············•·········•··············•······· 89 The Detector-Based Candela Scale and Related Photometric Calibration Procedures at NIST Y. Ohno, C.L. Cromer, ].E. H~dis, and G. Eppeldauer THIS PAPER IS AN IESNA TRANSACTION. IT WAS ORIGINALLY PRESENTED AT THE 1993 IESNA ANNUAL CONFERENCE. Introduction tions, such as those for luminous intensity, illumi· Since the redefinition of the candela in 1979, na· nance, luminance, and total luminous flux, have also tiona! standards laboratories have been free to realize been drastically revised, and the absolute accuracy of the candela by use of whatever radiometric means calibrations have been significantly improved. The they found most suitable. Different national new calibration procedures developed and being laboratories1- 8 and other research facilities9•10 have developed are described here. Uncertainty statements employed various techniques to realize their lumi· in this paper follow Taylor, 15 which recommends the nous intensity scales. use of 2u values for calibrations and 1u values for most Until recently, the luminous intensity scale at the other results including the uncertainty of scales. National Institute of Standards Technology was based on a gold-point blackbody. 1 The blackbody radiation The detector-based candela scale at the gold point (1337.33 K; determined at NIST from absolute radiometric detectors12) was used to Principles calibrate a variable temperature blackbody, which Let us assume a photometer as shown in Figure I is provided the spectral radiance scale. 13 From this, the constructed, using a silicon photodiode, a V(A) filter, spectral-irradiance scale was derived.14 The luminous· intensity scale was realized through spectral irra· Photometer diance measurements of a primary reference group of candela lamps using the spectral irradiance working Ught source standards. A secondary reference group, calibrated ~ r[m) against the primary group, was used for routine ~------------------------------~ calibrations. The final uncertainty of 0.8 percent (2u) 15 contained a relatively large component due to the uncertainty in the gold-point temperature at the V(A) filter Silicon top of the chain, and additional uncertainty ac· photodiode cumulated in the long calibration chain using sources. Figure I -Geometry for the detector-based candela scale realization Recently at the Radiometric Physics Division of NIST, the candela scale has been realized using a and a precision aperture. When the absolute spectral group of absolutely calibrated photometers. Other na· responsivity s(A) (in amps/watt) of the photometer is tiona! laboratories have employed absolutely measured, the responsivity R.r (in amps/lumen) of the calibrated radiometers or photometers using either photometer for luminous flux (lm) is given by thermal detectors or self-calibrated silicon photodiodes. NIST chose calibrated silicon photo· diodes with filters to make photometers on account of 1A P(A) s(A) dA (Ailm) (1) their wide dynamic range and simplicity of operation. R-r= -------- The photometers had their relative spectral respon· Km A P(A) V(A) dA sivity matched to the spectral luminous efficiency 1 function V(A), and were calibrated against the NIST where P(A) is the spectral power distribution of light absolute spectral responsivity scale, which is currently to be measured, V(A) is the spectral luminous efficien· based on a cryogenic radiometer.16 The design, cy function, and Km is the maximum spectral efficacy characterization, and calibration of the photometers (683 lm/W). If the area S(m2) of the aperture is are described in this paper. known and the responsivity R,.1 is uniform within the The procedures of various photometric calibra· aperture opening, the responsivity R,.; of the photometer for illuminance (lx) is given by Authors' affiliation: National Institute of Standards and Technology, Radiometric Physics Division, Gaithersburg, MD. R,.; = S • R,.1 (A/Ix) (2) JOURNAL of the Illuminating Engineering Society Winter 1994 90 •···•·······•········•··•········•··········· When a photometer calibrated for R,; is used to photometers are obtained by Equations 1 and 2 for measure the illuminance from a point source, the given P(A), thus providing the illuminance scale. Using luminous intensity Iv (in candelas) of the source is the photometers, the luminous intensity of a candela given by lamp is determined from the illuminance measure· ment and the given distance r in Equation 3. This pro· (3) cedure for a candela scale realization is simpler than where r is the distance (in meters) from the light the conventional source-based method. The details of source to the aperture surface of the photometer and each calibration step are discussed below. I is the output current (amps) of the photometer. To realize the candela scale, although the principles Construction of standard photometers are simple, uncertainties in measuring each of the A group of eight photometers has been developed. quantities- and uncertainties associated with the Figure 3 depicts the photometer design. A silicon