Effects of baffle reflection and shadow in an integrating sphere on the total luminous flux measurement of a linearly-shaped lamp
The measured total luminous flux of a linearly-shaped lamp by using the integrating sphere substitution method against a standard spherical lamp can be deviated from accurate by the presence of a baffle in the sphere. The baffle introduced two main effects on the sphere response: the baffle reflection, or high-signal region, and the baffle shadow, or low-signal region. Once the baffle condition changed, the two effects changed, causing the measured value to change differently regarding the lamp alignment. In the perpendicular alignment, increasing the baffle length increased the measured flux value. This was due to the dramatic increase in the magnitude of the signal in the baffle reflection behind the baffle. In a coaxial alignment, on the contrary, increasing the baffle length resulted in the decrease of the measured flux value. This was due to the increase in the area of the baffle shadow on the opposite hemisphere from the baffle, which increased at the higher ratio than the area of the baffle reflection. In both alignments, the measurement uncertainty increased with the baffle length due to the increasing magnitude of the signal fluctuations. The trends were similar for all linearly-shaped test lamps with different lengths and diameters.
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