Daylighting Overview

What is Daylighting?

The application of daylighting in AGi32 allows you to consider the effect of sun, sky and ground as a light source in the environment. The daily rotation of  the earth as well as its location in its orbit about the sun produce a predictable amount of sunlight at a given location. As the sunlight enters the atmosphere, a portion of the light is scattered by the atmospheric conditions. The user may select one of two methods to emulate the sky conditions: the Perez All-Weather Sky Model, or IES/CIE general sky conditions, such as Clear, Partly Cloudy and Overcast and the 15 CIE general sky conditions.

AGi32 performs single moment in time calculations and can batch a number of single scenes using the Daylight Study command. Annual daylight simulations can be prepared in AGi32 but require our separate simulation tool Licaso to process. Please visit www.licaso.com.

How do I Apply Daylighting?

Daylighting can be applied to interior and exterior environments in AGi32. In exterior environments, the exterior surfaces must first be designated Daylight Exterior surfaces. Then daylighting can be enabled and your geographical location and analysis information selected. For interior applications, the daylight will need to be "let into" the space by creating transition surfaces, such as windows, and/or daylight openings in the Room or Object. Exterior surfaces that obstruct and/or reflect light into the interior environment can also be considered.

Once the environment specification is complete, the application of daylighting can be calculated. AGi32 provides two methods of daylighting analysis. Both take place in Full Radiosity Method:

Daylighting - This method allows you to calculate the daylight contribution for one particular moment in time (e.g., Friday, November 29, 2010 at 9:35 AM) with one sky condition. The option to consider the effect of electric lighting as well is offered.

Daylight Study - This method allows you to set up multiple instances of daylight application to the environment. You can analyze several times within one day and/or several dates, per your specifications. Multiple sky conditions may be specified, as well as the application of electric lighting. The images created within the daylight study are saved and can be viewed and shared with the any image-viewing software.

Daylighting Calculations

When daylighting is enabled in AGi32, two separate radiosity solutions are processed: exterior and interior. The following steps are applied:

1. Direct - Sunlight and electric light are emitted to all elements visible to them, including normal and daylight exterior surfaces. Sunlight is transmitted directly into interiors only through Daylight Transition Glass and Daylight Transition Openings. Simply creating rooms and cutting holes in the surfaces is insufficient (see Surface Types section below).
2. Adaptive Subdivision - If adaptive subdivision is enabled (for interior and daylight exterior surfaces), the adaptive subdivision for both daylight and electric light is processed at this time.
3. Indirect Daylight - Radiosity (exterior) - The Sky Dome, Virtual ground plane and any other daylight exterior surfaces proceed through radiosity calculations until the specified Radiosity Stopping Criterion is reached. Direct light from the Sky Dome and interreflected light from other surfaces is collected on the external surface of the Daylight Transition Glass and Daylight Transition Openings.
4. Transition - The Daylight Transition Glass and Daylight Transition Openings serve as transition zones to bring the external interreflected component inside, essentially acting as a virtual luminaire. The photometric distribution of each window (based on the direction and intensity of the luminances calculated in the exterior radiosity solution) is computed.
5. Indirect Interior - Radiosity (interior) - The Radiosity solution is computed for the interior environment, with the Daylight Transition Glass and Daylight Transition Openings acting as luminaires. The calculations are processed, as described above, until the specified Radiosity Stopping Criterion is achieved. Note: Light hitting the interior of Daylight Windows and Openings is not emitted to the exterior; it is absorbed.

 

Surface Types and Properties

To consider the effect of daylighting in exteriors as well as interiors, surfaces imported into or created within AGi32 must be designated appropriate surface types (and assigned attributes) specifically for daylighting analysis. Exterior surfaces considered in daylighting computations are handled differently, and separately, from other surfaces. Once the model is created, use the Surface Edit command to change the surface types for all relevant surfaces.

• Single Sided and Double Sided Objects and Rooms may be used to create both interior and exterior surfaces.
• Windows, skylights and openings should be modeled with Planar objects (or their CAD equivalent). One of the following surface types should be assigned to each:
• 20 - Daylight Transition Glass (Transparent)
• 21 - Daylight Transition Glass (Diffuse)
• 22 - Daylight Transition Opening
• Room surfaces can also be designated as a transition surface to model a window wall.
• Surfaces exterior to the fenestration need to be assigned the Daylight Exterior attribute.

 

Adaptive Subdivision

We strongly recommend that Adaptive Subdivision be enabled and appropriate settings used when Daylighting is considered. This technique will make your shadowing elements look more realistic and produce a much "crisper" image. Adaptive Subdivision is available within the Calculate - Adaptive Subdivision command.

Adaptive Subdivision is applied separately to Non-Daylight and Daylight surfaces. The settings within each category are independent and may differ, if the application warrants more detail in one or the other.

• In interior environments, where no exterior surfaces other than the virtual ground plane are specified, only the default (upper) section in the dialog needs to be enabled.
• In interior environments where exterior surfaces are to be considered, both the default and Daylight Exterior sections may need to be enabled. However, to save calculation time, you may not want to subdivide exterior daylight surfaces unless they are directly visible in the rendering of the interior environment.
• In exterior environments, both sections needs to be enabled, although no interior subdivision will occur.

For daylight applications, it is recommended that the Adaptive Subdivision parameters be set to the High Level: Maximum Subdivision Level = 5, Minimum Element Area = 0.1, Luminance Threshold = 1.1.