assumptions must be evaluated. Figure 2 shows the photodiode, a V(A) filter, and a precision aperture are calibration chain for the candela scale as revised by mounted in the front piece of a cylindrical housing. The photodiode is plugged into a socket with a teflon this work. A cryogenic radiometer which has recently base of low electrical conductivity. On the front side of the filter, the precision aperture is glued to a holder Absolute Cryogenic carefully machined so that its front surface (the Radiometer reference surface of the photometer) is 3.00 mm from the plane of the aperture knife edge. Abs. Spectral Res ponsivity Transfer Under this front piece, an electronic assembly con· taining a current-to-voltage converter circuit with a high sensitivity and a wide dynamic range17 is built· Spectral Response Scale in to minimize noise. The circuit has a switchable gain (Silicon Photodiodes) [AIW] setting from 104 to 10 10 or 10 11 V/A. An input equivalent noise of -1 fA is achieved at the gain set· Abs.Spectral Re sponsivity calibration 11 Aperture Area measurement ting of 10 V/A with an integration time of 1.67 s and Calculation bas ed on Candela Definition a measurement bandwidth of 0.3 Hz. This high sen· sitivity feature allows precise measurement of s(A) Illuminance Scale even in the wings of the V(}l.) curve. ( Standard Photometers) [ A/lx] Because characteristics of the filter and photodiode change with temperature, a temperature sensor is in­ Distance Measu rement stalled in the front piece of the housing to monitor the photometer temperature.ts Luminous Intensity Scale Removable aperture holder (Standard lamps) [ cd] Precision Figure 2-Calibration chain for the detector-based candela scale i1s revised by the present work 24mm been installed at NIST acts as the absolute radiometric base at the top of the chain. The radiometer, (called HACR; High Accuracy Cryogenic Radiometer), cooled by liquid helium to 5 K, works on the principles of electrical substitution. The HACR's measurement uncertainty in the calibration of a light-trapping 16 detector is 0.026 percent (2cr) at 633 nm. The ab· 63.5mmdia. solute responsivities at several laser wavelengths are transferred to the Spectral Comparator Facility (SCF), Figure 3-Photometer design where the spectral responsivity scale is realized with a light-trapping detector. It is described below. The ab· * Specific firms and trade names are identified in this paper in order to solute spectral responsivity s(A) of each photometer in adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of the group is determined using this spectral response Standards and Technology, nor does it imply that the materials or equip· scale. The photometric responsivities R,; of the ment identified are necessarily the best available for the purpose. Winter 1994 JOURNAL of the Illuminating Engineering Society ••••••••••••••••••••••••••••••••••••••••••••• 91 Characterization of components radii, the area was calculated by a polygonal approx­ imation. The uncertainty of this area measurement Hamamatsu* SI226 and S1227 series silicon photodiodes were used for the photometers. They was given as 0.04 percent for the larger apertures and were selected for the largest shunt resistance that the 0.1 percent (both 2a) for the smaller. The transfer gains of the current-to-voltage con­ manufacturer could provide, 2.5-7.0 GO, in order to verters in the photometers were calibrated electrically minimize noise and drift in the amplifier circuit.17 by replacing the photodiode with a computer­ The reduced infrared sensitivity of these photodiodes controlled voltage source and calibrated resistors in compared to others is advantageous for application in series. The amplifier gains were determined by a photometry. SI227-IOIOBQ photodiodes with l-cm2 linear fitting of data for many combinations of input areas were used for photometers I and 2, which current and measured output voltage. The gains so possess larger-area V(}..) filters. SI226-8BQ photodi­ determined have an uncertainty of less than 0.01 per­ odes with 0.3-cm2 areas were used for the other six cent (2a). photometers. The photodiodes were screened for their uniformity of response over their active areas. Characterization and calibration of photometers The uniformities of the photodiodes were measured After characterizing the components, the pho­ at several wavelengths using the SCF (described in the tometers were assembled as shown in Figure 3. The next section), and the maximum deviation from the photometers were characterized for overall per­ mean value was less than 0.2 percent. The temperature formance and calibrated for photometric responsivity dependence of the photodiode responsivity was also as follows. measured in a temperature-controlled housing. It was An essential part of the calibration is to determine less than 0.03 percent/°C in the 400-700-nm region. the absolute spectral responsivity s(}..) using the spec­ For V(}..) filters, colored glass filters from different tral comparator facility (SCF) (Figure 4). A detector sources were used.