Preprint: Paper to be published in Proc., CIE Expert Symposium on LED Light Sources, June 2004, Tokyo MEASUREMENT OF TOTAL RADIANT FLUX OF UV LEDS ZONG, Y., MILLER, C. C., LYKKE, K. R., OHNO, Y. Optical Technology Division National Institute of Standards and Technology, Gaithersburg, MD 20899, USA ABSTRACT We have developed a source-based method and a detector-based method for total radiant flux measurement for deep-blue and UV LEDs, using a 2.5 m integrating sphere. Several UV LEDs with peak wavelengths of 375 nm and 390 nm were measured using both the source-based method and the detector-based method with a relative expanded uncertainty (k=2) of ~6 % and ~5 %, respectively. The results of the two methods agreed within 2 %. Keywords: integrating sphere; LED; total radiant flux. 1. INTRODUCTION There is an increasing need for accurate measurement of the total radiant flux (W) and efficiency of LEDs in the deep-blue to UV region. NIST has already established a calibration facility for the total luminous flux (lm) of LEDs using our 2.5 m integrating sphere [1]. The current absolute integrating sphere calibration method cannot be used for radiant flux measurement in the UV region or even the deep blue region because the photometer signal is very low and the uncertainty is too high. Total radiant flux measurements, in general, can be realized by radiometric measurements with a goniophotometer or by measurements of total spectral radiant flux (W/nm) with a spectro-goniophotometer. However, these facilities are yet to be established at NIST. To accommodate the urgent need of industry for calibration of UV and deep-blue LEDs, we have established two calibration methods using our 2.5 m integrating sphere facility. 2. MEASUREMENT METHODS Two independent methods have been developed for the measurement of the total radiant flux of deep-blue and UV LEDs. One is a source-based method using a spectral irradiance standard lamp. The other is a detector-based method using a spectral irradiance responsivity reference detector. 2.1 The source-based method Figure 1 depicts this source-based method. It employs a spectroradiometer and a spectral irradiance standard FEL lamp as an external calibration source. The same principles as the Absolute Integrating Sphere method [2] for the luminous flux measure-ment at NIST is used but applied spectrally. Since the flux from an LED is relatively low and the sphere throughput is low (due in part to the large size), we need an instrument with a very high sensitivity. We used an array spectroradiometer employing a back-thinned CCD array, which gives sufficient signal-to-noise ratio for the LEDs measured. The total sphere system (spectroradiometer and the integrating sphere) is calibrated against the spectral radiant flux of the beam introduced from the external spectral irradiance standard FEL lamp, which was calibrated at the NIST Facility for Automated Spectro-radiometric Calibrations, in the direction of its optical axis at a distance of 0.5 m. The external beam and the LED emission have very different spatial profiles and illuminate different parts of the sphere; thus, a uniform sphere responsivity is critical to reduce the error from the spatial dissimilarity. In order to achieve a uniform sphere responsivity, the cosine-response of the fiber probe of the spectroradiometer on the sphere wall is extremely important. We use the same diffuser, a surface-ground opal glass, as was used for total luminous flux calibrations [2]. The surface-ground 2.5 m Integrating Sphere LED Precision Aperture Baffle “Hot” spot Optical Fiber Spectroradiometer Spectral Irradiance Standard FEL Lamp d = 0.5 m Figure 1. Source-based method 1