 

Rendering Exposure and Daylighting

As humans, we often perceive the luminance of an interior space to be similar to that outdoors as we look out the window from within. However, it is generally several orders of magnitude different. While the human eye can adapt by changing the aperture of the pupil, AGi32 must attempt  to render a scene while restricted to a limited exposure setting (similar to the F-stop on a camera lens). This results in an image that is similar to what one might see with a digital camera attempting to correctly expose both interior and exterior in one image: If the interior exposure is “normal”, the exterior is overexposed. Similarly, if the exterior is correctly exposed, the interior is usually dramatically underexposed. AGi32 has the ability to view the Exposure setting for both interior surfaces and exterior daylight surfaces individually and attempt a single setting to optimize the appearance of both.

AGi32 provides three exposure settings to handle the interior (Non-Daylight Surfaces) vs. exterior (Daylight Surfaces) exposure settings.

1. All Surface Types - This method treats the Non-Daylight and Daylight surfaces individually as discussed below, and then sets the exposure for the different surface types individually for the rendered image. The result is an image that appears closer to that of human vision.
2. Non-Daylight Surfaces - The average Luminance of all non-daylight surfaces is calculated. The average Luminance value is divided by 18% reflectance to determine the average Illuminance value. Exposure is set to properly display this average value.
3. Daylight Surfaces - The average Luminance of all exterior daylight surfaces is calculated. The average Luminance value is divided by 18% reflectance to determine the average Illuminance value. Exposure is set to properly display this average value.
4. If the average reflectance of the current environment differs greatly from the assumed 18% value, the image may appear under- or overexposed. Exposure modification may need to be applied to properly display the image.

Tone mapping may also be used to help correct the overexposure of daylit images. Tone Mapping is only available for Ray Traced images.

 

Daylight Application Caveats

The following caveats should be taken into account when considering daylighting availability in AGi32.

• It is recommended that Adaptive Subdivision be enabled with the High Settings for daylighting calculations (Illuminance, Exitance, Daylight Factor, etc.) when computing daylight component to react to the dramatic luminance ratios on incident surfaces. For crisp renderings with no point-by-point calculations, post- process ray tracing may be used instead of a finer mesh.
• If selected General Sky is selected, AGi32 uses the IES and CIE accepted sky models to calculate the sky's contribution in the daylight calculations. The CIE recognizes the Kittler (CIE Clear Sky) model and the Moon and Spencer (CIE Overcast Sky) model, as well as 15 additional Standard General Sky models. The IESNA further recognizes the Pierpoint (Partly Cloudy) model.
  • The IES Handbook states that the traditional sky models (Clear, Partly Cloudy and Overcast) should not be compared to instantaneous sky conditions (especially for a partly cloudy sky, where the sky luminance distribution can change rapidly and in large amounts as the sun is revealed, partially obstructed or fully obstructed). It is not unusual for the instantaneous measured sky luminance to differ from the average mean value by a factor of 2 from measurement to measurement.
• If Perez All-Weather is selected, AGi32 uses Weather Station data for the date and time selected. The Weather Station nearest the project location may be selected, or a different Station if it is more applicable. See the Weather Stations - Procedures topic for information on downloading and installation the Weather Database.
• A Daylight Transition Glass (Transparent) surface emulates a Fresnel reflector with an index of refraction of 1.5. It may be assigned a Transmittance color to tint the glass, and the tinted windows can be seen in the renderings. The transmission and reflectance of light through transparent materials such as glass and plastic (which are referred to technically as dielectric materials) is dependent on the angle of the incident light. When the direction of the incident light is normal to the material surface, approximately four percent of the light is reflected. However, at grazing incidence angles, nearly all of the light may be reflected. This Fresnel reflection is important for accurate daylight calculations.
• The Daylight Transition Glass (Transparent) surface type can be assigned to any surface to model glass, with appropriate transmission and color. Daylight Transition Glass surfaces are visible in radiosity and ray-traced images. The color of the altered light will be evident on any incident surfaces.
• The geometric altitude of the site locations is assumed to be 0 (sea level) when General Sky Condition is selected. The IESNA and CIE sky models used in AGi32 include exponential atmospheric extinction parameters that model Rayleigh scattering by air molecules, haze, and particulate matter.
• Including daylight occluders and reflectors (e.g., surrounding buildings), modeled as Daylight Exterior surfaces, will make your daylighting calculations more accurate, as will modeling any exterior fenestration for blocking and reflection purposes.
• Daylighting calculations will not proceed if Roadway Calculation grids are present. Either the Roadway points must be disabled (Project Manager), or Daylighting must be disabled (Daylight Parameters dialog).

 

Sunrise, Sunset and Twilight

For Standard CIE or IES Sky Models: Daylight calculations cannot preformed for dawn or twilight conditions (sun below horizon) because there are no data available in the sky models.

For the Perez Sky Model - Daylight calculations can be performed if Diffuse Horizontal Irradiance and/or Direct Normal Irradiance is greater than 0, AND if the sun is above the horizon.

In addition, in accordance with accepted practices within the daylighting research community, AGi32 limits the minimum solar altitude to 3 degrees above the horizon. This is because the optical air mass equation used to calculate atmospheric absorption of direct sunlight becomes inaccurate near the horizon due to varying moisture content, aerosol concentrations, etc. The diffuse sky light contribution is, however, still calculated for any solar altitude greater than 0.1 degree.