IES LM-72-20

This document is intended to provide a clearly defined set of terms to be used to designate the angular positioning of photometric data. The terms are intended to be used by both providers and users of lighting software so as to unambiguously and consistently specify how photometric data is to be "rotated" before it is used. Two similar but fundamentally irreconcilable types of positioning have existed for years. The "Clockwise from North" ("CN") system starts by rotating luminaire photometric data clockwise from north. The "Counter-Clockwise from East" ("CCE") system starts by rotating luminaire photometric data counter-clockwise from east. A third positioning method, called "aiming luminaire at a point," has also existed for years and is also discussed in this document.