Daylighting Modeling Technique in Built-environment Study

Dr. Ernest K W TSANGSustainability Consultant

AGENDAWhy Daylighting

Assessments / Terms

Common Global Illumination Program

RADIANCE System

Common Parameters

Common Misunderstanding in Global Illumination

Why Daylight?

Enhances the Phase Synchronising Ability of LightImproves Circadian Photobiological ActivationHas Positive Effects on Sociability and Hormone PatternsPrevents Sick Building SyndromeProvides Energy Saving Opportunities

Assessments and Terms

Sun and SkyDiffuse 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

CIE Standard Overcast Sky

Completely Overcast SkySky being 3 times Brighter Overhead than Horizon

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)

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

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.

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

Maxwell’s Electromagnetic wave equationRadiosityRay-tracing

Global Illumination

Maxwell’s Electromagnetic wave equation

Treating Light as EM WaveCould model the behaviours (Reflection, Refraction, Diffraction, Interference) of LightUncommon neither in Daylighting Research or Design

Radiosity

Originally developed for Energy Calculation (Energy Balance for a set of Surfaces exchanging Radiant Energy)Surfaces must be subdivided into Finite ElementsAll elements are assumed to be Perfectly DiffuseView IndependentPre-eminence of major light sources

RaytracingDetermine 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 EquationCan consider specular material

RADIANCEIntroduction of a Simulation

Package

What RADIANCE is?Collection of 50+ programsRun Under UNIX SystemProvide Correct Numerical Result and Renderings that are Indistinguishable from PhotographAble to Predict RealityLatest Version 4.1

Overview of RADIANCE

Sky Description

Material DescriptionLight-emitting material Non light-emitting

material Virtual material

Light Mirror Mist

Illum Prism AntimatterGlow Plastic

Spotlight MetalTrans

DielectricGlass

Interface

Material Reflection

Material Descriptions

Transmission Characteristic for Transparent Surface

Geometry

Polygon Ring Cylinder

Tube Cone Cup

Sphere Bubble

Geometry Descriptions

Lighting

Descriptions similar to that for Other MaterialBuilt-in an ies2rad Program to assist Designers converting IES File (IESNA) to RADIANCE Description File

Examples of RADIANCE Output

Physically-based Rendering, False Colour Diagram, Sunpath Diagram

Physically-based Rendering

Falsecolour Presentation

Glare and Sunpath Analysis

Common ParametersSky, Ambient Settings

Sky ModelA simple program gensky is included in Radiance. This program can create a sky description file for the following sky:CIE overcast skyUniform skyCIE clear sky (1973)CIE clear turbid sky

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)

Gradation Function

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

Scattering Indicatrix

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

CIE Standard General Skies

No Type of sky

1 Overcast with the steep gradation and azimuthal uniform

2 Overcast with a steep gradation and slight brightening toward sun,

3 Overcast moderately gradated, azimuthal uniformity

4 Overcast moderately gradated and slightly brighteningtoward sun

5 Overcast or cloudy with overall uniformity

CIE Standard General Skies

No Type of sky

6 Partly cloudy with a uniform gradation and slightbrightening toward sun

7 Partly cloudy with a brighter circumsolar effect anduniform gradation

8 Partly cloudy, rather uniform with a clear solar corona

9 Partly cloudy with a shaded sun position

10 Partly cloudy with brighter circumsolar effect

CIE Standard General Skies

No 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

15White – blue sky, turbid with a wide solar corona effect

Sky Luminance Distribution

Ambient Bounces (-ab)

Control the number of Bounce (Reflection)Zero implies no Indirect CalculationDoubling -ab doubles the Rendering Time

Effect of -ab

Ambient Divisions (-ad) and Ambient Super-samples (-as)

To reduce the Errors in Monte Carlo Calculation of Indirect IlluminanceError reduce in a Inversely Proportional to the Square Root of this ValueAddition Sampling will be done if -ad shows a significant ChangeDoubling this value doubles the Rendering Time

Effects of -adand -as

Incorrect -ad Setting

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

Effect of -arand -aa

Incorrect -ar Setting

Common Misunderstandings in Global Illumination

Use Large Surface as Possible

YES.... but also NORecall the Definitions of -aa and -arThe -ar Setting is proportional to Minimum/Average SizeAn Oversize Surface in the Scene, -ar need to be increased. And Sometimes it requires a very large -ar.

Using a single Surface for Multiple Surfaces

Sure!

Using a single Surface for Multiple Surfaces

How about these?

Luminance Variation on a Non-flatted Surface

ConclusionsReview 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

Q&AThank you!