# MDI *AWt Calculator*

## Information

The MDI *AWt Calculator* provides a simple form for calculating average reflectance values for planar surfaces using the Area-Weighting (AWt) approach and storing the specified data in a file. This calculation is typically used in Lumen Method calculations.

Area Weighting (AWt) is a method for estimating the averaged reflectance of a set of (rectilinear) co-planar areas of specified dimensions and using individual reflectance values for the background and for each insert. For example, a area corresponding to a wall (background) with 40% reflectance with a glass window (insert with 1 to 5% reflectance) and door (insert with 1 to 80% reflectance) would probably have an average reflectance not equal to the 40% of the wall.

The MDI *AWt Calculator* uses reflectance and dimension specifications for the background and a variable number of inserts to calculate an average reflectance value.

The available actions are presented in the line of command buttons across the top. 'Calc' uses the current form data to calculate the corresponding AWt values. New or existing files can be opened or saved using the corresponding command buttons 'New', 'Open' and 'Save' (which also calculate the AWt value).

The number of inserts is initially set to 0 (and reset for New files). To add an insert, click on the '+' button and then an additional insert row will be shown with zeroes for values. Since the new insert has zero area, it will not change the calculation result until the area value is changed. To remove an insert, click on the '-' button and when prompted enter the number for the insert to remove. When the form is shown with the reduced number of inserts, the calculation is changed accordingly.

The total area for all inserts may not exceed the area of the background.

W and H are width and height values respectively for background or insert, and A is the corresponding area. The program requires these two values - or an area value - for the background and each insert for the calculation to proceed. If width and height are provided, the area will be calculated. If area is provided and width or length are omitted, the width will be set equal to the area and the height set to 1. These values would be saved in a file but have no effect on the calculation result.

Rho is the reflectance of the background or of each insert. Reflectance values must be between 0 to 1.

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