This document contains recommended guidelines for the optical measurement of LED Clusters. The 'LED Clusters' Focused Interest Group (FIG) was set up within the National Physical Laboratory's Optical Technology and Measurement Network to promote a better understanding of optical measurement issues in connection with LED Clusters in signalling applications. The LED Clusters FIG has developed this Guideline document to provide guideance for the measurement of the optical radiation properties (luminous intensity, colour, etc.) of LED cluster products, and is aimed at LED cluster suppliers, measurement organisations, standards authorities, and approval bodies.