According to the particularity of LED luminous flux measurement, unique optimization is adopted in the design of the
integrating sphere for LED measurement combined with the diffuse materials of high reflectivity, which makes the
system stability and accuracy have great improvement. The experimental results show that the system stability and
consistency is much higher than other common LED test system. It is the system which is really suitable for LED
optical parameters measurement.
Different with the traditional light source, the
luminous flux measurement of LED light source has posed a big challenge to the equipment in testing the veracity
in the process of using the integrating sphere to test luminous flux. On one hand, compared with the traditional
light source, usually, LED has much stronger directivity, and will not shine evenly in whole space. This feature
makes the distribution of LED direct light in the surface of the integrating sphere uneven
This uneven distribution will cause the direct light of different LED has different reflection features of
the detector. Because the position of the detector mouth and the position of the baffle are fixed, the
direct performance of various reflection distributions is signal fluctuation. In the ordinary testing
system, there exist the differences in LED of different positive divergence angle, the same LED of different
placed direction, the same direction with different position. Even the rated luminous flux is the same; the
actual measured value is different. Based on the customer’s verification result, the effect of LED placed
direction of ordinary LED test system on the luminous flux measurement result is always more than 50 % (the
difference of the maximum signal and minimum signal of the same LED measured in different direction).
Different lighting angle has different effect on LED measurement
When measuring the different lighting angle of different LEDs, since the distribution difference of the
surface of the inside integrating sphere makes the distribution of the direct reflection have different
effect on the detector, it directly affects the difference of the accuracy of the measurement
he inside reflection characteristic of the integrating sphere is one of the crucial factors that make the LED
directivity have the impact on the measurement accuracy. In the ordinary LED test system, the reflectivity
and the Lambert character of the integrating sphere surface coating are not ideal. One reason is the low
reflectivity, and the other reason is the poor diffuse characteristics. The result of the integrating sphere
surface of low reflectivity is that the direct light of LED gradually attenuates after few times of
reflection. However, during the whole process of mixing light, the direct irradiation light and reflection
light have held a very large proportion, which has made a leading role. And in some conditions, low
reflectivity materials will cause strong shadow effect on the back of the baffle probe. However, it is the
straight reflected light and shadow effect that leads to the inaccurate measurement.
Moreover, lower diffuse reflectivity will seriously affect the attenuation of the signal. Since the light has
been reflected for many times in the integrating sphere during the process of the light measurement, each
reflection will cause certain attenuation, but the impact of the reflectivity degree on the light intensity
has been strengthened after many-times reflection. For example, the reflected light has been reflected for
15 times in the integrating sphere, if there is 5% difference between their reflectivity, the signal
attenuation may exceed more than doubled. Actually, the reflectivity difference in the integrating sphere is
far more than this.
The current LED test system has not been used as the standard LED for the standard light source. In the
process of measurement, we still choose to use the standard halogen tungsten lamp with stable driver as the
standard light source. Since there is a big difference in the external structure between the standard lamp
and the measuring LED, including the light absorption effect of LED holder and the difference between the
standard lamp installation position and LED installation position, all these are the important factors that
affect the accuracy of the test result.
