Abstract
The standard for measuring solar irradiance utilizes the units of watts per meter squared (W/m2). Irradiance meters are both costly and limited in the ability to measure low irradiance values. With a lower cost and higher sensitivity in low light conditions, light meters measure luminous flux per unit area (illuminance) utilizing the units of lumens per meter squared or lux (lx). An effective conversion factor between W/m2 and lx would enable the use of light meters to evaluate photovoltaic performance under low solar irradiance conditions. A survey of the literature found no definitive and readily available “rule of thumb” conversion standard between solar irradiance and illuminance. Easy-to-find Internet sources contain conflicting and widely varying values ranging from 688449 to 21000 lx for 1000 W/m2 (1 Sun) of solar irradiance. Peer-reviewed literature contains Luminous Efficacy equivalent values ranging from 21 to 131 lx per W/m2. This manuscript explores the relationship and establishes a theoretical and laboratory measurement guide for the conversion between solar irradiance and illuminance. The conversion factor includes standards data, equipment calibration accuracy, and uncertainty estimates. Solar Irradiance of 1 Sun (1000 W/m2) for an LED-based solar simulator is (116 ± 3) klx and (122 ± 1) klx for outdoor sunlight.
Highlights
Harvesting of electrical energy using photovoltaic (PV) systems is an essential part of renewable energy development
Solar irradiance is measured in watts per meter2 (W/m2) [1] and light illuminance in the unit of lux [2]
The Eq (1) example is for Commission Internationale de I’Eclariage (CIE) 1988 data, where k is conversion constant, V λ is solar irradiance AM1.5 data, and E(λ) is the CIE illuminance data: Illuminance = kVλEλ dλ
Summary
Harvesting of electrical energy using photovoltaic (PV) systems is an essential part of renewable energy development. A key issue in PV system operation is the ability to measure performance effectively. Of concern is how to effectively measure the performance of PV solar cells and modules under low solar light conditions. A simple and cost-effective method to measure low solar irradiance would support this research. Solar irradiance is measured in watts per meter (W/m2) [1] and light illuminance in the unit of lux (lx) [2].
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