CIE Expert Symposium 2008 on Advances in Photometry and Colorimetry 38 CIE x033:2008 f 1 ’ EVALUATION AND MEASUREMENT COMPARISON Jiangen Pan 1 , Haiping Shen 1 , Yuqin Zong 2 , and Yoshi Ohno 2 1 EVERFINE PHOTO-E-INFO CO., LTD. 2 National Institute of Standards and Technology ABSTRACT The mismatch of spectral responsivity to the CIE V(λ) function, i.e., the f 1 ’ index, is the most critical characteristic of photometers and tristimulus colorimeters. The f 1 ’ value varies with measurement conditions, which is often omitted in the f 1 ’ evaluation. We investigated the variations of the f 1 ’ value with different measurement conditions such as angle of incidence, spatial uniformity, and ambient temperature, by measuring eight high-grade commercial photometer heads from four manufacturers. The f 1 ’ values measured by three different facilities as well as manufacturers’ data were compared. The results indicate that f 1 ’ value can change considerably under different measurement conditions depending on the design of photometer. Keywords: Photometer heads, spectral responsivity, f 1 ’ index, measurement condition, photometry. 1. INTRODUCTION High-accuracy photometers are increasingly important in many applications, e.g. for measurement of LED lighting products. In the International Commission on Ilumination (CIE) Technical Committee TC 2-40 draft [1] under discussion, it is proposed that the photometers be classified as Class L (1.5%), Class A (3%), Class B (6%), or Class C (9%) * Pan is also a guest researcher at National Institute of Standards and Technology at the time of this research. Email: everfine@everfine.net by the f 1 ’ number. In such classification, especially for Class L, measurement uncertainty and variation of results for the f 1 ’ are in question. The f 1 ’ index is calculated from the spectral responsivity of the photometer head according to publication of CIE 69-1987; thus the uncertainty or variation in the measurement of spectral responsivity is the key factor for the f 1 ’ evaluation. Simulation analyses were first conducted to estimate measurement errors due to difference in monochromator bandpass and other conditions. Then, eight high-grade commercial photometer heads were measured to evaluate the effects of different measurement conditions - The photometers were measured at NIST and Everfine using a laser-based facility and monochromator based facilities for the comparison of measured f 1 ’. The results were also compared with the data provided by the manufacturers. 2. MEASUREMENT GEOMETRY AND SETUP Monochromators are normally used for the measurement of spectral responsivity detectors, but facilities using interference filters or tunable lasers are also used. An example of the laser-based system, the Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) developed at NIST, is shown in Figure 1.