Standardization of LED Measurements

Dr. C. Cameron Miller and Dr. Yoshi Ohno, National Institute of Standards and Technology

Today, standards development remains a critical concern for LED manufacturers. One standard publication of note is Publication No. 127, Measurement of LEDs, which was published in 1997 by the Commission Internationale de L'Eclairage (CIE) to establish guidelines primarily for measuring the luminous intensity of LEDs.¹ This is perhaps more complex than it sounds because luminous intensity is a quantity that describes a point source, and most LEDs are not point sources.

Hence, the measured luminous intensity varies, depending on the distance and size of the photometer's aperture. To solve this problem, the publication introduced standardized geometries: The size of the photometer aperture is set at 1 cm² (circular), and the distance between LED and photometer is either 316 or 100 mm. The quantity measured is not a true luminous intensity, so it is called averaged LED intensity and is designated I_{LED A} and I_{LED B}, depending on the geometry used (in condition A, d = 316 mm; in B, d = 100 mm) (see table).

This recommendation has proved successful in solving the problem in luminous intensity measurements and is already widely used in the industry. To extend this recommendation further, CIE Technical Committee 2-46 is preparing a CIE/ISO standard on the measurement of averaged LED intensity.

Publication No. 127 also describes the measurement of total luminous flux (integration of flux over 4Π steradians) using integrating spheres. However, the sphere geometries described have flaws that could lead to inaccurate measurements. A revision to CIE Publication 127 currently in the works will include improved geometries for total luminous flux measurements.

In addition, partial LED flux (flux emitted within a given cone angle) is being considered. Such a quantity is needed for applications where only forward emission of LEDs is utilized. The revision also is to include recommendations on using spectroradiometers for photometric and color measurements, including those used with an integrating sphere.

Many questions and concerns also relate to the measurement of LED clusters and arrays. Two groups are active in preparing recommendations for these categories. CIE Technical Committee 2-50 is preparing a report on the measurement of the optical properties of visible LED clusters and arrays that will suggest recommendations for measurement methods and conditions for various clusters and arrays, including static displays and signs.

Also of note is Project 78, Guide for Measurement of LEDs, under the auspices of the Testing Procedures Committee of the Illuminating Engineering Society of North America. Its purpose is to help the community come to a consensus on appropriate methods and equipment to measure the output of LED fixtures.

In addition, CIE is investigating the visual aspects involved with LEDs through Technical Committee 1-62. This group is investigating the color-rendering properties of white LED light sources to test the applicability of the CIE Color-Rendering Index to white LEDs.

On the side of photobiological safety, two documents are of note: IEC 60825-1-2001, Safety of laser products part 1: equipment classification and user’s guide, and CIE S009/E: 2002, Photobiological safety of lamps and lamp systems. Both include LEDs in their scope. The problem is that LEDs are treated as lasers in IEC 60825. In line with this, CIE Technical Committee 6-55 will report on the different methods of assessing the photobiological safety of LEDs. This will entail a review of and report on the known effects from a physiological standpoint, and a determination of the dose relationships that pose a potential risk for eye injury from excessive irradiation.

One aspect that makes the photobiological safety evaluation difficult is the measurement of LED radiance. CIE technical committee 2-58 has just begun work on a technical report that sets out recommendations for measurement of the radiance and luminance of LEDs, taking particular account of the specific requirements for evaluation of photobiological safety.

Reference

1. CIE (1997). Publication No. 127. Web site: www.cie.co.at/cie.

Meet the authors

C. Cameron Miller is the photometry project leader and Yoshi Ohno is the Optical Sensor group leader in the Optical Technology Div. of the Physics Laboratory at the National Institute of Standards and Technology in Gaithersburg, Md.; e-mail: c.miller@nist.gov. Ohno serves as secretary of CIE Div. 2.