LED近场测试

kengLEDs’ near-field distribution testing

Abstract:

With the development and application of LEDs,it is becoming increasingly important to be able to measure the performance of LEDs and LED-based systems in accurate and meaningful ways.The intended purpose of the measurement to inform development, to characterize or evaluate finished products, for production quality control, or for in-use performance assessment determines by the photometric and colorimetric quantities which are important to measurement.Considering the evolution of LED from point light source to surface light source,far-field distribution measurement can’t satisfy the measurement requirement.So that the selection of the new measurement about near-field testing method and system is necessary.Near-field testing contains more spatial photometric and colorimetric information than far-field testing,also the far-field characteristic distribution can be extrapolated from the near-field one.In addition,the precise measurement will support LED luminaire design.

Near-field testing has many advantages,such as measuring LED dies and package,supporting optical engineering package design,optical engineering package design,producing headlamps, and so on. Near-field testing measures luminance (spatial distribution) of the LED at many view angles, which includes luminous intensity data. with an imaging detector (CCD camera) rotated about the LED, this test will provides luminance matching view angle data.

A testing system named goniophotometers are designed to very accurately measure the near-field luminance distribution of a light source. It can capture the output light distribution at the source from multiple viewing angles ,gain the corresponding data,then make a near-field model to analyze by a software. Off course this system has a number of physical configurations . With two independent axes of rotation, this system maintains precise positioning between the DUT (device under test) and the imaging colorimeter. With the help of this system, near-field testing becomes easy and accurate.

Key words:

near-field distribution, measurement, an extended light source, goniophotometers.

1. Introduction:

Measurement considerations consist of clearly understanding the meaningful accuracy required for application, assessing the trade-off between measurement speed and volume of data collected, accounting for the measurement environment, and accounting for device attributes (e.g. LED warm-up). The flowing picture depicts the common ways applicated in LED relative testing. Different programs need different testing methods. For example, luminaires composed of LED arrays with narrow beams can’t use far-filed testing ,because it dosen’t meet the far-field standards a distance of five times the maximal dimension of a light source. In the following context,I’ll show the near-filed testing system.

Figure 1Measurement Domain Chart

2. Near-field testing

Usually,a point measurement from one viewing angle or a single integrated measurement will suffice.In fact,when it comes to the package,even a little variation of LED’s place will lead to variation of luminance and color with angle.As for LED luminaire,it can’t be treated as point light,instead,consideration of the spatial distribution affected by light output from point to point on the source.Therefore,accurate measurement requires more detailed distribution.

Near-field Imaging goniophotometers (figure 2 below) are especially designed to measure non-point source,such as large light sources, Surface light source. In general though, the potential complexity of a light source as a system and the potential for error in theoretical understanding are sufficient to make a measurement preferred, as it is a real description of the source. Of course for evaluation, inspection, or control, having the real data specific to a device is necessary as the measurement is the point. So goniophotometers should have a sufficiently small field of view to allow enough CCD pixels to map to the surface of the light source to see any relevant fine scale detail on the LED die or device.

Figure 2Goniophotometers

By using a CCD-based imaging photometer as a detector on a goniophotometer, modern near-field goniophotometers allow the determination of the starting point and the direction of the emitted radiation of a light source expressed by a set of rays (near-field model). The various near-field goniophotometers available on the market are nowadays used for generating ray data (“ray files”), which are commonly employed for optical simulation tools for the computer aided design of modern luminaries, like modern headlamps which use e.g. LEDs as light sources.

Near-filed testing will collect light data at a distance less then five times of light source’ diameter whi ch is used in far-filed testing. In this case, light distributions are complicated and ample. Also complete description of light radiation on the basis of the ray data and luminous flux measurement with integrated photometer through integration of the illuminances will add to validation.

Goniophotometers near-field models are the most comprehensive representation of the luminance and color output of LED die and devices as a function of angle. These measurements, since a scan consists of thousands of images, generally require several hours to complete for a single source. Faster measurement times are possible by compromising on angular resolution, imaging optics, or the allowed error, but this is really unacceptable for LED die and device characterization. If accuracy dominates, it will take much time to Generates large files.

In detail, when the testing starts ,moving luminaire devices which rotate the object to be measured around two axes(figure 3). Goniophotometers which rotate the object to be measured around only one axis (e.g. moving mirror goniophotometer). Goniophotometers which do not move the object to be measured, and which implement the positioning of the detector around the test item by means of two axes bedded one inside the other.

Figure 3.measurement around two axes Concrete step: 1.Easy alignment of the objects to be measured by means of the measuring camera. Image grid which can be activated, and metric coordinate system.

2.Measurement of luminous intensity distributions with the camera in the case of large measurement objects in relation to the sensor distance (near-field mode) or with the photometer in the case of small objects to be measured (far-field mode).

3.Saving in the Techno Team – format(.TTL), conversion into various standard formats (LDT, IES). The measurements will be evaluated using LumCAT.

4.Angular step sizes 0.1° ...

2.5°(camera),0.1°... 90°(photometer). 5.Capturing ray data, saving in the TechnoTeam –format (.TTR). Conversion into various standard formats using the Converter801 program. 6.Spectrometer measurement (option) 7.Protocolling the pole illuminances for stability monitoring (pole monitoring) 8.Protocolling the burn-in process and automatic start of the measurement. 9.Controlling the filter wheel of a color measuring camera (option). 10.Data acquisition of external devices (e.g. power analyzer or data logger). 11.Synchronisation of external data acquisition software to the measurement by triggering 12.Batch processing of several measurements.

3. Data analysis

Ray data describe the complete radiation characteristic of a light source and are often used for simulating optical components. In many cases, ray data derived from mathematical/physical models describe the real light sources insufficiently, which makes measured ray data indispensable for the realistic simulation of optical components. Furthermore, generation of near-field models for import into optical design software named ProSource? Radiant Source Model, and the data can be saved as in all common formats (IES, EULUMDAT, TM14, Calculux, ...).

ProSource? Radiant Source Model Analysis ( Figure 4 ) consists of near-field luminous intensity and color distribution analysis, it can act as view and analyze any angular measurement image and be used as export of importance sampled ray sets for all major optical design software packages. In addition, this software has the function of automated conversion of near-field distribution into far-field distribution.

Figure 4.ProSource? Radiant Source Model

Concrete steps: 1.Visualization of all data (ray data, luminous intensity distribution, luminance images, alignment of the object to be measured, burn-in protocol as well as the logged measurement data of external devices such as power analyzer) contained in TechnoTeam ray files (.TTR). 2.Generation of various ray data formats (ASAP, Optis, LightTools, LucidShape, Zemax, TracePro, SimuLux). 3.Raytracing to basic geometries (sphere,cylinder, cuboid). 4.Rotation and displacement of the ray data.

5.New calculation of the luminous intensity distribution in other angular resolutions.

6.Output of the luminous intensity distribution in various formats (EULUMDAT, IES).

7.Provision of customized formats possible. 8.Batch processing of conversion processes. 9.API for accessing the TechnoTeam ray data format.

4. Summary

LED near-field goniophotometer will lead to more confidential results. And Measurement of an extended source using a goniometric measurement is more accurate. This Direct measurement of illumination distribution will improve the LED relative industry.

5. REFERENCES

1.Brochure-RiGO801

2.LED_Near_field_goniophotometer_at_ptb_Publication_Lopez_1

3.LED_Optical_Measurement.pdf

4.Near-field and far-field goniophotometry of focused LED arrays

5.Radiant_Zemax-Choosing-Measurement-Systems-WP_130312