As well as we know that for the LED luminaires lumen test, the CIE121: 1966 Clause 6.1, CIE127-2007 Clause 6.2 and IES-LM-79-08 Clause 9.0 are mentioned two luminous flux test method: the first is adoption integrating sphere+photometer or spectrometer(recommend by CIE121: 1966 Clause 6.1.1, CIE127-2007 Clause 6.2.and IES-LM-79-08 Clause 9.0), this kind of test method is relative measurement method on lumen test; the second is photometric method which is adoption goniophotometer, this kind of test way is absolute measuring method. If adoption integrating sphere and photometric method to test same one LED lamp, we will find luminous flux data of two test method have big difference. This article is mainly discuss luminous flux test difference at integrating sphere and goniophotometer.
The principle of integrating sphere measuring total luminous flux by calibrated standard lamp. Due to there will be use a calibrated standard lamp, so there’s no need to know the integrating sphere’s output luminous flux, we can calculate the SSL product’s luminous flux by compare with standard lamp. Generally speaking, the integrating sphere test method suitable for small size LED lamp to measuring lumen and color parameters, this is lumen relative test method, moreover, the integrating sphere test method have quick test speed and it needn’t dark room advantages etc. Normally, if lamp size is small or like single LED, the test result and accuracy will be much better.
If adoption integrating sphere method testing big size LED luminaires, it have some better disadvantages compared with photometric method. When use integrating sphere test LED luminaires, there have a fact that the LED luminaires have different types, such as single LED, LED bulb, LED luminaires and others, the type of LED luminaires have big influence for final total luminous flux measurement. Meantime, integrating sphere test method need to do calibration procedure. Normally, if test LED luminaires, the standard lamp have to keep same emission characteristics with tested LED luminaires, the standard lamp adoption white LED will be best way. Of course, other type lamp also can used as standard lamp, but the test accuracy will be impacted. The test way will bring some difference, usually we use the 4π test way to test a lamp which emitting the lighting at 360° direction and the tested lamp should be placed in the centre position of the integrating sphere(recommend by IESLM-79-08 Clause 9.2.5). This is the best way to test LED luminaires; if the tested luminaries emitting the lighting at fixed direction, such as LED panel, LED street light or others, we need use 2π test way which means test the luminaries should be mounted on the one side of integrating sphere(recommend by IESLM-79-08 Clause 9.2.5). For the 4π test way; if the tested luminaries output power is too big or the the lamp shell have large size, there will be happen self-absorption effect more or less, then we need to use a auxiliary lamp(recommend by IESLM-79-08 Clause 9.1.5) to avoid the error. Generally speaking, the integrating sphere method is suitable for the compact LED and small size LED lighting. The test result of the luminaries flux can be achieve high accuracy and stability by this way; if use integrating sphere testing large size LED luminaries, the integrating sphere method have obviously defect, this time, the the luminaries flux data is not accurate and stable.
Adoption goniophotometer measuring total lumen which is photometric method, and this way have small limitation on the lumen test. The test principle of photometric method is use goniophotometer measuring all direction’s intensity distribution (or show the illuminance of light source at limited distance), through collection the intensity data at different direction to calculate the total lumen. Compared with the integrating sphere method, due to the difference of test light source’s intensity distribution, the photometric method don’t exist error in the theory, as the photometric method is absolute test method on the lumen test. It doesn’t need calibrated total luminous flux standard lamp, but it need longer test time for each sample. The photometric method which will adoption goniophotometer device, and there will be mentioned the type C Goniophotometer(recommend by IES-LM-79-08 Clause 9.3.1 and CIE121:1996 Clause 3.2 etc), dark room, test distance(mentioned by IES-LM-79-08 Clause 9.3 and CIE121:1996 Clause 18.104.22.168).
The difference of testing total lumen output consist of type of goniophotometer, test method(CIE121: 1996 Clause 3.4.2, Clause 3.4.1 and Clause 3.4.3), test distance and detector level etc. According to different type of LED luminaires, we can adjustment the relevant test method or testing device, such as if tested luminaires belong to narrow beam angle SSL product, we can use compact goniophotometer; selection Type C Goniophotometer, adjust test distance, choose higher level Class L detector(detector details classification description here: article-id-70.html) which can help achieve high precision measurement. In the practice testing, the photometric method can achieve luminous flux data’s highest measurement, due to some limitations inherent of integrating sphere test method, it’s impossible to remove the test tolerance, the only we can do is try best to reduce the test tolerance; but for the photometric method, there almost haven’t too much limitations and we can replace the test instrument, adjustment system configuration and change test method to fix this test tolerance.
As explained above, the most simple way to measuring LED luminaires luminous flux is adoption integrating sphere and photometric meter, this way is meet visual flux spatial demand, we can use a fixed front-end photometric meter to measure the total flux and the test time is fast and convenient. By comparison with a standard light source which keep have similar spatial and spectral distribution characteristics of tested luminaires to measuring luminous flux, so this way will be need use a calibrated standard lamp. Comparing with goniophotometer, the integrating sphere and photometric meter have high test speed, but when tested LED luminaires have different spatial intensity distribution characteristics from standard lamp, it’s easy to cause test tolerance. This kind of test tolerance is hard to fix, so it should be adoption use a good design geometry integrating sphere and use standard LED standard lamp which have similar emission characteristics with tested LED luminaires to reduce this error.
As mentioned previously, if the shape of tested lamp is similar with tested luminaires, then the integrating sphere method will have better test accuracy. When measuring luminous flux, for the LED bulb, small size LED luminaires and tube which beam angle more than 180°, we should adoption integrating sphere and spectrometer to do 4π test. As for the big size LED panel, LED street lamp and traffic lamp which beam angle less 180°, if need adoption integrating sphere method, then the integrating sphere should have side opening structure to do 2π test or use auxiliary lamp for assistance test, but this way have complex test procedure and test result is uncertainty. For these small beam angle luminaires, the best way is adoption goniophotometer method and working on the dark room, this can make sure high accuracy. But when adoption goniophotometer method, we have a clear understanding for test difference of different test method. Normally, like the LED panel, the best test method is C-γ; for the traffic lamp and spotlights will choose B-β test. At the same time, both test methods are need at dark room environmental, comparing with integrating sphere, the goniophotometer need more professional test environmental and professional engineer. In summary, the measurement principle, environment and test methods of the integrating sphere and the distributed spectrometer are different, and the measurement results are not comparable. We can choose the right way to test according to different standards and different requirements.
In summary, both integrating sphere and goniophotometer method have different measuring principle, environmental and testing methods, the both measurement results are not comparable. We can based on different standards and requirements to selection suitable test way.
In order to better compared test result, we used the spectrometer, integrating sphere and goniophotometer to testing LED bulb and street lamp. The test report as below, we can refer this.