application note LED measurement radiant power measurement

Transcription

1 application note LED measurement radiant power measurement

2 Contents 1 Introduction Setup Introduction Measurement setup Integrating spheres LED measurement procedure CIE 127 & Admesy calibration procedure Overview Spectral and spatial distribution Remarks for measurement procedure References

3 1 Introduction This document is intended to clarify the procedure for accurate measurements of LED and SSL (Solid State Lighting) products by means of an integrating sphere. In perspective of CIE 127: 2007 recommendations, the entire procedure including calibration, absorption correction and led measurements is listed. Special attention is given to the enhancements of the calibration procedure with an Admesy tungsten broadband reference standard compared to the conventional calibration procedure. At last a number of general remarks concerning LED measurements are provided to ensure accurate and repeatable LED measurements. These measurements can be carried out using Admesy s Iliad software application. Latest Iliad Version can be downloaded from the Admesy website for free. 3

4 2 Setup 2.1 Introduction A typical LED measurement procedure with integrating sphere consists of three steps: calibrating the test setup, absorption correction for device under test (DUT) and actual measurements. Special attention is given to enhancements of the sphere setup calibration procedure with Admesy products compared to conventional calibration procedures as defined by CIE 127. Details are discussed in chapter CIE 127 & Admesy calibration procedure. 2.2 Measurement setup In order to conduct fast and accurate LED and SSL measurements, an integrating sphere setup as shown in figure 1 is recommended (CIE 127: 2007)]. An integrating sphere (1) connected to a spectrometer (2) by a fiber-optic flex cable (3) to measure a LED (4). For absorption correction an additional correction connecting an auxiliary lamp (5) may be necessary. Various reference standard lamps (not shown in the scheme below) are needed for spectral and luminous flux calibration as defined by CIE 127. This Admesy test setup is developed for luminous flux (lm) measurements and provides information regarding X, Y, Z parameters as well as x,y, u,v, CCT, dominant and peak wavelength, CRI and spectral distribution Fig 1 Integrating sphere test setup for LED measurements. 4

5 2.3 Integrating spheres An integrating sphere is used to spatially distribute the unidirectional radiation emitted by the LED into homogeneous radiation. Depending on properties of the LED, an appropriate sphere size has to be chosen. Large spheres typically have better light distribution compared to small spheres due to the relative small geometrical errors objects in the sphere cause. While small spheres have the advantage of a relatively high throughput, resulting in an increased signal-noise ratio. Table 1 provides an overview of Admesy spheres and typical applications. Model Diameter Auxiliary lamp Application Type of DUT 1 75mm No Production Standard single LEDs 2 150mm Yes Production Laboratory Standard single LEDs High power LEDs 3 250mm Yes Laboratory, CIE 127 compliant for 2π measurements Standard single LEDs High power LEDs Small LED modules Table 1 Admesy integrating spheres and applications. 5

6 3 LED measurement procedure The scheme below shows three steps of a typical LED measurement procedure, starting with sphere calibration procedure according to CIE 127 (left column) in two steps. The Admesy procedure with only one step is shown in the right column. The proposed adjustments in calibration procedures are described in detail in chapter 4. Followed by DUT and fixture absorption by means of an auxiliary lamp according to CIE 127 procedure. At last the actual LED measurement procedure. Procedure overview CIE 127: 2007 procedure Admesy procedure CIE 127 calibration: step 1 Admesy calibration Spectral wavelength calibration with tungsten broadband reference standard. Spectral wavelength calibration with Admesy tungsten broadband reference standard. Compensated for enhanced blue region spectral response (blue filter) and spatial distribution (cosine corrector). CIE 127 calibration: step 2 Absolute luminous flux calibration with LED reference standard (compensate for spectral and spatial distribution of tungsten). Absorption correction: step 1 Applicable to sphere with auxiliary port only (150 and 250 mm). Measure spectral radiant distribution without objects attached to sphere. Absorption correction: step 2 Applicable to sphere with auxiliary port only (150 and 250 mm). Measure spectral radiant distribution with test LED (turned off) attached to sphere. Start LED measurement Measurement of LEDs Fig 2 Overview of complete LED measurement procedure including calibration procedure. Fig 2 Overview of complete LED measurement procedure including calibration procedure. 6

7 4 CIE 127 & Admesy calibration procedure 4.1 Overview CIE 127 LED sphere setup calibration procedure describes two steps: Spectral wavelength calibration for spectral response of sphere. Absolute luminous flux calibration to compensate for poor blue spectral response of tungsten reference standard and typical spatial distribution of LED. Admesy enhanced calibration procedure in one step: Spectral calibration with blue enhanced broadband reference standard (poor blue region compensation) and cosine corrector (spatial LED distribution). 4.2 Spectral and spatial distribution CIE 127 requires the second step with a LED reference standard to calibrate the setup for LEDs based on their typical spectral and spatial distribution. Due to the wide range of variations in LEDs, CIE 127 states that a large amount of LED reference standards should be maintained. For each type of LED to be measured, a corresponding LED reference standard is required for calibration. Concerning the spectral deviations, the spectral distribution of the tungsten reference standard (fig 3) shows relatively low spectral power in the blue region compared to the longer wavelengths. In the shorter wavelength region, the signal noise ratio therefore is non optimal. Measuring white and blue LEDs which have typically high luminous flux in this region (fig 4) with a poor signal noise ratio may result in inaccurate measurement results. Second reason for calibrating with a LED reference standard is because of typical spatial distribution of LEDs. Fig 3 Spectral distribution of broadband tungsten reference standard used for CIE 127 calibration step 1. Fig 4 Spectral distribution of a (white) LED reference standard used for CIE 127 calibration step 2. Fig 5 Spectral distribution of Admesy enhanced broadband tungsten reference standard. To overcome all inconveniences in both calibration procedure as well as time and cost efficient LED measurements, Admesy has developed an enhanced halogen reference standard with blue filter and cosine corrector. This reference standard allows integrating both calibration steps of the CIE 127 recommendation. Because of the integrated blue light balancing filter, a more balanced spectral distribution is created (fig 5) causing a better signal noise ratio in the short wavelength region. Thus, allowing accurate measurements of white and blue LEDs. In addition, a cosine corrector ensures optimal Lambertian distribution to match the spatial distribution of a wide range of LEDs. Instead of maintaining multiple reference standards, one Admesy reference standard is sufficient for calibrating integrating sphere setups. 7

8 5 Remarks for measurement procedure High speed measurements (i.e. production settings) are technically speaking not as sophisticated compared to laboratory measurements. To ensure accuracy and repeatability of production measurements as recommended by CIE 127: 2007, a number of remarks are listed in this chapter with a brief explanation. Control sphere for dust and dirt. Control calibration (date) of spectrometer. Control calibration (date) of reference standard: due to maximum lifetime. Control stabilization time of reference standard before calibration. Control power supplies (current, voltage, AC/ DC, waveform). Control measurement parameters: i.e. measurement integration time, resolution. Control ambient temperature and humidity of test environment. CIE 127: 2007 states an ambient temperature of 25ºC (77F) unless otherwise specified. Control ambient lighting (no ambient light should enter the sphere). Control LED position during measurement (see examples below) Fig 6 Undesired situation (left) light beam is affected by obstructions. Desired situation (right) light beam is not affected by any obstructions. Fig 7 Generally undesired situation (left) where light from LED leaks outside the sphere. Desired situation (right) where all light emitted from LED enters the sphere. Fig 8 Components of LED and/or fixture may absorb light 8

9 6 References CIE, Publication No Measurements of LEDs, Central Bureau of the CIE, Vienna, Austria

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