![]() The test socket is inserted into the tube of the condition B adapter during measurement. Intensity probe that conforms to the standard CIE condition B for 100-mm distance together with a precision test socket for mounting the LED. A fiber bundle is located behind the diffuser to guide the light into a calibrated spectroradiometer.įIG. Two baffles in the beam path help to reduce stray light. The distance between the tip of the LED and the detector is exactly 100 mm in the case shown in Fig. The intensity probe comprises a tube with a length corresponding to the desired CIE condition into which an LED test socket can be inserted. 1 shows the realization of this concept in practice. This guarantees that the same geometry is always used when measuring luminous intensity in different laboratories irrespective of the design of the LED.įig. The front tip of the LED is always taken as the reference point for the distance. Condition B is the most commonly used geometry since it is also suitable for weak LED light sources. The CIE gives two recommendations for the distance between the LED and the detector surface (see table). The LED is positioned in such a way that its mechanical axis is directly in line with the center point of a round detector with an active area of 1 cm 2, and the surface of the detector is perpendicular to this axis. This concept no longer corresponds to the physically precise definition of luminous intensity but relates more to a measurement of illuminance at a fixed distance and detector size. Therefore, measurements done with different geometrical setups will most likely lead to different results and are difficult to compare.īecause of this, the CIE developed the concept of "averaged LED intensity" to solve the problem that occurs under near-field conditions. The irradiance measured at the detector is not easily related to the intensity of the source. A point source cannot be assumed and therefore the inverse square law no longer holds. Lenses, if present, may dramatically shift the apparent position of the emitting center. Many LEDs have a relatively large emitting area compared to the short distance that is generally used for a measurement. ![]() The many different designs available make it difficult to determine the precise position of the emission center (also known as the goniometric centroid) of the LED. One method of determining luminous intensity I v involves calibrating the detector in illuminance E v and calculating luminous intensity using the inverse square law:Īpart from maintaining the far-field condition, the validity of this calculation requires the precise measurement of the distance r between the detector and LED. The minimum factor, given by the ratio of the distance to the detector and the maximum extent of the light emitting surface, varies between 5 and 15 depending on the applied standard and the prevailing spatial radiation pattern. It varies with the size of the light source to be measured. The distance of the detector from the test specimen required for conformity with this criterion is known as the photometric distance. According to the definition, luminous intensity must be measured at a distance where the sample can be considered as an approximated point light source. Luminous intensity is the most frequently measured parameter for low power LEDs. Interested in articles & announcements on SSL performance & testing? The prior article and this article have been excepted from a chapter of the newly-published Handbook of LED and SSL Metrology reference book. This article continues the metrology theme, covering luminous and radiant intensity measurements and providing a discussion of uniformity and glare. In the October issue of LEDs Magazine, the article " Understand how to measure luminous flux and radiant power" covered some of the basics of metrology for the solid-state lighting (SSL) industry. In this excerpt from a reference book entitled Handbook of LED and SSL Metrology, GÜNTHER LESCHHORN and RICHARD YOUNG explain the fundamentals behind luminous and radiant intensity measurements and how to characterize uniformity and glare.
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