A.7.
Sh,hf
— Vertical projections, correction factor I
Sh,vf
Figure A.8 — Cross-section (“top view”) 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 section
Sh,vf
being evaluated for lighting aspects (on the plane of the external wall surface) as shown in Figure A.8.
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.8.
Sh,vf
— Courtyards and atria (glazed forecourts), correction factor I
Sh,In,At
© ISO/CIE 2024 – All rights reserved
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 I :
well
ha ()+b
In ,,At In At In ,At
I = (A.10)
well
2  × ab
In ,,At In At
NOTE In daylight literature wi is used as symbol for the well index.
When determining the well index I for an area being evaluated, the height measured from the floor of the
well
respective area is considered to be the height of the courtyard or atrium (Figure A.9).
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 evaluated;
In,At
I is the well index used to account for the geometry of the courtyard or atrium.
well
Figure A.9 — Illustration of the geometrical parameters used to define the well index I
well
The correction factor for taking into consideration building shading in internal courtyards, light wells, or
atria is:
I = 1 − 0,85 I                                             for internal courtyards
Sh,In,At well
I = τ k k k (1 − 0,85 I )      for atria (A.11)
Sh,In,At Sh,In,At,D65 Sh,In,At,1 Sh,In,At,2 Sh,In,At,3 well
I = 0                                                         for I > 1,18
Sh,In,At well
© ISO/CIE 2024 – All rights reserved
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 is
Sh,In,At,3
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 incidence;
Sh,GDF,D65
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, 0,85
Sh,GDF,3
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 I .
Sh,vA Sh,hA
In glazed double facades, it is assumed that pollution of the space between the outer glazing and 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 factor for frames
Sh,GDF,2
and subdivisions is calculated as follows:
area of structural components transparenta rea
k =−1 = (A.13)
Sh,,GDF 1,j
area of rawb uilldingc arcass opening area of rawb uildinggc arcass opening
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
...