How to read a photometric report.

Introduction

Every luminaire manufacturer’s claim about product performance should be backed by a photometric report. These reports, produced by accredited laboratories like LightLab, contain the data specifiers need to design with confidence and regulators need to verify compliance. But to the uninitiated, a photometric report is a wall of charts, tables and coefficients.

This guide walks through the key components of a standard photometric report and explains what each element means, why it matters, and what values to look for.

Anatomy of a report

A standard NATA-endorsed photometric report includes product information, test conditions, electrical measurements, the polar intensity curve, utilisation table, spacing-to-height ratio, and either isocandela or isolux diagrams depending on luminaire type.

The polar curve

The polar curve, shown in Figure 1, is typically the first chart a specifier looks at. It plots luminous intensity (in candela) against angle from the luminaire’s reference axis. The shape tells you how the luminaire distributes light in space.

A narrow, peaked curve indicates a directional luminaire like a spotlight or downlight. A broad curve indicates wider distribution suitable for general area lighting. Two curves are usually shown, one for the C0–C180 plane (longitudinal) and one for the C90–C270 plane (transverse), which together characterise the full three-dimensional distribution.

Understanding LOR

Luminaire Output Ratio (LOR) expresses what percentage of the lamp’s total light output actually leaves the luminaire. An LOR of 0.85 means 85% of the light produced by the lamp reaches the room, and 15% is absorbed within the fixture.

LOR is critical for energy efficiency calculations and for predicting real-world performance. A luminaire with a high lumen-per-watt lamp but low LOR may deliver less usable light than a less efficient lamp in a well-designed optical package.

Spacing-to-Height Ratio

SHR tells a designer the maximum ratio of fixture-spacing to mounting-height at which acceptable uniformity can be maintained. If a report gives an SHR of 1.5, fixtures can be spaced up to 1.5 times the mounting height apart before uniformity drops below acceptable levels.

Summary

A photometric report is a rich technical document, but the key data points are well-defined and consistent across accredited reports. Once you understand the polar curve, LOR, SHR and utilisation table, you have the core tools to interpret any manufacturer’s claim.

Questions about a specific report or testing your own product? Talk to a LightLab technician.