IES LM-75-01

The information presented in this Lighting Measurements Guide describes the three types of goniophotometers available for photometric testing and the three coordinate systems used to describe photometric data. Many current IESNA documents mention goniophotometers and coordinate systems, but none offer the complete, concise explanations or provide the harmonious definitions presented in this Guide.

A goniophotometer measures the directional light distribution characteristics of sources, luminaires, media, and surfaces. Goniophotometers and coordinate systems are divided into three categories (A, B, and C), the different types originally designed to match the type of source being measured. But the advent of position sensitive lamps introduced a further complexity: Such sources could not be properly evaluated if physical changes in lamp burning position were required during a test.

This Guide features three main section including spherical coordinates explaining how the three coordinate systems used in photometry are really variations on the standard spherical coordinate system used in mathematics; coordinate systems describing Type A, B, can C coordinates with an illustration of the photometric angles applicable to each system; and goniophotometers presenting the Type A (horizontal axis), the Type B (vertical axis), and the Type C (moving detector or moving mirror) versions. Detailed figures are used to illustrate either (1) the directions of luminaire rotation or (2) how a moving photodetector or moving mirror can be used to test a fixed light source. Since any goniophotometer can be used to generate any coordinate system (with varying degrees of difficulty), all the equations needed to convert Type A and Type B coordinates to their Type C equivalent are presented in a separate annex.