LED/SSL Testing

An overview of the possibilities of LED/SSL testing in production, laboratories, and different applications.

 


Content

1. Brief description

2. Introduction

3. Overview of measurement requirements

3.1 Standards, norms, and guidelines

3.2 Difference in measurement requirements depending on the application

3.3 Key parameters

4. Metrological solutions

5. Summary

6. Literature

 


  

1. Brief Description 

This article will first describe the requirements of LED/SSL (LED = light emitting diode, SSL = solid state lighting) testing and the possible metrological solutions for different applications, ranging from applications in the production field and laboratory measurements, to on-site tests by users/customers. Gigahertz-Optik has over 30 years of experience in light measurement technology, and has taken a very innovative path in spectral measurement with the BTS technology.

 


 

2. Introduction 

LEDs are increasingly being used for illumination purposes and as design elements. With many new applications and enormous market comes an increase in demands for LEDs and SSL in terms of the LED quality (i.e. spectral properties, such as color rendering, color fidelity, dominant wavelength, color location), energy efficiency (efficacy lm/W), and price. This means increased product quality with the increasing global competition, at falling prices. In terms of the quality, very precise, stable, and cost-friendly measurement techniques are required in addition to high-end and precise manufacturing methods, since LEDs cannot be produced with exactly the same optical properties due to the manufacturing processes involved. Optical properties can vary even within the same production batch. This thus necessitates comprehensive LED tests (intensity (W, lm, W/sr, cd) and color), as well as tests within the production process for various LED classes. In addition, the production cycle of LEDs is also decisive for their technical connection in applications. For instance, the heat sink has a decisive influence on the product stability and lifetime.

 

LED Technik im Wandel

 

Figure 1 The LED technology is changing

 


 

3. Overview of measurement requirements

3.1 Standards, norms, and guidelines

Certain specialist committees have developed guidelines in order to satisfy the metrological, electronic, and optical requirements. These offer a very good framework for users in terms of both the choice of a measurement technique, as well as key parameters. The following documents are hereby worth mentioning:

Released:

CIE84 Measurement of Luminous Flux
CIE127

Measurement of LEDS

CIE198 Determination of Measurement Uncertainties in Photometry
CIE214 Effect of Instrumental Bandpass Function
CIE S014 Colorimetry
CIE S025 Test Method for LED Lamps, LED Luminaires and LED Modules
LM-79-08 Electrical and Photometric Measurements of Solid-Sate Lighting Products
LM-80-08 Measuring Lumen Maintenance of LED Light Sources
DIN 5032-9

Measurement of Photometric Quantities of Incoherently Radiating Semiconductor Light Sources


Still being processed, but to be released soon:

CIE TC2-64 High-Speed Testing Methods for LEDs
CIE TC2-63

Measurement of LEDS

CIE TC2-51 Calibration, Characterization and Use of Array Spectroradiometers
CIE TC2-50 Measurement of the Optical Properties of LED Assemblies

 

Info: As a Silver Supportive Member of the CIE, Gigahertz-Optik GmbH supports the norm processing and helps ensure that metrological concepts are technically feasible. We also participate in DIN specialist committees. The process enables us to quickly respond to new requirements of our customers.

 


3.2 Difference in measurement requirements depending on the application

The measurement techniques must be adapted to the corresponding application. This means that industrial measurement in a production line has very different requirements from an on-site product quality check by the customer. The following basic requirements apply:

Applicable to all applications:

Testing in production lines

Laboratory applications/incoming goods inspection/Production-assistant QA

On-site application tests by the customer/user


 

3.3 Key parameters

Specialist committees (e.g., CIE and IES) have specified different standards, norms, and guidelines for essential device parameters based on scientific publications. Below are some of the parameters:

Optical light meter:

Spectroradiometers are largely used in LED/SSL measurement technology.


