Tutorial abstracts

The tutorial is intended mainly for those who are involved in LED measurements and want to learn more generally about photometry and radiometry, and those who have some knowledge and experience in photometry and radiometry but are new to LED measurements.  

 
 

Tutorial 1

Introduction to Photometry and Radiometry of LEDs
Yoshi Ohno
National Institute of Standards and Technology
Gaithersburg, Maryland USA
 
The quantities and units in photometry and radiometry, with important principles including Planck's law and Lambert's cosine law will be first introduced. Then, the fundamentals of luminous intensity measurement and luminous flux measurement for LEDs will be presented. The outlines for luminous intensity measurement include calibration methods (source-based and detector-based), spectral mismatch correction, f1' of a photometer, reference plane of a photometer head, and stray light. Special issues for LED measurement such as reference plane and reference axis of LEDs, and use of standard LEDs will be discussed. The outlines for luminous flux measurement include basic integrating sphere theories, calibration methods (sphere photometry, goniophotometry), fundamentals of sphere photometry including self-absorption, near-filed absorption, spectral mismatch correction, and spatial nonunniformity errors. Special issues for LED measurements such as LED mounting geometires (4 and 2), backward emission, and use of standard LEDs will be discussed. Finally, the measurement of radiometric quantities using a photometer or a radiometers is discussed, including conversions between photometric and radiometric quantities.
 
 
 
 
Tutorial 2
Introduction to Spectroradiometry and Colorimetry of LEDs
Teresa Goodman
National Physical Laboratory
Teddington, Middlesex UK
 
 
LEDs have many special features which not only make them particularly useful for a wide range of applications, but can also make them especially difficult to characterize. For example they:
Often the spectral and colour properties are the most significant feature, and hence spectroradiometric and colorimetric measurements are frequently required for the selection of LEDs for a specific application, quality control, new product development etc.
 
This presentation will introduce the basic principles of spectroradiometry and colorimetry, including the CIE chromaticity diagram and (x, y) coordinates. Methods for spectroradiometric and colorimetric measurements will be studied, including scanning monochromators, diode arrays and tristimulus colorimeters. The selection and use of reference standards will be discussed and potential sources of measurement error will also be reviewed, taking account of the particular properties of LEDs which can have a significant influence on the magnitude of these errors.
 
 
 
 
 
Tutorial 3
Uncertainty principles and their application to LED measurement
Georg Sauter
Physikalisch-Technische Bundesanstalt
Bundesallee 100, D-38116 Braunschweig, Germany
 
The light of LEDs varies from saturated colors to "broadband white" with angular distributions ranging from the "classical Lambertian" to narrow solid angles and it is emitted from areas formed arbitrarily in shape, structure and size - especially, if LED-clusters are regarded, too. These multiple distributions of the radiant power have to be weighted and integrated to get the characterizing radiometric, photometric and colorimetric quantities. The principles to determine measurement uncertainty are summarized as a basis to develop the models for the evaluation of uncertainty associated to the measured quantities and, as examples, LED intensity, flux and chromaticity coordinates are demonstrated. It is shown, that traceability and calibration of the measurement setup depend on type and operation of the reference standards, and how measurement method, procedure and minimum uncertainty of the LED-measurements are effected., Examples of uncertainty budgets for individual measurements are shown and compared additionally with the tolerance interval of LED-characteristics stated in catalogues and used for selection and classification of the products.
 
 
 
 
Tutorial 4
CIE Work on LED Measurements
Kathleen Muray
INPHORA Inc. California
 
The tutorial will start by reviewing the recommendations as in CIE Report 127. The most important result of how to measure LED intensity will be emphasized. The remaining issues not generally accepted by the measurement community will be discussed, such as spectral and luminous flux measurements; problems still not resolved will be mentioned. The newly formed technical committees in CIE Div. 2 will be described and their present work in order to complete the measurement recommendations of individual LEDs will be detailed. The first attempts to solve LED measurement problems were concentrated on individual LEDs. During the last 5 years the efficiency of LEDs was increasing very rapidly, and they became available on the market by increasing numbers in all colors including white. Applications of these unique light sources has been growing rapidly as well; in most cases clusters of LEDs are put into different type of configurations. It became important to study LED cluster measurement problems, therefore one more CIE Div.2 Technical committee was born; also within CORM Radiometry section a technical committee collects all the existing measurement standards of LED clusters in applications such as traffic lights and signs, automotive lighting, general lighting and many others. At present many different standards are used at different locations in the world. This technical committee hopes for being able to generalize eventually the most important measurement types to where it applies world-wide.
 
 
 
 
Tutorial 5
Visual aspects connected with LED measurement
J Schanda
University Veszprém, Hungary
 
The present photometric system is based on the visibility (V()) function adopted by the CIE in 1924. During the past decades many investigations have shown that the function is in error in the blue part of the spectrum. A correction has been implemented in 1990 in the form of the VM() function. This has never been accepted by the metrological authorities as a valid photometric weighting function. For very low light intensities, for the scotopic region, the V'() function has been agreed upon, but between the photopic and scotopic region, in the mesopic one, there is no approved photometric system either.
The situation has been further complicated by the findings that the V() based photometry is a poor descriptor of brightness perception, but can be applied to describe light levels needed to perform work on foveally seen objects. If the object (sign or signal) is seen parafoveally, i.e. it is not in the direct line of sight, V() fails again. For such visual tasks the V10() function &endash; suggested some 40 years ago &endash; could be an alternative, but this function has no official status yet. (A CIE TC is currently working on the subject.)
In the tutorial we will show the advantages of using a metric based on the V10() function, describe how it was derived and where the limitations of its use are. For LED technology this is a very important question, because the visibility of a blue signal is underestimated using the current photometric system, compared to a V10() based one by a factor of two.
Further issues to be covered are the new CIE proposed colour boundaries of signal lights, and the implication of this on LED signals.