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BS8206-2 Glazing Calculator for Radiance
British Standard 8206-2:2008 Lighting for buildings – Part 2: Code of practice for daylighting defines a so-called maintenance factors accounting for the reduction of daylight transmittance due to dirt. In BS8206-2, the light loss from dirty glazing is calculated from values in three different tables (A.3 to A.5). An additional correction factor for frames and glazing bars is given in table A.6. It needs to be applied if the framing (frame, mullions and transoms) is not part of the actual 3d model used in the computer simulation.
Additionally, BS8206-2 presents a list of typical glazing types and their visible light transmittance in table A.2. Glazing transmittance values are also presented in CIBSE Lighting Guide 10: Daylighting and Window Design (1999), where they are listed in table A 1.15. Since the choices of glazing types for which typical transmittance values are given differ between BS8206-2 and LG10, both tables are included here for convenience.
Please note that table A.3 lists a range of percentages for some of the configurations. Where this is the case, the mean value is chosen in this calculator. Also note that some combinations of variables, e.g. 'Urban' - 'Polluted' - 'Leaded' - 'Horizontal' - 'Snow' lead to negative maintenance factors.
For the difference between 'transmittance' and 'transmissivity', as used in the Radiance glass material type, please refer to the Radiance Tutorial, Section A.1.1, and the Radiance Cookbook, Section 2.5.2, both of which are available from our Radiance Documentation page.