How is the luminous efficacy of a lighting system measured?
April 26, 2018
Luminous efficacy is the ratio between luminous flux expressed as lumens (lm), and power expressed as watts (W).
The two principal methods for measuring total radiant power and luminous flux are using either an integrating sphere or a goniophotometer/goniospectroradiometer.
The integrating sphere method
The luminous flux quantity is sometimes called total luminous flux to emphasize the fact that it is the total for all directions. It is also referred to as 4π flux since a complete sphere has 4π steradians of solid angle. To collect all light within the 4π steradians the source needs to be at the center of the sphere. This 4π geometry is the conventional configuration for measuring luminous flux. The radiation emitted in all directions is captured and the total luminous flux is measured.
The goniophotometer method
Although measuring luminous flux or radiant power with a goniophotometer is more time-consuming compared to using integrating spheres, it is much more precise. This measuring procedure does not require luminous flux standard lamps as a reference value as it is the case in sphere photometry. It is the method of choice if lamps with different luminous intensity distributions have to be measured and it is the baseline for calibrating luminous flux standard lamps which provide the reference value for other test procedures. Another distinguishing feature of goniophotometry compared to sphere photometry is the capability to measure partial luminous flux and angle of half intensity. These values need to be determined when measuring characteristics relating to energy efficiency and conformity to Zhaga specifications.