2. Agenda
Evaluation method for daylighting design
Sophisticated computational method
Standard skies
Comparing of existing assessment
methods
Shortcomings of existing method and the
use of climate-base daylight modeling

3. Evaluation method for daylighting
design
• Scale-model measurement
• Simplified calculation method
• Sophisticated computational method

4. Scale-model measurement
• Building professionals use scale models
as design tools to study various aspects
of building design and construction
• Widely recognised by building
professionals for years
• Scale models portray the distribution of
daylight within the model room almost
as exactly as in a full-size room.

5. Suggested scale of model
Scale Application(s)
1:200 - 1:500 For preliminary design and concept development
To provide a gross sense of the massing of the project
To study the shadow created by the future building or from a
neighbouring building
1:200 - 1:50 To study direct sunlight penetration into a building (e.g. efficiency
of solar protection
To study diffuse daylight in a very big space (e.g. atrium)
1:100 - 1:10 To consider detailed refinement of spatial components
To have highly detailed inside view (e.g. video or photos)
To study accurately diffuse and direct dayligth penetration
1:1-1:10 To integrate critical industrial components
To consider daylighting devices that cannot be reduced in scale
To proceed to final evaluation of advanced daylighting systems
through monitoring and user assessment

6. Real-sky measurement
• Overcast Sky
– 0.36 < foc < 0.44 (True Value = 0.396)
– Any time of a year
• Clear Sky
– winter solstice (+/- 4 weeks, 1 day)
– equinox, either spring or autumn (+/- 4 weeks, 1
day). In locations where there is a significant
discrepancy between the spring and the autumn
equinox, it is recommended to measure during
both equinox.
– summer solstice (+/- 4 weeks, 1 day)

7. Artificial sky measurement
• Artificial sky
• Sky-simulator

8. Artificial Sky

9. Sky simulator

10. Simplified calculation method
• Daylight factor
• Average daylight factor
• Vertical daylight factor

11. Daylight factor
Where,
DF = daylight factor (%)
Ev = illuminance level of a point (lx)
Evd = horizontal diffuse illuminance (lx)

12. Average daylight factor
Where,
DFave = average daylight factor (%)
τ = transmittance of window
(dimensionless)
W = window area (m2)
θ = angle subtended by the visible sky
(rad)
A = area of internal surface (m2)
R = average reflectance (dimensionless)

13. Vertical daylight factor
Where,
VDF = vertical Daylight Factor (%)
Ews = illuminance for light reflected onto the
window opening from unobstructed
sky(lx)
Ewr = illuminance for light reflected onto the
window opening from surrounding
building (lx)
ρg = ground reflectance (dimensionless)
Evrg = ground illuminance (lx)

14. Sophisticated computational
method
• Maxwell’s electromagnetic wave
• Radiosity
• Ray tracing

15. Maxwell’s electromagnetic wave
• Treating light as an EM wave
• Could model the behaviours (reflection,
refraction, diffraction, interference) of
light
• Uncommon neither in daylighting
research or design

16. Radiosity
• Originally developed for energy calculation
(energy balance for a set of surfaces
exchanging radiant energy)
• Surfaces must be subdivided into finite
elements
• All elements are assumed to be perfectly
diffuse
• View independent
• Pre-eminence of major light sources

17. Ray tracing
• Determine the visibility of surfaces by
tracing imaginary rays of light form
viewer’s eye.
• Account for every optical phenomenon that
can analytically expressed by physical
equation
• Can consider specular material
• Include forward and backward ray tracing
approaches

18. Examples of simulation package
Algorithm Package
Radiosity AGI32 (www.agi32.com)
Lumen Micro (www.ltioptics.com –
discontinued)
Forward ray tracing Photopia (www.ltioptics.com)
TracePro (www.lambdares.com)
OptiCAD (www.opticad.com)
Backward ray tracing RADIANCE (radsite.lbl.gov)
Desktop Radiance (radsite.lbl.gov –
without any update since 2002)

19. Major components in Radiance
system
Climate dependent parameter Climate independent parameters
Sky Material Geometry Lighting
Converter
Binary file
Calculation Rendering
High dynamic
ASCII results
range image

20. Sky description
• A program gensky is included in
Radiance. This program can create a
sky description file for the following
skies:
– CIE overcast sky
– Uniform sky
– CIE clear sky
– CIE intermediate sky

