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

Evaluation method for daylighting design

• Scale-model measurement

• Simplified calculation method

• Sophisticated computational method

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.

Suggested scale of modelScale Application(s)

1:200 - 1:500 For preliminary design and concept developmentTo provide a gross sense of the massing of the projectTo 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 protectionTo study diffuse daylight in a very big space (e.g. atrium)

1:100 - 1:10 To consider detailed refinement of spatial componentsTo 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 componentsTo consider daylighting devices that cannot be reduced in scaleTo proceed to final evaluation of advanced daylighting systems through monitoring and user assessment

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)

Artificial sky measurement

• Artificial sky

• Sky-simulator

Artificial Sky

Sky simulator

Simplified calculation method

• Daylight factor

• Average daylight factor

• Vertical daylight factor

Daylight factor

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

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)

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)

Sophisticated computational method

• Maxwell’s electromagnetic wave

• Radiosity

• Ray tracing

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

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

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

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)

Major components in Radiance system

MaterialSky Geometry Lighting

Converter

Binary file

Calculation

ASCII results

Rendering

High dynamic range image

Climate dependent parameter Climate independent parameters

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

Sky description

Sun brightness

Solar position and size

Sky brightness, distribution pattern and solar position

Sky brightness multiplier and colour correction

Sky vault direction and angular size

Material description

Light-emitting material

Non light-emitting material

Virtual material

Light Mirror Mist

Illum Prism Antimatter

Glow Plastic

Spotlight Metal

Trans

Dielectric

Glass

Interface

Major materials in Radiance:

Material description

Diffuse reflection

Specular reflection

Incident light

Material descriptionSurface type

Colour of material

Roughness

Specularity

Geometry

• The following geometries are available in Radiance:

Polygon Ring Cylinder

Tube Cone Cup

Sphere Bubble

Geometry description

Coordinate of vertices

Material

Type of surface

Lighting

• Descriptions similar to that for other material

• Built-in an ies2rad program to assist designers convert IES file (IESNA) to radiance description file

Renderings – simulation model

Physically based renderings

Daylight Artificial light False-colour image

Daylight factor analysis

Contour lines overlay on modelFalse-colour image of daylight factor on working

plane

Glare and sunpath studies

Glare-source identification Sunpath diagram

Sky type

• CIE overcast sky

• CIE clear sky

• CIE standard skies

Lights coming from sun and sky

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)

CIE clear sky

Where,f(χ) = indicatrix function (dimensionless)φ(Z) = gradation function (dimensionless) χ = scattering angle (rad)Zs = solar zenith angle (rad)

CIE standard skies

Where,f(χ) = indicatrix function (dimensionless)φ(Z) = gradation function (dimensionless)

Daylighting requirements for LEED and BEAM Plus

Sky type Time Criteria

LEED (2.1 and 2.2)EQ Credit 8.1

Clear sky 12:00 on equinox 75% of area achieves 25 fc (269 lx) or more

LEED (3.0)EQ Credit 8.1

Clear sky 9:00 and 15:00 on equinox

75% of area falls between 25 fc (269 lx) and 500 fc (5381.9 lx)

BEAM Plus (1.1)IEQ 15

Overcast sky N/A 80% of area achieves a daylight factor 2% or more

Questions?

• Which clear sky should we choose?• Does the CIE overcast sky really represent

the worst scenario?• Does the current assessing criteria sufficient?

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

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 clear sky

86,024 9,018

At solar noon on the equinox

Sky luminance distribution

CIE Clear Sky 11 Sky 12

Sky 13 Sky 14 Sky 15

Relative luminance (%)

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”

Does the CIE overcast sky really represent the worst scenario?

CIE Clear Sky CIE Overcast Sky

North-facing window at the noon on equinox(Both skies are generated by gensky)

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?

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)

Questions & Answers Session