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

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.


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.

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.


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.