1. Daylighting Modeling
Technique in Built-
environment Study
Dr. Ernest K W TSANG
Sustainability Consultant

2. AGENDA
Why Daylighting
Assessments / Terms
Common Global
Illumination Program
RADIANCE System
Common Parameters
Common Misunderstanding
in Global Illumination

3. Why Daylight?
Enhances the Phase Synchronising Ability of
Light
Improves Circadian Photobiological
Activation
Has Positive Effects on Sociability and
Hormone Patterns
Prevents Sick Building Syndrome
Provides Energy Saving Opportunities

4. Assessments and Terms

5. Sun and Sky
Diffuse illuminance (Evd)
also called Skylight - Solar Radiation reaches the Earth as
a Result of Scattering in the Atmosphere
Direct illuminance (Evs)
also called sunlight - Solar Radiation reaches the Earth’s
Surface as Parallel Rays, directly from the Sun’s Disc,
after Selective Attenuation by the Atmosphere
Global Illuminance (Evg)
Evg = Evd + Evs

6. CIE Standard
Overcast Sky
Completely Overcast Sky
Sky being 3 times Brighter Overhead
than Horizon

7. Daylight Factor (DF)
Illuminance received at a Point
Indoors, expressed as a Percentage of
Evd from an UNOBSTRUCTED sky.
Containing Sky Component (SC),
Externally Reflected Component (ERC)
and the Internally Reflected
Component (IRC)

9. Vertical Daylight
Factor (VDF)
Illuminance received at a Point on a
VERTICAL OUTDOOR Surface, expressed
as a Percentage of Evd from an
UNOBSTRUCTED sky. Containing SC and
ERC.
Applied in PNAP APP-130 Lighting and
Ventilation Requirements –
Performance-based Approach

10. Daylight Glare Index
(DGI)
Glare from Windows can arise from
Excessive Contrast between the
Luminance of the Visible Sky and the
Luminance of the Internal Surfaces
within the Field of View.

11. Daylight Autonomy
“The % of Aggregate Floor Area of Regularly
Occupied Spaces which achieves a minimum
highly Illuminance value of 300 lux at Task
Level for at least 50% of the hours between
8am to 6pm, Local Clock Time, after accounting
for Typical Weather Conditions, Exterior
Obstructions, Attached Furniture Systems and
after Blinds have been operated Hourly to
block Direct Sun predicted to enter the Space
that would fall on more than 2% of the
Calculation Grid.” LEED 2012 Draft 3

12. Global Illumination
Maxwell’s Electromagnetic
wave equation
Radiosity
Ray-tracing

13. Maxwell’s Electromagnetic
wave equation
Treating Light as EM Wave
Could model the behaviours
(Reflection, Refraction, Diffraction,
Interference) of Light
Uncommon neither in Daylighting
Research or Design

14. 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

15. Raytracing
Determine the Visibility of Surfaces
by Tracing Imaginary Rays of Light
from Viewer’s eye.
Account for every Optical Phenomenon
that can Analytically expressed by
Physical Equation
Can consider specular material

16. RADIANCE
Introduction of a
Simulation Package

17. What RADIANCE is?
Collection of 50+ programs
Run Under UNIX System
Provide Correct Numerical Result and
Renderings that are Indistinguishable
from Photograph
Able to Predict Reality
Latest Version 4.1

18. Overview of RADIANCE

19. Sky Description

20. Material Description
Non light-
Light-emitting
emitting Virtual material
material
material
Light Mirror Mist
Illum Prism Antimatter
Glow Plastic
Spotlight Metal
Trans
Dielectric
Glass
Interface

21. Material Reflection

22. Material
Descriptions

23. Transmission Characteristic for
Transparent Surface

24. Geometry
Polygon Ring Cylinder
Tube Cone Cup
Sphere Bubble

25. Geometry
Descriptions

26. Lighting
Descriptions similar to that for
Other Material
Built-in an ies2rad Program to assist
Designers converting IES File (IESNA)
to RADIANCE Description File