21. Sky description
Sky brightness,
distribution pattern and
Sun solar position
brightness
Sky brightness
multiplier and
colour correction
Solar
position
and size
Sky vault direction
and angular size

22. Material description
Major materials in Radiance:
Light-emitting Non light-emitting Virtual material
material material
Light Mirror Mist
Illum Prism Antimatter
Glow Plastic
Spotlight Metal
Trans
Dielectric
Glass
Interface

23. Material description
Incident
light Specular
reflection
Diffuse
reflection

24. Material description
Surface type
Colour of material
Roughness
Specularity

25. Geometry
• The following geometries are available
in Radiance:
Polygon Ring Cylinder
Tube Cone Cup
Sphere Bubble

26. Geometry description
Material
Coordinate of vertices
Type of surface

27. Lighting
• Descriptions similar to that for other
material
• Built-in an ies2rad program to assist
designers convert IES file (IESNA) to
radiance description file

28. Renderings – simulation model

29. Physically based renderings
Daylight Artificial light False-colour image

30. Daylight factor analysis
False-colour image of Contour lines overlay on model
daylight factor on working
plane

31. Glare and sunpath studies
Glare-source identification Sunpath diagram

32. Sky type
• CIE overcast sky
• CIE clear sky
• CIE standard skies

33. Lights coming from sun and
sky

34. CIE overcast sky
Where,
L = sky luminance in an arbitrary sky
element (cd/m2)
Lz = sky luminance at the zenith (cd/m2)
Z = zenith angle of a sky element (rad)

35. CIE clear sky
Where,
f(χ) = indicatrix function (dimensionless)
φ(Z) = gradation function (dimensionless)
χ = scattering angle (rad)
Zs = solar zenith angle (rad)

36. CIE standard skies
Where,
f(χ) = indicatrix function (dimensionless)
φ(Z) = gradation function (dimensionless)

37. Daylighting requirements for
LEED and BEAM Plus
Sky type Time Criteria
LEED (2.1 and 2.2) Clear sky 12:00 on equinox 75% of area
EQ Credit 8.1 achieves 25 fc (269
lx) or more
LEED (3.0) Clear sky 9:00 and 15:00 on 75% of area falls
EQ Credit 8.1 equinox between 25 fc (269
lx) and 500 fc
(5381.9 lx)
BEAM Plus (1.1) Overcast sky N/A 80% of area
IEQ 15 achieves a daylight
factor 2% or more

38. Questions?
• Which clear sky should we choose?
• Does the CIE overcast sky really represent
the worst scenario?
• Does the current assessing criteria sufficient?

39. Which clear sky should we
choose?
• CIE clear or CIE standard skies
Sky model Type of sky
11 White – blue sky with a clear solar corona
12 Very clear / unturbid with a clear solar corona
13 Cloudless polluted with a broader solar corona
14 Cloudless turbid with a broader solar corona
15 White – blue sky turbid with a wide solar corona effect

40. Which clear sky should we
choose?
Sky model Direct sun (lx) Diffuse sky (lx)
11 86,737 28,539
12 102,048 12,408
13 82,164 34,743
14 77,831 34,743
15 86,739 37,225
Radiance
built-in CIE 86,024 9,018
clear sky
At solar noon on the equinox

41. Sky luminance distribution
CIE Clear Sky 11 Sky 12
Relative
luminance
(%)
Sky 13 Sky 14 Sky 15

42. Which clear sky should we
choose?
• ANSI/ASHRAE/USGBC/IES Standard 189.1-
2009 “Standard for the Design of High-
Performance Green Buildings Except Low-
Rise Residential Buildings”
• Clause 8.5.1.1
– “Simulation shall be done using either CIE
Overcast Sky Model or the CIE Clear Sky Model”

43. Does the CIE overcast sky really
represent the worst scenario?
North-facing window at the
noon on equinox
(Both skies are generated
CIE Clear Sky CIE Overcast Sky by gensky)

44. Does the current assessing
criteria sufficient?
• Does the monitoring period long
enough?
• Is this city dominated by clear or
overcast sky?
• Does the weather data enough for
conducting long-term analysis?

45. Climate-base daylight
modeling
• Based on measured / modeled outdoor
illuminance/luminance distribution data
• Algorithm – Daylight coefficient approach
• Define a range of useful daylight level (e.g.
300lx – 2,500lx)
• Provide annual daylighting performance
analysis
• Example of a Radiance Based program is
DAYSIM (www.daysim.com)

46. Questions & Answers
Session