May 19, 2017: Luminous flux and radiant power are referred to as the vital optical parameters for LEDs even though sometimes the spatial intensity division is also required. Partial LED flux is a growing measurement, which still needs some time for global acceptance. In case of SSL sources, the colorimetric and photometric radiation characteristics are essential.
In order to measure total luminous flux and radiant power, it is important to use a goniophotometer/goniospectroradiometer or an integrating sphere method.
Integrating sphere method
The luminous flux measure is also known as total luminous flux to highlight the fact that it is the entire amount for each and every direction. Sometimes, it is also called 4π flux as an entire sphere consists of 4π steradians of solid angle. In order to collect all light within the 4π steradians the source needs to be at the center of the sphere.
The integrating sphere needs to be calibrated completely on the basis of measuring geometry in agreement with the substitution principle. According to the principle, the test light source should preferably be measured by evaluation to a standard source, which has similar spectral and spatial distributions.
How to select right size
The test sample needs to be smaller than the internal diameter of the sphere, so that the interference factor caused by the sample itself remains low. However, as the sphere gets bigger, the incident light intensity on the detector reduces.
In order to effectively balance between high measuring quality and good throughput, choosing the right relationship between the size of the test object and the size of the sphere is extremely important.
Using the 4π geometry, the entire surface of the test sample needs to be smaller 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. In case of 2π geometry, the diameter of the measuring port and thus the highest extension of the test sample should not go beyond 1/3 of the sphere diameter.
How to correct self-absorption
The test object contributes to the absorption of light radiation in the integrating sphere. This form of interference called self-absorption can lead to a vital attenuation of light radiation and result in deviations in measurement.
The attenuation becomes more pronounced as the test sample becomes darker and larger. Self-absorption can result in a correction up to several ten%. Hence, a self-absorption correction with the help of an appropriate auxiliary light source is necessary for accurate measurements.
An object such as a socket that is in close proximity of any source light absorbs light significantly, which may cause big errors. This is referred to as near-field absorption that cannot be corrected by a self-absorption dimension. Hence, the object needs to be placed in a distant place from the lamp and the formation of cavities needs to be avoided. Further, it is recommend to coat the object surface with a high-reflectance material.
Even though measuring luminous flux and radiant power with a goniophotometer is time-consuming in comparison to integrating spheres, it is much more accurate. This measuring method does not need luminous flux standard lamps as a reference value. It is the process of choice if lamps with varied luminous intensity distributions have to be measured and it is the foundation for calibrating luminous flux standard lamps that offer the reference value for other test methods.
Another highlight of goniophotometry is that it can measure partial luminous flux and angle of half intensity. The values are required to be determined when measuring characteristics regarding energy efficiency and compliance to Zhaga specifications.
Large distances are a condition for luminous intensity distribution to meet the far-field condition. For measurements of total flux using a goniometer, the large distances are not required. Providing the detector has good cosine response the irradiance can be measured precisely at all angles.
Irradiance is referred to as light falling onto a surface. By measuring the irradiance at adequate locations around a virtual sphere enclosing the lamp, the total flux can be drawn from integration. Providing that no interactions between source and detector occur, the size of the source can be roughly the size of the virtual sphere.