Near-field optical distribution test of LED light source

LED optical design is a very important part of lighting products. Before the advent of the near-field goniophotometer, the model of the primary light source can only be "theoretical modeling" by the optical design engineer's ability and simulation software. Because there are differences between theoretical modeling and actual models, there are two major problems in this type of modeling. First, most of the lenses need to be re-opened, which is expensive; second, product development takes a long time. Therefore, the advent of the near-field goniophotometer is the best gift for optical design engineers. In recent years, as our optical quality requirements for LED products have become higher and higher, the demand for optical field near-field optical distribution models has continued to grow. Both LED light source manufacturers and lens manufacturers require optical optics. Distribution model. So, what is the near-field optical distribution model of the light source? How to use the near-field goniophotometer to obtain the near-field model? What types of files can be generated by near field testing and how can I use the various documents generated? In addition, for unconventional lamp beads, such as high-power modules, bare crystal, 5-sided CSP and other light sources, how can it be handled? This article will answer you one by one.

1. Near-field optical distribution model and test principle of light source

At present, in the LED optical design process, two models are generally used to simulate the light source, namely "light source far field model" and "light source near field model". Before you understand the near-field model of the light source, let's briefly introduce the familiar far-field model of the light source.

The far-field model of the light source is to treat the light source as a point source, and all the light rays are emitted from the same point. Generally, the light output from the point source is isotropic. The far-field model of the source is measured by a far-field distribution photometer, which typically includes a mechanical structure (turntable) and a photometric detector for supporting and positioning the source under test. According to the requirements of CIE70, the distance between the light source and the detector needs to be far enough when measuring (generally, the measurement distance should be at least 5 times of the maximum luminous surface of the LED light source), and the light source can be regarded as a point light source.

For LED light sources, especially white light sources, the brightness and color of the surface are not evenly distributed due to the effects of electrode design, chip structure, and phosphor coating. As shown in Figure 1, it can be seen that the brightness of each chip and the color distribution of the surface of the light source are not completely identical. The secondary optical design of the LED light source is relatively rough, and the information of the light source obtained by the far-field model is relatively rough, which cannot accurately reflect the brightness and chromaticity spatial distribution difference of the LED light source surface, and it is difficult to achieve accurate secondary optical design for the light source. Therefore, accurate measurement of the luminescence model of the source itself is critical to the accuracy of the optical design and simulation results.

a) true color map b) pseudo color map

Figure 1 White LED light source surface brightness distribution

That is to say, the most essential difference from the far-field model of the light source is that the far-field model of the light source considers the light source as a point source, while the near-field model of the source considers the source as a complex surface source. The shape of the light source is represented by a plane, and all light rays are emitted from the surface of the light source. The near-field model is closer to the actual light-emitting condition of the LED light source. The measurement can obtain the brightness and chromaticity values ​​of each point in the measured plane, and provide more accurate and detailed data for the optical design of the LED light source.

The near-field model of the source can be measured by a near-field distribution photometer. As shown in Figure 2 a), the near-field distribution photometer consists of a distribution photometer and an imaging luminance meter, which replaces the photometric detector in the distribution photometer. The imaging luminance meter uses a two-dimensional optical receiving component (such as a CCD), and one sampling can measure the brightness value of each point in the measured plane. The imaging luminance meter in the near-field distribution photometer directly receives the light beam of the LED light source facing the LED light source under test. The light beams emitted by the measured light source all have measurable distance-independent brightness values. By measuring the brightness values ​​of the respective light-emitting points on the surface of the tested LED light source in all directions in the space, the ray tracing method can accurately obtain each of the LED light sources. A luminosity parameter such as a illuminance distribution, a spatial light intensity distribution, and a total luminous flux, and is independent of the test distance, direction, or radius of curvature of the LED surface. If the chromaticity information is to be measured, the imaging illuminance meter is replaced by an imaging chromaticity meter to obtain the spatial chromaticity distribution of the LED light source.

As shown in Figure 2 b), the light source can be rotated around its own mechanical axis during the measurement process. The imaging light/colorimeter shoots the light source image from various angles of the space. The measurement results of each specified angle contain brightness and color information, and the light source is constructed. A three-dimensional image of the brightness and chrominance output. At the end of the measurement, the measurement software integrates these images into a near-field model that describes the brightness and color distribution of the source and gives it in the form of light intensity. The intensity I (x, y, z, θ, φ) is the position (x, y). , z) and the function of the angle (θ, φ). This function also contains color coordinate values ​​or spectra if color and spectral measurements are taken. The near-field model of the source generates a set of rays for optical design and far field distribution of the extrapolated source.

Figure 2 a) Schematic diagram of the structure of the near-field optical distribution photometer Figure 2 b) Schematic diagram of the near-field optical distribution photometer