Calculations_Obtrusive_Light-Threshold

Obtrusive Light - Threshold Increment

AS/NZS 4282:2019 defines Threshold Increment (TI) as: "the measure of disability glare expressed as the percentage increase in contrast required between an object and its background for it to be seen equally well with a source of glare present. Note: Higher values of TI correspond to greater disability glare." The definition in CIE 150:2017 is similar.

The formula for calculating TI in these two documents is: TI = 65 L_v /( L_ad)^0.8, where

L_v = veiling luminance

L_ad = adaptation luminance

Important note: In this formula, adaptation luminance is taken from a table. This differs from AS1158.2 and BSEN 13201-3-2015, which use the calculated average roadway luminance. In addition, veiling luminance is calculated differently here than in ANIS-IES RP-8-18 Roadway. Therefore, TI calculated with the Obtrusive Light - TI command may generate different results from that calculated in Roadway Optimizer or with the Roadway Calculations dialog.

Additional notes regarding the calculation of Threshold Increment in the Obtrusive Light command:

CIE 150:2017 requires the computation of Veiling Luminance meet specific standards in tandem with TI. AGi32 can include the Lv grid as well as TI with a check box (below, #4).
Only luminaires that lie within the range 1.5 - 60 degrees from the observer's line of sight are taken into account in the calculation of TI.
The driver's eye height is 1.5m above the road surface.
The driver's line of sight is parallel with the center line of the road and 1 degree down relative to the plane of the road surface.
The cutoff angle of the windshield (windscreen) should be as assumed in AS1158.2 (20 degrees above horizontal).
For pedestrians, the eye height is 1.7m above the footpath, and the line of sight is parallel with the direction of the footpath and 1 degree down relative to the plane of the footpath.
Only direct light is considered in the TI calculations. (No inter-reflected light, even if calculated with Full Radiosity Method.)

To add an Obtrusive Light - Threshold Increment calculation in AGi32:

From the Main toolbar, select Add - Calculations - Obtrusive Light - Threshold Increment, or in the Calculations Toolkit, click the down arrow next to the Obtrusive Light button and select Threshold Increment.
Enter a label and Description, if desired. The default label will be ObtrusiveLight_TI_1. Unless changed, subsequent labels will be ObtrusiveLight_TI_2, ObtrusiveLight_TI_3, etc. If a number is used for the label, subsequent labels will be incremented accordingly.
Set the Point Spacing according to the requirements of the standard that you're following.
Check the box to include the Lv (Veiling Luminance) grid if desired.
Set the Observer Height. The default is 1.5 meters (4.92 feet).
Set the Viewing Direction of the Observer. This will be either along the line of points as drawn, or in the opposite direction.
Set the Windshield (windscreen) cutoff angle according to the requirement in the standard you are following. (20 degrees for CIE 150:2017 and AS/NZS 4282:2019)
Select the appropriate Adaptation Luminance from the drop-down list. The values offered in this list are taken from CIE 150:2017 and AS/NZS 4282:2019.
Select the veiling luminance formula to use based on the standard. CIE 150:2017 uses the "Standard" formula, AS/NZ 4282:2019 requires the AS/NZS formula.
In the Summary section of the dialog, select the items to calculate. By default, average, maximum and minimum illuminance are displayed along with uniformity ratios (Max/Avg ratio is not selected by default). If Number of Points is selected, the number of points that are in the calculation (not including points that have been removed) are shown.
In the Display Options section, you may set the decimals for calculation. The default is 0, meaning whole percentages will be calculated, e.g., 2%, 5%, etc. You may change the color of the grid here, and set any Labeling option desired. By default, the points themselves, the line they are associated with, and the Meter Indicator (showing observer's direction of view) will be displayed when the line is drawn or selected, but they may be de-selected here if you don't wish for them to be displayed.
Select whether you will Manually Specify the Line or Select an Existing Drawing Entity to define the calculation points.
You may choose to create an Array of lines. The array may be created in a horizontal plane on one or both sides of the original line, or vertically above it. The Number of Array Lines does not include the original drawn or selected line. The Total Number Of Lines does include the original line. If you are creating an array, select the type of array and set the Array Spacing.
If you would like Isolines or Highlighted Values to be shown on the grid after it is calculated, you may specify them from this dialog by clicking on the Isolines button or the Highlight button.
Click OK to exit the dialog.
Now you are ready to specify the line, by either drawing it or selecting an existing line. If drawn manually, click the first point and then subsequent points as appropriate. The line may be composed of multiple segments. After clicking on the last point, right click to end the command.
If an existing drawing entity was selected, AGi32 will display it as bold. If it is correct, press Enter or right click. Pressing Esc will cancel the command. Clicking the Undo button (or pressing Ctrl-Z on the keyboard) will allow you to select again.
The observer's tentative direction of view will be shown. You may press the F5 key on your keyboard to flip the direction the other way. Press Enter or right click your mouse to accept the direction.
If you chose to create an Array, AGi32 will display the tentative direction of the array. You may press the F5 key on your keyboard to flip the direction of the array to be on the other side of the line. Press Enter or right click your mouse to accept the direction. AGi32 will create the line (or array) of points. It will be located above the line drawn or selected at the distance that was specified in the dialog. For example, if the line is drawn at Z=0, and the height in the dialog was the default, Z=1.5m, the points will be at 1.5m