Near-field Goniophotometer vs Integrated Sphere System|
Article - February 23, 2016 By LarsonElectronics.com
Near-field Goniophotometer vs Integrated Sphere System
Lighting manufacturers have several options for measuring a fixture’s characteristics and luminous properties. But in order to adhere to guidelines set forth by the Illuminating Engineering Society of North America (IESNA), one must use an integrated sphere or a near-field goniophotometer when measuring an SSL’s total luminous flux or lumen (ref: IESNA-LM-79-80).
This article aims to compare the uses, advantages and suitability of both measuring tools. One can also find recommendations for committing to a specific instrument based on the accuracy of results generated by both devices (separately).
Near-field Goniophotometry Mechanisms and Advantages
A near-field goniophotometer is a device that gathers information about a luminary’s performance in relation to total luminous flux, illuminance and luminous intensity distribution. The measuring technique is composed of several parts: a goniometer, an analysis software and other equipment (spectrometer, spectroradiometer, photometer or colorimeter) used to capture signals generated from the instrument. The analysis software is used to process and display actionable measurement variables, such as lumen, candela, lumen per watt, CRI, color temperature, beam angle and etc. Testing luminaries with a near-field goniophotometer must be done in a temperature-controlled dark room to achieve consistent readings. Furthermore, the machine must be calibrated properly for accuracy.
According to the International Commission on Illumination, below are three types of goniophotometers and their unique features:
• Type A- fixed horizontal axis with an attached vertical axis (both components perpendicular to the direction of the sample)
• Type B- fixed vertical axis with an attached horizontal axis (both components perpendicular to the direction of the sample)
• Type C- Single column configuration; fixed vertical axis perpendicular to line of measurement and the horizontal axis is parallel to the direction of the sample
The advantages of using a goniophotometer are numerous. First, testing can be conducted in a small dark room, as small as a lab measuring 5 x 5 meters. Compared to far-field testing methods, such devices are also more cost effective. Lighting manufacturers that have unrestricted access to this type of equipment are able to release products to the market more quickly (since one would not have to wait for test results to come back) and with less financial barriers by shortening the development cycle. When it comes to accuracy, near-field systems are able to gather information from multiple points, instead of being limited to a single point source. They are ideal for measuring both large SSL and compact SSL luminaries and are not greatly affected by heat emission from the sample fixture.
Integrating Sphere 101
An integrating sphere system (also known as an Ulbricht sphere) is designed to measure a light source’s total luminous flux and colorimetric quantities. Like a near-field goniophotometer, a traditional sphere is used with other light measuring devices, such as a spectroradiometer or spectrometer, and analysis software for data processing and management. Moreover, it has to be calibrated to achieve accurate measurements, which is done every time a different light or color is used. Inside the sphere, one can find a highly reflective (typically greater than 97 percent reflectivity) surface with very little to no absorbing surfaces. This allows the light to be distributed uniformly, spectrally and accurately. During testing, the light source is installed on the side of the device with the light shining internally. It can also be mounted to maintain a stable position in the center of the sphere.
A major drawback with this configuration is the requirement of different size adapters to accommodate a wide range of light sources. For instance, a compact, rectangular LED bulb may not be suitable for a large adapter designed for a circular incandescent bulb. Forcing incompatible light sources and adapters will generate light leakage during testing, which will lead to erroneous or inaccurate results. Also, the size of the sphere determines the range of light source sizes one can test using the equipment. Small spheres are generally compact and can be used on a work table, while large spheres may take up a good amount of space in a room.
Integrated sphere systems come with several notable advantages. Because the light source is tested in an enclosed environment (i.e., the sphere), testing does not need to be conducted in a dark, temperature-controlled room. As a result, other parts of the room can be utilized for other tasks. The main issue with relying on an enclosed environment is the possibility of increasing the ambient temperature inside the unit due to heat emission from the light source.
Suitability, Applications and Recommendations
Choosing between a near-field goniophotometer and an integrated sphere comes down to the type of measurements one is seeking out. The former instrument is suitable for comprehensive characterization of spatial distribution of a luminary’s colorimetric and photometric features. While the latter is ideal for measurements related to a light source’s luminous flux and colorimetric properties. It is critical to consider that based on formal testing standards, an integrating sphere is applied when testing a light source and a near-field goniophotometer is deployed when testing a luminary.
Interestingly, some results generated by both instruments are almost identical. In a comparative study led by Denan Konjhodzic, a Germany-based applications engineer, researchers tested a selection of LED assemblies using both types of measuring tools. When comparing total luminous flux results, the difference between the results generated by the near-field goniophotometer and the integrated sphere only varied by 3.7 percent. In some of the results, the difference was as low as 0.55 percent. Individuals may also take note that integrated sphere systems are capable of completing tests faster than a near-field goniophotometer. This could be beneficial for manufacturers that conduct light testing measurements on a regular basis.