TP(a) and TP(b)

Understanding thermoplastic diffusers, their applications and differences

The choice between TP(a) and TP(b) panels can be an interesting one to make. There are subtle, yet important, differences to consider when specifying the appropriate product for the application. This article will provide an overview of each, as well as the differences between the two options.

Firstly, TP is a measurement related to flammability of material of the lighting diffuser and the degrees to which each product performs under this stress. TP stands for Thermoplastic and can be a material such as polycarbonate (PC), acrylic (PMMA) and polystyrene (PS). Judging TP as a variable in a lighting design is only relevant when it forms part of the overall ceiling construction or penetrates the ceiling barrier. This means that should not be considered as a relevant variable for surface mounted products or suspended products.

Any diffuser that is over 3mm thick is automatically classified as TP(a), as are materials that are self-extinguishing within 5 seconds of a flame being removed. Usage of TP(a) rated products are unlimited in general use, except for protected stairways and exits enclosed with fire-resisting construction.

The benefits of TP(a) rated panels can come at the expense of either the efficacy or beam angle, when compared to TP(b). This is because of the impact on light output that a thicker diffuser creates.

Restrictions on use do apply to TP(b) rated diffusers, particularly on product spacing and maximum unit quantity in a given room. Specifically, there is a maximum of 5m2 for a group of recessed panels with a minimum 3m space between each group of panels. TP(b) rated panels most not burn at a speed of more than 50mm per minute. TP(b) panels have an opposite effect on efficacy and beam angle, compared to TP(a). For example, in the Collingwood range, TP(a) rated panels have a 90° beam angle and TP(b) panels have a 110° beam angle.

The current guidance on spacing tends to lead lighting designers to select more TP(a) rated panels, but there are distinct advantages to both types.