What Do the Designations 'front' and 'back' Mean?

Last Updated: 
Wednesday, September 20, 2017

Software/Versions Affected:

'front' and 'back' reflectance Rf and Rb are arbitrary designations simply used to distinguish the reflectance from each side of a glazing in the spectral data. The position and orientation of a glazing in an IGU determine how these values match up to surface positions #1, #2 — the relationship of Rf/Rb to surface position cannot be standardized because some coatings can be used in more than one position (for example). When spectral data is used to simulate a glazing system using Optics or WINDOW, each glazing must be oriented correctly by the user (use the 'Flip' command to change the orientation of a layer / swap Rf and Rb values). The correct position and orientation for a glazing can be checked by taking note of the position of any coatings (illustrated by a dashed line in Optics and WINDOW) and/or by checking the reflectance values of each surface. The product literature from the manufacturer of the glazing can help to establish the most appropriate orientation for a given glazing.

If you are preparing spectral data, we recommend that you choose a convention for defining Rf and Rb that will be consistent throughout your own data. Some companies always use Rf to denote the reflectance of a coated surface, some companies always use Rf to mean the surface closer to the exterior, and Rb to mean the surface closer to the interior when the glazing is oriented in its 'usual' manner so that simulators rarely have to 'flip' the glazing when simulating it. Any convention that makes good sense to you is acceptable.