Measurement Seminar – from SSL to Displays

Measurement is never a trivial task. The effort only makes sense if it is thoroughly executed with the appropriate equipment, well calibrated and adjusted. Otherwise, the results are not reproducible and therefore of questionable value. This is true for any measured parameter, but in light measurement especially, the effort, costs and requirements have become especially demanding. Measurement basics in regards to the measured parameters remain the same as before with conventional light sources, but this is not the whole story.

New quality criteria have to be documented that did not exist before or that were given constraints of the technology with minimal variation between competing brands. First, LEDs are directional light sources and any SSL design has to take this into account. Secondly, white LEDs emit a mix of light that is generated by the LED itself and some kind of color conversion material, which is at least slightly dislocated from the emission layer of the LED, having a completely different emission characteristic. In conjunction with other factors, this causes the spectral distribution to vary over emission angle. Third of all, not unique to LEDs but still relevant, the electronics to operate the LEDs, and here especially the digital character of the SSL system, have to be taken into account. By analogy, modern display technology also has some specialties that make measurement no trivial task.

The presenters not only sensitized the audience to the facts mentioned in the introduction, but on both days they presented measurement basics and lighting basics as well as practical issues. A practical demonstration of the appropriate measurement equipment was offered in the closing session of each day. Going into detail for every interesting and important point would go far beyond the scope of this short report and therefore the following sections will only be an overview of the comprehensive information that was provided with notes on the most important remarks and highlights.

Summary and Lesson Learned from the SSL Lectures

The SSL measurement session started with a lecture by Dr. Ðenan Konjhodžić on lighting basics. Dr. Konjhodžić explained what light is, how humans perceive light, what the special characteristics of different light sources are, which light measurement methods exist, and which photometric units exist – their meaning, how they are correlated and how they are measured. He explained the relation between radiometric units and photometric units as well as colorimetry with its indices and metrology. 

While measurement uncertainties, failures and failure-correction as well as reproducibility were a big topic throughout all lectures, Dr. Günther Leschhorn especially turned the spotlight on those points in his talk about goniometer measurement and integrating sphere measurement. Dr. Leschhorn explained the main requirements of the essential measurement standards, explicitly, the CIE S 025, and introduced the audience to standard-conform measurement. One interesting remark in this context was that any test method is allowed as long as the equivalence to established methods is proven. He presented some very useful tables that summarized the standard test conditions and requirements on the equipment before he came to the comparison between goniometer measurement and integrating sphere measurement in any detail, explaining the underlying theory and giving advice for best practice.

Martin Senft, R&D optronik line leader in Berlin, held a lecture on “Light Measurement in Automotive”. He focused on the differences to general lighting and made the audience aware of some pitfalls. At the beginning he gave a brief history of automotive lighting and legal regulations with a focus on the ECE-regulation. After explaining the photometric basics, he presented the national and international standards and recommendations for goniophotometers and goniophotometer measurement. As a sophisticated example, he related an idea about     candlepower measurement of headlights and how to use and transform the data for reports. Finally, Mr. Senft explained the requirements for a measuring room for automotive exterior lights and gave some safety instructions.

After our communal lunch, Dr. Tobias Roesener talked about laboratory measurement with a strong focus on spectral specification of the Device Under Test (DUT). Dr Roesener explained the available methods – based on photometer, colorimeter and spectrometer - their opportunities and limitations. One key message was that photometer and colorimeter based measurement are not sufficient for accurate LED measurement. The filter mismatch in combination with the narrow bandwidth of LEDs may cause blatant failures. As a consequence, he concentrated on spectro-radiometers, their working principle, technologies, requirements and quality criteria. He identified stray light to be very critical and showed methods to reduce this failure. Furthermore, he addressed the dynamic range and sensitivity of the sensor and the proper choice of the measurement range, because wrong settings or insufficient parameters lead to wrong results. As a special topic, Dr. Roesener elaborated on UV LED measurement and its unique requirements on the equipment. He demonstrated the difference between BaSO4 and PTFE coated integrating spheres, the measures to correct fluorescence related failures, and in addition, explained the stray light correction and suppression mechanisms.   

Dr. Elisabeth Bothschafter covered the topic which is crucial for the significance and credibility of any published measurement result: Calibration and reproducibility. She talked about “accuracy”, “validity” and “reliability” of measurements. - Three terms with distinct different meanings: While “accuracy” is determined by the parameters of the equipment, “validity” is characterized by the reproducibility and calibration. Both together determine the “reliability” of a measurement result. Dr. Bothschafter informed the audience about the calibration chain, certification and accreditation. In addition, she presented the LED calibration standards of the company. She discussed uncertainty and the uncertainty-budget as required for regulation conform measurement in detail on the example of an ILED-B measurement.

After a short coffee break, integrating sphere measurement and goniometer measurement were demonstrated by Mr. Peter Laepple and Dr. Günther Leschhorn respectively.

Peter Laepple demonstrated and explained the correct use of an integrating sphere in his esteemed competent and charming manner

Vigorous and vivid, Dr. Leschhorn demonstrated how to work with a goniometer and explained what one has to consider

Summary and Lesson Learned from the Display Measurement Lectures

While display measurement is a different field and the questions to be answered by measurement tasks are usually different from SSL measurement and its goals, there are still similarities, especially in regards to some equipment.
In a very interesting introduction, Dr. Michael E. Becker answered the why, what and how to measure and showed that there are many standards around the world, from which he identified four to be most relevant: CIE, IEC, ISO and SID-ICDM. CIE delivers the basics for a characterization, IEC defines the basic technical properties of the different display devices, ISO has a focus on the ergonomic properties, and SID-ICDM provides practical guidelines. The fact that display measurement is quite costly and a time-consuming task is nothing new, but one impressive slide showed why display testing has to be so complex and thorough: Luminance and chromaticity variations over angle should both be measured and documented. There might be distinct differences between them and a mixed result might therefore look very similar, but could appear to be different. 

