How to select the right integrating sphere for luminous flux measurement?
One of the most important tests in light measurement is the assessment of the luminous flux and radiant power. The two principal methods for measuring total radiant power and luminous flux are using either an integrating sphere or a goniophotometer. The most common of which is by using an integrating sphere. The interior of an integrating sphere is coated with a diffusely reflecting material, eg. BaSO4 which guarantees complete integration and homogenization of the emitted radiation. The integrated light is then measured at the detector port.
The integrating sphere method and measuring geometries
The luminous flux is the measurement of the total output of light from the source in all direction. The conventional method of measurement is known as 4π flux since a complete sphere has 4π steradians of solid angle. To collect all light in this geometry, the illumination source must be placed in the center of the sphere.
However, typical downlights today have almost no light radiation directed backwards. Hence, the total flux can be measured as the forward flux or 2π geometry. In this case, the light source is placed at the wall of the sphere and the sphere will only collect the light emitting out from the front of the light source.
Guidelines for selecting the correct size
An important criterion for integrating sphere selection would be the size. There are a range of sizes from 50mm up to 2000mm spheres.
- The test specimen should always be significantly smaller than the internal diameter of the sphere as it will reduce the interference caused by the sample itself.
- The incident light intensity on the detector decreases as the sphere gets bigger. As a rule of thumb, the light throughput of an integrating sphere is a function of the inverse square of the sphere’s radius.
- Using the 4π geometry:
- Total surface of the test sample should be less than 2% of the surface of the sphere.
- The length of a linear lamp should be less than 2/3 of the diameter of the sphere.
- Using the 2π geometry: the maximum extension of the test specimen should not exceed 1/3 of the sphere diameter.
Burning position
Measurements of passively cooled SSL sources should be performed in the burning position defined by the manufacturer. When measuring in the 4π geometry, it is convenient to use an internal lamp post that can mount the light source to the required burning position. However, in the 2π geometry spheres, a rotatable sphere will be required to position the measuring port in the required position to achieve the burning position.
Solutions available for flux measurement?
To ensure the precise luminous flux and color measurement of SSL Lighting, Instruments Systems GmbH offers a wide variety of spectrometers and integrating spheres. Instrument Systems is a frontrunner in developing state-of-the-art light measurement equipment, software, and accessories.
Instrument Systems offers a complete series of integrating spheres available in a wide range of sizes to provide functionality for a broad spectrum of applications. The size ranges from 50-2000mm covering both 2π & 4π configuration. Pair with its flagship spectrometer CAS-140D, the system can measure photometric and radiometric quantities, chromaticity, CCT, CRI and spectral power distribution data.
Contact us for a free consultation with our application specialists and let us assist you on your lights and color measurement needs.
