Advantages and disadvantages for total flux test of Integrating Sphere System and Goniophotometer
According to IESNA-LM-79-80, the total luminous flux (lumen) of SSL products shall be measured with an integrating sphere system or a goniophotometer. The method may be chosen depending on what other measurement quantities (color, intensity distribution) need to be measured, the size of SSL products, and other requirements.
The integrating sphere system is suited for total luminous flux and color measurement of integrated LED lamps and relatively small-size LED luminaires. The integrating sphere system has the advantage of fast measurement and does not require a dark room. Air movement is minimized and temperature within the sphere is not subject to the fluctuations potentially present in a temperature-controlled room. It should be noted that the heat from the SSL product mounted in or on the integrating sphere may accumulate to increase the ambient temperature of the product under test. Two types of integrating sphere systems are used, one employing a V(l)-corrected photometer head, and another employing a spectroradiometer as the detector. Spectral mismatch errors occur with the first method due to the deviation of the relative spectral responsivity of the integrating sphere photometer from the V(l), while there are theoretically no spectral mismatch errors with the second method.
The spectroradiometer method is preferred for measurement of SSL products because spectral mismatch errors with the photometer head tend to be significant for LED emissions and correction is not trivial, requiring knowledge of the system spectral responsivity as well as the spectrum of the device under test. In addition, with the spectroradiometer method, color quantities can be measured at the same time as total luminous flux.
Goniophotometers provide measurement of luminous intensity distribution as well as total luminous flux. Goniophotometers can measure total luminous flux of SSL products of relatively large size (correspondingto dimensions of traditional fluorescent lamp luminaires) while they can measure small SSL products as well. A goniophotometer is installed in a dark room, normally temperature-controlled, and is not subject to heat accumulation from a source being measured. Care must be taken, however, for drafts from ventilation that might affect measurement of SSL products that are sensitive to temperature. Measurements with a goniophotometer aretime-consuming compared to a sphere photometer. Goniophotometers using broadband photopic detectors are susceptible to the spectral mismatch errors noted above. In fact, correction for spectral mismatch can be more difficult if there is significant variation in color with angle.
Generally, when integrating sphere system and Goniophotometer test light source, the difference of data is not large. If test luminaries, the test result of Goniophotometer should be as the criterion, of course, it should ensure that the calibration of Goniophotometer accurate. According to formal test requirements, integrating sphere system is to test for light source, Goniophototmeter is to test for luminaires.
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