The type and rating of plastic diffusers used in luminaires, for compliance with The Building Regulations 2010 Fire Safety Approved Document B, Volume 2: Buildings other than Dwelling Houses. Only applies to recessed fittings, where the diffuser forms part of the ceiling. Diffusers are rated TP(a) or TP(b) according to their ability to restrict the spread of flames/fire.

TP(a) – Self-extinguishing. No restrictions on their use, except in protected zones.

TP(b) – Restrict the spread of flame. Total surface area of diffuser cannot exceed 15% in circulation areas and cannot be closer than 3m edge to edge. In open areas, surface area of diffusers cannot exceed 50% and spacing cannot be less than the largest plan dimension of the largest diffuser.

Neither can be used in protected stairways or zones. Variations apply between UK nations.