As a fiber-optic probe that can be used for a wide range of applications, the LED 25 is a genuine innovation in the field of light measurement. For the first time ever, it is now possible to measure averaged LED intensity (ILED-A and ILED-B), illuminance and (using a goniometer) luminous flux with a single measurement head. At the same time, only a single calibration is required. Application Note Fiber-optic Probe for Averaged LED Intensity LED 25 as a fiber-optic probe for ILED-A and ILED-B measurements Single LEDs for the visible spectral range are typically described in terms of the following photometric quantities: luminous intensity IV , luminous flux F V, and the dominant wavelength. In the case of white LEDs, the ”correlated color temperature“ (CCT; unit: Kelvin) is usually stated. Since the luminous intensity is defined by the derivate dF/dW, the surface area of the detector should be as small as possible and the distance between the tip of the LED and the detector as large as possible. In general, such conditions can be set up only on a laboratory scale. For this reason, CIE publication 127 introduced the optical quantity of ‘averaged LED intensity‘ (ILED-A or ILED-B) for measuring single LEDs in 1997. According to this recommendation, a locally homogeneous detector with a surface area of 100 mm2 and V(l)-shaped spectral response should be positioned 316 mm or 100 mm (for ILED-A and ILED-B respectively) away from the tip of the LED to be measured. Fig. 1a: Measurement geometry for ILED-A and ILED-B Detector‘s surface area (100 mm2) Fig. 1b: LED 25 in combination with an ILED-B spacer tube and an LED mounted in a test socket Integrating sphere Since in most cases the tip of an LED does not correspond to the LED‘s point of light emission, ILED-A and ILED-B represent independent measurement quantities that are defined by the average illuminance at a specific distance from the respective light source multiplied by the square of this distance. ILED–A, B = Ev · r 2A,B Fig. 1a shows the basic measurement geometry, Fig. 1b a sectional drawing of the LED 25 in combination with an ILED-B spacer tube and an LED in a test fixture.

Although ILED-A and ILED-B cannot be compared directly with luminous intensity, the requirements regarding measurement geometry and detector specifications are nevertheless comparable: The spectral and absolute responsivity of the detector must be homogeneous over the entire area. Particularly in the case of high-brightness LEDs, which usually have a narrow-angled radiation pattern, this is crucial because the irradiance along the detector area can vary significantly. Detectors with poor homogeneity lead to slightly inconsistent results, which makes it far more difficult to compare them. To keep...

diameter. In this case, the percentage variation amounts to just 2%. In the event of even smaller distances or higher requirements regarding measurement uncertainty, it is possible to use cosineoptimized fiber-optic probes, such as the ISP 40. Fig. 4: Potential measurement geometry for testing LED modules Fig. 5: Percentage variation of the measured illuminance from the actual value as a function of the aperture ratio for light sources that take the form of a circular area (e.g. certain LED modules) In this application, the LED 25 is used as a fiber-optic probe in conjunction with the LEDGON...

Fig. 6: Top: LED module as is used in traffic light systems Right: Examples of radiation patterns in the near field (top diagram) and the far field (bottom diagram) Fig. 7: Schematic drawing illustrating luminous flux measurement using the LED 25 and LEDGON For correct luminous flux measurement, it must be ensured that the angle scan of the LEDGON covers all directions in which light is emitted. The luminous flux is calculated from integration of the radiation pattern over the measured solid angle: Furthermore, the good cosine response of the LED 25 makes it possible to determine the luminous...