Daylight Parameters

This dialog allows you to enable daylighting calculations for a known geographical location at a specific date and time. Within this dialog, you will also be able to select the sky conditions, choose whether or not to consider the contribution of electric lighting, and manage multiple site locations.

Enable Daylighting - This checkbox provides the On/Off switch for daylighting calculations. To consider daylight availability, check this selection box.

Consider Electric Lighting in Daylight Calculations - This checkbox provides the On/Off switch for electric lighting when daylighting is enabled. To consider electrical lighting, check this selection box. This selection is disabled by default.

Site Information

This section allows you to specify the geographical location of your site and True North orientation.

Site Name - Choose a saved site location from this list, or add a new site location by clicking the Sites button. This will open the Site Locations dialog.

Clear Site - Clicking this button will clear the site currently displayed, if any. Latitude and longitude are set to 0.

Site Latitude - This text box echoes the Latitude coordinates for the chosen site location. Alternatively, the latitude may be entered manually. Latitude information is entered in decimal degrees. Latitude is measured north and south from the equator. Compass direction must be specified after the angle. Example: 40 N. If no compass direction is specified, latitude defaults to north.

Site Longitude - This text box echoes the Longitude coordinates for the chosen site location. Alternatively, the longitude may be entered manually. Longitude information is in decimal degrees. Longitude is measured east and west from the Prime Meridian, (the meridian that runs through Greenwich, England). Compass direction must be specified after the angle. Example: 105 W. If no compass direction is specified, longitude defaults to west.

It is highly recommended that one obtain the exact geographical coordinates (latitude and longitude) for the intended daylighting analysis. This will make your daylighting calculations more accurate.

True North - Specify the direction of True North for the site location. This orients your site location locally in AGi32 to the site's location on the planet.

When setting True North you are provided with two options:

  1. Enter the True North orientation in the text box. The default True North position is 90, which orients your site location using the standard North direction in AGi32 (top of the screen in Plan View).
  2. Click the Set North button to orient the North direction of your site graphically in AGi32. Locate two reference points parallel to your North orientation to set the compass direction. You must be in Model Mode to set the compass graphically.

Date and Time

Select the calendar date and the time of day for the daylighting analysis. Time of day is based on a 24-hour clock (no AM or PM). Example: 1:14 AM would be entered as 01:14, while 1:14 PM would be entered as 13:14.

AGi32 will warn if the sun is below the horizon for the date and  time selected.

For Standard CIE or IES Sky Models: Daylight calculations cannot preformed for dawn or twilight conditions (sun below horizon) because there is no data available in the sky models. Therefore, the sun must be above the horizon. AGi32 will check to make sure that the sun is at least 0.1 degree above the horizon.

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. AGi32 performs a check to make sure that the sun is at least 0.1 degree above the horizon.

Daylight Saving Time (DST) - If Daylight Saving Time is used for the project site, select the check box for Apply Daylight Saving Time and set the range of dates for which it applies at this location. (Dates vary with year and country.)

For example: If Apply DST is specified for March 13 through November 5 (the 2016 dates in North America), and the Date for the analysis is April 16, the time for each calculation for this date will be adjusted for Daylight Saving Time. If the Time is set at 10:00, the time for the calculation will be 09:00.

Solar Position - Displays the calculated solar position based on site location, date and time. (Displayed in Render Mode only.)

TIP: For information on how AGi32 handles sunrise, sunset and twilight conditions, see Daylighting Overview.

Sky Condition

The sky condition may be specified as either General, which uses a CIE or IES general sky type, or Perez All-Weather, which will use data from a selected Weather Station.

General - If General is selected, AGi32 will use 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 IES further recognizes the Pierpoint (Partly Cloudy) model.

The CIE mathematically defines fifteen sky types in CIE Publication S 011/E:2003, Spatial Distributions of Daylight - CIE Standard General Skies. The relative sky luminance distributions are described in the following table:

Type

Gradation Group

Indicatrix Group

Description of luminance distribution

1

I

1

CIE Standard Overcast Sky

Steep luminance gradation towards zenith, azimuthal uniformity.

2

I

2

Overcast, with steep luminance gradation and slight brightening towards the sun.

3

II

1

Overcast, moderately graded with azimuthal uniformity.

4

II

2

Overcast, moderately graded and slight brightening towards the sun.

5

II

1

Sky of uniform luminance.

6

III

2

Partly cloudy sky, no gradation towards zenith, slight brightening towards the sun.

7

III

3

Partly cloudy sky, no gradation towards zenith, brighter circumsolar region.

8

III

4

Partly cloudy sky, no gradation towards zenith, distinct solar corona.

9

III

2

Partly cloudy, with obscured sun.

10

IV

3

Partly cloudy, with brighter circumsolar region.

11

IV

4

White-blue sky with distinct solar corona.

12

V

4

CIE Standard Clear Sky, low luminance turbidity.

13

V

5

CIE Standard Clear Sky, polluted atmosphere.

14

VI

5

Cloudless turbid sky with broad solar corona.

15

VI

6

White-blue turbid sky with broad solar corona.

Gradation Group - indicates the gradation between horizon and zenith.

Indicatrix Group - indicates the scattering function, which relates the luminance of a sky element to its angular distance to the sun.

These models provide mean data averaged over a variety of time, location and measurement conditions. 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 significantly 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. The CIE states that the Standard General Sky models (1 - 15) give an approximation to clear, overcast, and skies of broken clouds that are sufficiently accurate for most daylight calculations.

Perez All-Weather - If Perez All-Weather is selected, AGi32 will use the data from the selected Weather Station to determine the likely sky and atmospheric conditions for that location, date and time.

Station Name - The selected Weather Station. The station may be selected by clicking on the Stations button or on the Find Closest button. When a Weather Station has been selected, its Latitude and Longitude are displayed.

Stations - Opens the Weather Stations dialog, with the stations nearest the site location listed. A Weather Station may be selected this way, or a previous selection may be changed.

Find Closest - Selects and loads the name, latitude and longitude of the nearest weather station to the specified site coordinates. Once selected, clicking on the Stations button will show the distance the selected station is from the site.

Clear Station - Removes the Station Name and all of the data associated with it.

Date/Time Retrieval - AGi32 enters the data for the closest date/time, rounding back to the nearest whole hour, per IES document LM-83-12. For example, if the Time is set to 08:15 and data are only available every hour, the data returned would be for 08:00. If the Time was 08:45, data for 08:00 would be returned.