The calculation and use of utilisation factors (TM5) has been withdrawn, as it was made obsolete by the publication of BS EN 13032-1 in 2004. This publication should not be used on current projects.
Technical Memorandum No. 5 replaced IES Technical Report No. 2. A number of major changes were made to extend the range of application of the report and to simplify the arithmetical processes. The underlying mathematical assumptions remained the same and accuracy is unaffected except for some minor improvements. The revised method of calculating utilisation factors is as easy to use as the BZ classified method, so no purpose would be served by including the BZ classified method in this TM. However, it did retain a description of the classification method, as it is needed for calculating glare indices and for the MCD method. Consideration was given to adopting the CIE method developed by CIE TC-1.5, which dealt with luminaires individually. Although a link is provided in this TM with the CIE method, it was not adopted; the one described in this TM provides continuity with the BZ method, is easier to understand and requires fewer tables. Moreover, the transfer factor tables provided in this TM can be used for other purposes than for calculating utilisation factors and form a useful adjunct to Technical Report No. 1 1.