27. Examples of RADIANCE
Output
Physically-based Rendering, False Colour
Diagram, Sunpath Diagram

28. Physically-based
Rendering

29. Falsecolour
Presentation

30. Glare and Sunpath
Analysis

31. Common Parameters
Sky, Ambient Settings

32. Sky Model
A simple program gensky is included
in Radiance. This program can create
a sky description file for the
following sky:
CIE overcast sky
Uniform sky
CIE clear sky (1973)
CIE clear turbid sky

33. CIE Standard General
Skies
5 Clear, 5 Intermediate and 5
Overcast Sky Types
Any Standard Sky can be considered as
combining the Gradation function φ(Z)
and Indicatrix Function f(χ)

34. Gradation Function
The Standard Gradation Equation
relates Sky Luminance to the Angular
Distance from the Zenith

35. Scattering
Indicatrix
The Relative Scattering Indicatrix
Function models Sky Luminance with
Angular Distance from the Sun.

36. CIE Standard General
Skies
No Type of sky
Overcast with the steep gradation and azimuthal
1
uniform
Overcast with a steep gradation and slight brightening
2
toward sun,
Overcast moderately gradated, azimuthal uniformity
3
Overcast moderately gradated and slightly brightening
4
toward sun
Overcast or cloudy with overall uniformity
5

37. CIE Standard General
Skies
No Type of sky
Partly cloudy with a uniform gradation and slight
6
brightening toward sun
Partly cloudy with a brighter circumsolar effect and
7
uniform gradation
Partly cloudy, rather uniform with a clear solar corona
8
Partly cloudy with a shaded sun position
9
Partly cloudy with brighter circumsolar effect
10

38. CIE Standard General
Skies
No Type of sky
White – blue sky with a clear solar corona
11
Very clear / unturbid with a clear solar corona
12
Cloudless polluted with a broader solar corona
13
Cloudless turbid with a broader solar corona
14
White – blue sky, turbid with a wide solar corona
15 effect

39. Sky Luminance
Distribution

40. Ambient Bounces (-ab)
Control the number of Bounce
(Reflection)
Zero implies no Indirect Calculation
Doubling -ab doubles the Rendering
Time

41. Effect of -ab

42. Ambient Divisions (-ad) and
Ambient Super-samples (-as)
To reduce the Errors in Monte Carlo
Calculation of Indirect Illuminance
Error reduce in a Inversely Proportional
to the Square Root of this Value
Addition Sampling will be done if -ad
shows a significant Change
Doubling this value doubles the
Rendering Time

43. Effects of -ad
and -as
and -as

44. Incorrect -ad
Setting

45. Ambient Accuracy (-aa) and
Ambient Resolution (-ar)
-aa controls the Error from Indirect
Illuminance Calculation
-ar determines the Maximum Density of
Ambient Values used in Interpolation.
The Maximum Ambient Value Density is the
Scene Size times -aa divided by -ar.
Doubling these Two Values quadruples the
Time for Rendering

46. Effect of -ar
and -aa
and -aa

47. Incorrect -ar
Setting

48. Common Misunderstandings
in Global Illumination

49. Use Large Surface as
Possible
YES.... but also NO
Recall the Definitions of -aa and -ar
The -ar Setting is proportional to
Minimum/Average Size
An Oversize Surface in the Scene, -ar
need to be increased. And Sometimes
it requires a very large -ar.

50. Using a single Surface
for Multiple Surfaces
Sure!

51. Using a single Surface
for Multiple Surfaces
How about these?

52. Luminance Variation on a Non-flatted
Surface

53. Conclusions
Review Major Simulation Technique involved
in Global Illuminance
Introduces RADIANCE System includes the
inputs and outputs
Review the Parameters and Items affecting
the Accurcy of simulation
Introduces two of the Major Misunderstanding
in Global Illumination

54. Q&A
Thank you!