Final manuscript, December 1, 1996 OSA Handbook of Applied Photometry Chapter 3 – Photometric Standards Dr. Yoshihiro Ohno Table of Contents 3.1 History of photometric standards ····································································· 1 3.2 Photometry, physical photometry, and radiometry ·········································· 2 3.3 SI units and the international legal metrology system ········································4 3.4 Luminous intensity standards ··········································································· 6 3.4.1 Detector-based candela realization ······················································· 6 3.4.2 Source-based candela realization ·························································· 9 3.4.3 Luminous intensity transfer standard lamps ······································· 11 3.4.3.1 Lamp types ·········································································· 11 3.4.3.2 Lamp seasoning ···································································· 12 3.4.3.3 Lamp characteristics and screening ······································ 13 3.4.3.4 Operation and handling of standard lamps ··························· 17 3.4.4 Illuminance transfer standard photometers ········································· 18 3.4.4.1 Requirements for standard photometers ······························ 18 3.4.4.2 Characterization of standard photometers ··························· 20 3.4.4.3 Operation and handling of standard photometers ················ 23 3.4.4.4 Determination of the reference plane ··································· 24 3.5 Luminous flux standards ················································································· 26 3.5.1 Goniophotometric method ·································································· 26 3.5.2 Absolute integrating sphere method ··················································· 29 3.5.3 Luminous flux transfer standards ························································ 31 3.5.3.1 Requirements for standard lamps ········································· 31 3.5.3.2 Seasoning and screening ······················································· 33 3.5.3.3 Operation and handling of standard lamps ···························· 34 - i - 3.6 Luminance standards ······················································································· 34 3.6.1 Detector-based realization of luminance unit ········································34 3.6.2 Method using a diffuse reflectance/transmittance standard ················ 36 3.6.3 Luminance transfer standards ····························································· 38 3.6.3.1 Opal glass ············································································ 38 3.6.3.2 Integrating sphere sources ··················································· 38 3.6.3.3 Luminance meters ······························································· 39 3.7 Color temperature standards ·········································································· 40 3.7.1 Definitions of terms ··········································································· 40 3.7.2 Realization of distribution temperature scale····································· 41 3.7.3 Color temperature transfer standards ················································· 42 Acknowledgement ··································································································· 45 Reference ················································································································· 46 - ii - 3.1 History of photometric standards The history of the standards for light dates back to the early nineteenth century, when the flame of a candle was used as a unit of luminous intensity that was called the candle. The candle power, the old name for the luminous intensity, originated from the use of candles. As early visual photometers were improved, it was determined that candles were not reproducible to the accuracy of the measurement even when the composition, form, and rate of burning were carefully specified. Numerous efforts were made to use controlled flame lamps. In the mid nineteenth century, the standard candles were gradually superseded by various other flame standards such as the carcel lamp, the pentane lamp, and Hefner lamp1. Despite careful specifications of manufacturing details and numerous determinations of the correction factors, none of the flame standards proved adequate for accurate photometry. In the late nineteenth century, suggestions were made to construct some form of standard depending on the radiation given by a specified area of surface at a given temperature, such as the melting platinum standard known as the Violle standard. This standard utilizing molten platinum, however, was found unsatisfactory because of variations in the surface emissivity and the freezing point caused by contamination. About the same time, the use of an incandescent filament lamp as a standard was proposed. But it was found to be impractical because it was not possible to specify and manufacture such a lamp to the extreme accuracy required for an absolute standard. In early twentieth century, to improve the Violle standard, investigations on platinum point blackbodies began at some national laboratories. The blackbody consisted of a cylindrical radiator made of pure fused thoria (about 45 mm long), which was immersed in pure molten platinum maintained at the temperature of solidification (2042 K). The entire blackbody was heated in a high-frequency induction furnace with 7 kW power to bring it to the melting point. An agreement was first established in 1909 among the national laboratories of France, Great Britain, and the United States to use this method. The unit was recognized as the international candle. This standard was adopted by the Commission Internationale de l’Eclairage (CIE) in 19212. After a successful realization of the candle in 19313, this method became universally recognized. In 1948, it was adopted by the Conférence Générale des Poids et Mesures (CGPM)4 with a new Latin name “candela.” In 1967, CGPM adopted a more precise definition of the candela5 as The candela is the luminous intensity, in the perpendicular direction, of a surface of 1/600000 square meter of a blackbody (full radiator) at the temperature of freezing platinum under a pressure of 101325 newton per square meter. The candela also became one of the base SI units (Systéme International d’Unités) when the SI was established in 19606. Although this definition served to establish the uniformity of photometric measurements in the world, difficulties in fabrication of the blackbody and in improving accuracy were addressed. Since the mid 1950s, suggestions were made to define the candela in relation to the optical watt so that complicated source standards would not be needed. There were many efforts to determine the - 1 - constant that would provide a numerical relationship between the photometric quantities and the radiometric quantities7,8. In 1979, the new definition of the candela was adopted by the CGPM9 defining the candela in relation to the radiant power (watt) by introducing the constant Km as described in the later sections of this chapter. The 1979 redefinition of the candela has allowed the use of appropriate techniques to derive the photometric units from the radiometric scales. After the new definition, most national laboratories have realized the candela based on the absolute responsivity of detectors rather than blackbody radiation. Before the international intercomparison of photometric units held by the Comité Consultatif de Photométrie et Radiométrie (CCPR) in 198510, many national laboratories realized the candela by using room temperature electrical substitution radiometers (ESRs). This intercomparison showed a ± 1 % variation of the national units of the candela, which was slightly better than previous intercomparisons, but the improvement was less than expected. In the early 1980s, the silicon photodiode self-calibration technique11,12 was developed and used extensively for realization of photometric units. Absolute cryogenic radiometers are now used in national laboratories to provide radiometric scales with uncertainties on the order of 0.01 %. The candela is now realized based on cryogenic radiometers at several national laboratories. With these recent improvements in technology, a smaller variation of national units is expected, and will be the subject of another CCPR international intercomparison of photometric units planned for 1998. 3.2 Photometry, physical photometry, and radiometry The primary aim of photometry is to measure visible radiation or light, in such a way that the results correlate as closely as possible with what the visual sensation would be of a normal human observer exposed to that radiation. Until about 1940, visual comparison techniques of measurements were predominant in photometry, where typically an observer was required to match the brightness of two visual fields viewed either simultaneously or sequentially. In modern photometric practice, almost all measurements are made with photodetectors, and is referred to as physical photometry. In order to achieve the aim of photometry, one must take into account the characteristics of the human vision. The relative spectral responsivity of the human eye is similar for most observers but can vary depending on individuals and on the viewing conditions. A relative spectral responsivity of the human