LEDs are used in many applications, and new applications are found every day. To address this market, LEDs often come in vastly different varieties, shapes, sizes, packages and modules. Measurement of these LEDs is required so that they can be compared and selected within a global market. This paper presents the different types of optical quantity that can be measured for these LEDs together with guidelines for measurement. In particular, the protocols for measuring Averaged LED Intensity, Partial LED Flux, luminance and illuminance are presented. Some of these quantities are new, and the reader may be unfamiliar with them. Definitions are provided where appropriate. Many LED measurements have associated standard measurement conditions, which apply to LEDs and not other sources. Other measurements depend critically on setup conditions but lack standardization and hence details of methods used must accompany results. Where standard conditions exist these are detailed and where they do not advice is provided on the best methodologies. Work in establishing standard conditions is on-going, especially in Commission Internationale de l'Eclairage (CIE) technical committees, and information on this work is provided.
A comprehensive illuminator has been designed and constructed for complete test or characterization of CCD and CMOS image sensors. By changing the position of a multi-grating turret of a Czerny-Turner type monochromator from a grating to a highly reflective mirror, two modes of operation can be achieved. Monochromatic flux is generated when the turret is set to the normal grating position and broadband light is generated when the turret is set to the mirrored position. The flux is directed through an aperture and collimated to form a uniform, wide-field, monochromatic or broadband source. An in-situ calibrated detector along with two automated filter wheels is used to monitor monochromatic irradiance, illuminance and color temperature. Testing of image sensors for parameters such as well capacity, sensitivity and linearity can be made for broadband illumination whereas spectral responsivity and quantum efficiency measurements of the image sensor can be made over the wavelength range of 380 to 800 nm. An in-situ calibrated detector is used for absolute light calibration in both intensity and correlated color temperature. Two filter wheels and high-speed shutter have been integrated into the light path of the monochromator to enable the automatic control of output light intensity, shape and color. Stages are also provided for slide pattern projection and aperture control. This illuminator is able to output a 2'-diameter beam with less than 5% non-uniformity. The maximum broadband light output is close to 400 lux, and has 90 intensity control steps. The spectral test mode can cover visible wavelength range with resolution up to 0.5 nm. Software has been configured to do automatic mode change and testing. By designing standard image sensor illuminator functionality into a spectroradiometer, we have achieved a compact, multi- functional, automated, low-cost illuminator for image sensor testing and characterization. The option of adding a fiber optic to the system allows easy integration of the illuminator into any laboratory or production equipment such as a prober station or a packaged parts handler.
Displays providing NVIS compatibility must meet exacting standards. Instruments to test these devices must also perform to set minimum requirements, given in MIL-L-85762A appendix B. Traditionally, these requirements have been difficult to achieve in practice, and then only by compromising measurement conditions. A combination of new technology and application- oriented optimization of the spectroradiometer has led to recent revolutionary changes in NVIS measurement instrumentation. The optimizations possible, the mechanisms of achieving these optimizations, and the impact of any differences on measurements are discussed for two commercial instruments.
SC657: Accurate Measurement of LED Optical Properties
This course provides attendees with a working knowledge of the optical properties of LEDs and how to measure them correctly. The course concentrates on techniques for controlling variables that can lead to large errors. Traceability to NIST and uncertainty are explained clearly. Many practical examples are included throughout, including actual measurements of die and packaged LEDs. Attendees will be able to identify and control critical variables to give high accuracy measurements.