The spherical photometer is an efficient and fast device for measuring the luminous flux and light efficiency of LED products . It is the most important equipment for LED luminous flux and light effect detection. It involves the measurement mechanism of luminous flux measurement, product testing center and enterprise quality control department. How to properly use a spherical photometer to obtain accurate luminous flux values ​​is a common concern of testers. This article will attempt to discuss the key technical points of the spherical photometer to measure the total luminous flux of LEDs.
1 Principle of spherical photometer
The spherical photometer is a photometer that uses the integrating sphere to measure the total luminous flux of the light source. The basic structure is shown in Figure 1. The light source S is placed in the integrating sphere (common position is the 4Ï€ measurement mode of the ball center position, or the 2Ï€ measurement mode near the ball wall), and the optical probe D is attached to the detection hole on the ball wall. The hole is on the wall of the ball and a diffuser is placed. If the probe D is a photometric probe, its output current is converted into a voltage by I/V, and finally displayed on the reading meter R; if the probe D is a fiber collecting surface, the light is collected and then connected to the spectroradiometer through the optical fiber, according to the measured The spectral power in the visible region is calculated using the spectral luminous efficiency function to calculate the total luminous flux. There is also a combination of the two, that is, using a photometric probe and a spectroradiometer, using a photometric probe to measure the luminous flux, and a spectroradiometer to measure the spectral power distribution of the light source.
Figure 1a. Schematic diagram of a spherical photometer (detection system is an illuminometer)
Figure 1b. Schematic diagram of a spherical photometer (probe system is a spectroradiometer)
The inside of the integrating sphere is uniformly coated with a white diffuse reflective material. The material requires good diffuse reflection characteristics and no spectral selectivity. Let the reflection of the ball wall be Ï, the radius of the ball is R, and the total luminous flux emitted by the light source S is φ. According to the integrating sphere theory, the reflected illuminance of the light source S on the ball wall is
For a certain integrating sphere, both R and Ï are fixed. Therefore, the above formula indicates that the indirect illuminance values ​​formed by the light source at each point on the inner wall of the ball are equal due to the multiple diffuse reflection of the inner wall of the integrating sphere, and are proportional to the total luminous flux of the light source. In Fig. 1, the function of the baffle B is to block the light source S from directly illuminating the probe D. If the baffle B is removed from the integrating sphere, the light emitted from the light source will directly illuminate the probe D. At this time, the illuminance at the position of the probe D will depend on the luminous intensity of the light source in this direction. The illuminance values ​​at the points on the inner wall of the ball are not equal and will not be proportional to the total luminous flux of the light source.
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