Input Optics

  • Luminous flux measurement:

    Integrating spheres → Different applications require different sphere geometries. 2pi measurement geometries are often preferred in production lines, since they allow for fast and easy placement of the measurement object in the sphere. Combined 2pi or 4pi integrating spheres are a good choice for laboratory applications since they offer a high flexibility level in terms of position of the measurement object. In addition, rotatable spheres are a good option for measurement of the lamp in its application position.

    Note: Gigahertz-Optik GmbH has decades of experience in manufacturing of integrating spheres, and offers the necessary expertise and versatile design options in the selection and configuration of the spheres.

     

  • Luminous intensity measurement:

    ILED-B or ILED-A adapters → The LED average intensity adapter (A or B) has established itself in LED measurement technology through its fast and reproducible results.  Both variants require a 1 cm² homogeneous responsivity area and a defined measurement distance (ILED-B = 100 mm → 0.01 sr, ILED-A = 316 mm → 0.001 sr). ILED-B is more widespread due to its additional advantages associated with its shorter measurement distance and the resulting shorter measurement times (factor of 10).

     

  • Wafer tester:

    Some production lines require inspection/sorting during manufacturing on the wafer. This uses the so-called wafer testers. These are special, high-sensitivity, fiber-based input optics that allow for the shortest possible measurement times and precise positioning directly on the wafer in front of the desired LED. They are mostly calibrated in irradiance and radiance. In principle, they are a modification of the ILED-B or ILED-A measurement geometry for a different requirement.

     

  • Illuminance:

    COS diffuser → COS diffusers that are perfectly adapted across the entire spectral range of the spectroradiometer are very important for precise illuminance measurements. If they are not integrated, spectral measurement errors can result for large incident angles, due to a different COS weighting of the measurement signal.

     


 

Electric measurement

Software

In order to avoid operational errors and additional costs, intuitive software and good support are required for integration of the measurement technology in application processes, and to ensure correct measurements. The software must also support customization to meet the specific requirements of users. It should allow for inspection and monitoring of different devices, as well as support logical operations and evaluations. In addition, it should be able to provide fast, reliable, and precise calculation parameters. Fast and easy integration of new parameters, e.g. the IES TM-25 or IES TM30-15, must also be taken into account.

 

Environmental Conditions 

CIE S025 specifies guidelines for laboratory and framework conditions. These include the ambient temperature (tolerance interval ± 1.2 °C), surface temperature, draft, etc. It also specifies guidelines for the accuracy of the electric measurements.

 


 

4. Metrological solutions

The different norms, e.g. those mentioned under 3.1, serve as a guide for the metrological implementation. However, a good solution is not just about meeting these stipulated norms, but also keeping the solution practical, cost-friendly, and fulfilling the requirements of users. At Gigahertz-Optik, we place great value in this and have over 30 years of experience in the production of measurement systems that meet the requirements of customers, their respective applications, and the applicable norms. Below are some examples:

 


 

 

Tests in production lines

The BTS2048-VL series is characterized by its optical properties, extensive electronics, compact design, and flexibility in these applications. In terms of its optical properties and quality, the device is in the upper class of spectroradiometers. It can be connected directly to an integrating sphere for measurement of the luminous flux and different color measurements. The connection to an ILED-B adapter allows for irradiance measurements as per CIE127. Both measurement geometries are commonly used by established LED manufacturers for LED binning.

 

Schematische Darstellung abb2 en2 

Figure 2 BTS2048-VL with ISD-xxx (top-right) in schematically illustrated in a production line (top-left). The BTS2048-VL with an ILED-B adapter (bottom)

 

 

Thanks to the integrated electronic shutter, the BTS2048-VL enables significantly shorter integration times (shorter than 2 µs) compared to other devices in this class (µs range). This increases the dynamics 1000 times without OD filters. Use of OD filters increases the dynamics even further. The modern and powerful electronics, which include a variety of interfaces (ethernet, USB, …), make it possible to perform fast measurement sequences, i.e. the waiting time between measurements is reduced. For instance you can have a 1 ms measurement with a delay of just 6 ms. This saves time in production and thus also costs (more information can be found here).