After this brief introduction and a coffee break, Dr. Becker talked about electro-optical displays and screens, giving an idea as to how versatile the topics and the requirements are. Starting with the basics like additive and subtractive color mixing, pixels, raster, direct versus projected observation, he gave a perfect overview of the available and recent technologies and their capabilities. Besides the measurement relevant information, he surprised the audience with his thorough understanding of the market, current, and upcoming trends. LED, OLED and QD technology, their advantages and disadvantages were explained as well as “visual flaws” of displays that are, in fact, caused by the human visual system. A rather big part of the presentation concerned the e-paper technology as an emerging technology and market. The different technologies from the origin to state-of-the-art and possible future solutions were explained in detail.

Dr. Martin Wolf’s “Measurement technologies for Interior” shed light on the diverging requirements safety, operability and esthetics. In this context, it is not just the physical spectrum that is relevant but the human perception defined by the tristimulus-color space (XYZ). Imaging colorimeters as well as spectrometers are used in this domain. Spectrometer technology and positioning systems including goniometers were discussed and a complete measurement system including temperature control that allows the measurement of every relevant parameter. While it was clearly expressed that filter based colorimeters can never reach the accuracy of spectrometers, Dr. Wolf demonstrated that an increased number of filters with a matrix-optimization algorithm significantly improves the measurement results and makes such a system reasonable for many applications. The company’s all-in-one workhorse combines a RGB CCD-camera with a spectroradiometer and photometer. Again, the calibration was emphasized to being crucial for the result. The last part of the talk concerned two topics which are more critical in automotive applications than in most other applications: “Mura” and direction dependent variations. Solutions to analyze both problems were shown and explained.

Finally a useful matrix for measurement equipment, properties and tasks and applications was provided.

Andreas Huber talked about measurement in the lab, quality management and production - tasks with completely different requirements and goals. He emphasized that while an important requirement for lab equipment is flexibility, for quality management, especially in-line at production, speed and integration are crucial. Lab equipment has to be capable to determine optical parameters like contrast, luminance, color point and homogeneity and electro-optical properties like switching characteristics, gamma and flicker, optical parameters as function of the temperature and the influence of ambient light as ambient contrast, transmission and reflection. Amongst these tasks, lab equipment is also used to define the pass/fail-criteria for the quality management in the production. In succession, Mr. Huber explained the different tasks and their challenges. In regards to measurement in the production, he addressed also “accuracy”, “validity” and “reliability” of measurements which require robust equipment, especially because, in many cases, in-line calibration of the displays is required. Consistence of the results is inevitable. In the context of the tasks, Mr. Huber also showed some interesting peculiarities of display technologies, especially OLEDs: Their spectra for instance often show huge variations from probe to probe and fluctuations in luminance over time. The last part of his presentation dealt with equipment auditing and calibration standards.

The last presentation before the closing demo-session was again held by Dr. Becker and concerned “New Challenges for the Display Measurement Technology”. It would go far beyond the scope of this article to go into detail on this complex and extremely important topic. Just his closing advice must be mentioned: Watch out! – Always take care to provide reproducible measurement conditions and document them carefully!

The two final demonstrations were made by Peter Laepple and Dr. Ðenan Konjhodžić. Both presentations were characterized by interesting discussions sparked by questions from the audience.

The first display measurement demo clarified the strengths of imaging measurement systems which are fast recording of photometric and colorimetric quantities with spatial resolution

The second demonstration helped to understanding the correct application and the opportunities of 2-in-1 or 3-in-1 measurement solutions consisting of a RGB CCD-camera, a spectroradiometer and/or a photometer

Final Thoughts and Remarks  

The presentations covered the whole range from light measurement basics and measurement basics, in general, to standardization to information on the devices, either SSL or displays, and finally to up-to-date measurement technologies and equipment with its application. While someone might probably find a recap of light measurement basics and general measurement basics redundant, it is crucial to always keep these rules and basics in mind and it is certainly no mistake to be sensitized to them.

The much bigger part of the presentations concerned practical advice for the different challenges as well as an introduction to the technologies of the measured devices and measurement equipment. The updates given on the topics of standards, latest technologies, its specialties and the consequences for measurement tasks are, to say the least, interesting, and a view beyond one’s own tasks is always useful. Young engineers got a condensed introduction on the topic and the provided folder with the presentations is a valuable piece. On top of that, every participant got the “Handbook of LED and SSL Metrology” authored by Guenther Leschhorn and Richard Young: A very valuable reference book - also for experienced engineers who got the expected comprehensive refresher course with the seminar and an update on the latest knowledge in light measurement technologies.

Just one question remains to be answered: Who should attend? All in all, this successful seminar is highly recommended to anyone that is new to light and display measurement but it can also absolutely be recommended to experienced measurement engineers for an update of their measurement knowledge and valuable background information beyond measurement about the latest SSL and display technologies and the related challenges.

Acknowledegement:

The author wants to thank the organizers for the perfect organization, the interesting program and for providing the PPT-presentations. For additional information on the seminar and future dates, please visit the Instrument Systems Website.