 

shutter electronic normal2postprocessingBTS2048

 

Figure 3 The electronic shutter of the BTS2048-VL shortens the waiting time between two subsequent measurements. The powerful electronics enable much faster calculations and data transfer. 

binning process en 

Figure 4: Basic sequence of a binning process in production 

 


 

Laboratory applications/WEK/production-assistant QA

The BTS256-LED series is a versatile technology for laboratory applications. Being a handheld device, the BTS256-LED is well suited for measurement of individual LEDs. A quick-mount diffuser lens and the device’s bayonet connector enable the device to be connected to a goniometer, or used as an irradiance meter for spot lamps. The bayonet connector also makes it possible to connect the device onto different integrating spheres. It is the ideal spectroradiometer for versatile measurement tasks in a laboratory. (More information can be found here).

 

BTS256 LEDKugel set

 

 

The same concept can also be used with the BTS2048 series for even higher quality measurements (more information can be found here.

 

The TPI21-TH is a plug & play test system for single LEDs (Star, SMD, etc.) or LED assemblies with diameters of up to 70 mm. All functions necessary for measurement of LEDs using the Keithley Source meter, the high-quality spectroradiometer (BTS2048-VL), and measurement with thermoelectric cooling and heating (LEDA-7-TEC) are run by the system software. Specifications for the measurement cycle can be set individually be the user. The S-BTS2048 software also supports a complete measurement data analysis. Generally, its properties allow this system to perform measurements in accordance with DINv5032-9 and CIE S025. The optional light-tight measurement chamber enables the measurement system to be used in illuminated production locations.

 

 TPI21 TH Aufbau Schrank

 

Figure 5 TPI21-TH measurement system in the optional measurement chamber

 


 

On-site application test by the customer/user

We have two devices designed for measurement of the illuminance, irradiance, luminous color, and the CRI. These are developed for different technical requirements to provide customers with the ideal measurement device for their application. The BTS256-E series is a high-quality, BTS-based handheld meter with good stray light suppression, wavelength accuracy, etc., for the highest possible precision in field applications. With this device for instance precise color coordinate measurements can be performed (Mac Adam ellipses, Binning fields, etc.) for quality control purposes or inspection. Furthermore the EF vesion is able to measure flicker parameters directly on site.

The MSC15 is more like the little brother to the BTS256. It is priced significantly lower with slight compromises in terms of optical and functional performance. This is compensated for by the ease of use and intuitive software.

MSC15 BTS256E 


 

5. Summary

The requirements in LED/SSL testing are changing rapidly due to the rapid development in LED technology. In terms of the measurement technology, the main challenge lies in fulfilling the stipulated norms and standards, and at the same time meeting the requirements of users. In addition, short development times and good customer support are necessary for measurement techniques to also match up to the speed of LED development. With over 30 years of experience, Gigahertz-Optik GmbH is an expert in the development of this measurement technology, and with its modular product portfolio and high-quality calibration laboratory, it is the ideal partner for all current and future requirements.

 


 

6. Literature

Nevas S, Wübbeler G, Sperling A, Elster C and Teuber A 2012 Simultaneous correction of bandpass and stray-light effects in array spectroradiometer data Metrologia 49 S43

Zong Y, Brown S W, Johnson B C, Lykke K R and Ohno Y 2006 Simple spectral stray light correction method for array spectroradiometers Appl. Opt. 45 1111-9

CIE84, Measurement of Luminous Flux

CIE127, Measurement of LEDS

CIE198, Determination of Measurement Uncertainties in Photometry

CIE214, Effect of Instrumental Bandpass Function

CIE S014, Colorimetry

CIE S025, Test Method for LED Lamps, LED Luminaires and LED Modules

IES LM-79-08, Electrical and Photometric Measurements of Solid-Sate Lighting Products

IES LM-80-08, Measuring Lumen Maintenance of LED Light Sources

DIN 5032-9 Messung der lichttechnischen Größen von inkohärent strahlenden Halbleiterlichtquellen