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Home Search Collections Journals About Contact us My IOPscience New source and detector technology for the realization of photometric units This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Metrologia 51 S276 (http://iopscience.iop.org/0026-1394/51/6/S276) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 176.9.124.142 This content was downloaded on 03/12/2014 at 14:56 Please note that terms and conditions apply. | Bureau International des Poids et Mesures Metrologia Metrologia 51 (2014) S276–S281 doi:10.1088/0026-1394/51/6/S276 New source and detector technology for the realization of photometric units 1,2 ¨ Timo Donsberg , Tomi Pulli2 , Tuomas Poikonen1 , Hans Baumgartner1,2 , Anna Vaskuri2 , Meelis Sildoja2 , ¨ a¨ 1,2 and Erkki Ikonen1,2 Farshid Manoocheri1,2 , Petri Karh 1 2 Centre for Metrology and Accreditation (MIKES), PO Box 9, FI-02151 Espoo, Finland Metrology Research Institute, Aalto University, PO Box 13000, FI-00076 Aalto, Finland E-mail: [email protected] Received 9 June 2014, revised 27 August 2014 Accepted for publication 28 August 2014 Published 20 November 2014 Abstract The production of incandescent light bulbs is bound to end, as incandescent lighting is being phased out globally in favour of more energy-efficient and sustainable solutions. Temporally stable light-emitting diodes (LEDs) are potential candidates to replace incandescent lamps as photometric source standards. However, traditional V(λ) filter based photometers may have large uncertainty when LEDs are measured instead of incandescent lamps. This is due to the narrow and complicated spectra of LEDs. When the spectra of LEDs are limited to the visible wavelength range, new silicon detector technology can be advantageously exploited in photometry. We present a novel method—based on the recently introduced Predictable Quantum Efficient Detector (PQED)—for the realization of photometric units which completely eliminates the need to use V(λ) filters. Instead, the photometric weighting is taken into account numerically by measuring the relative spectral irradiance. The illuminance values of a blue and a red LED were determined using the new method and a conventional reference photometer. The values obtained by the two methods deviated from each other by −0.06% and 0.48% for the blue and red LED, respectively. The PQED-based values have much lower standard uncertainty (0.17% to 0.18%) than the uncertainty of the values based on the conventional photometer (0.46% to 0.51%). Keywords: photometry, light-emitting diode, silicon photodetector, induced junction, optical standards (Some figures may appear in colour only in the online journal) 1. Introduction Incandescent lamps are widely used as measurement standards in photometry [1–3]. However, their mass production will end as incandescent lighting is being phased out globally in favour of more energy-efficient, sustainable and cost-effective light sources [4–7]. Eventually, the industry and the know-how needed for the manufacturing of incandescent lamps will be lost, and incandescent standards may not be available—or the costs will be exceedingly high. Therefore, it is worth looking at new photometric source standards to replace the traditional incandescent lamps. Light-emitting diodes (LEDs) are widely used in photometric applications [8–11]. Prospects for the new technology as photometric standards seem promising, as 0026-1394/14/060276+06$33.00 certain types of commercial LED light sources have been shown to be temporally stable within 0.2% in luminous flux over a time period of 20 000 h of operation [12]. However, the uncertainties of the measurement methods based on V(λ) filtered photometers may increase when measuring LEDs instead of incandescent lamps. This is due to the narrow and complicated spectra of LEDs. The situation is problematic in the red and blue regions of the visible spectrum, where the relative uncertainty of the V(λ) weighted spectral responsivity is high [13, 14]. We present a novel method for the realization of photometric units based on the recently introduced Predictable Quantum Efficient Detector (PQED) operated at room temperature [15–18]. The PQED is a primary standard of optical power that consists of two induced-junction S276 © 2014 BIPM & IOP Publishing Ltd Printed in the UK ¨ T Donsberg et al Metrologia 51 (2014) S276 photodiodes with almost unity internal quantum efficiency (IQE), meaning that almost all absorbed photons produce a collectable charge carrier. The photodiodes are mounted in a wedged trap configuration [19] for the elimination of specular reflectance losses. At room temperature, the IQE of the PQED can be modelled with an estimated standard uncertainty of 70 ppm in the visible wavelength range [20], whereas the uncertainty due to reflectance of unpolarized radiation is less than 30 ppm for most of the visible wavelength range [15, 21]. The predicted responsivity of the PQED has been confirmed experimentally with measurements against cryogenic radiometers [16]. The new method to realize the photometric units completely eliminates the need to use V(λ) filters. Instead, the photometric weighing is carried out numerically using separately measured relative spectral irradiance of the light source and the predicted spectral responsivity of the PQED. The method is applicable to light sources whose emission spectra do not extend outside the visible wavelength range, such as LED light sources. In order to validate the new method, illuminance values were determined using the new method and a conventional reference photometer. Such measured illuminances of a blue and red LED deviated by −0.06% and 0.48%, respectively. Moreover, the PQED-based illuminance values have a factor of three lower standard uncertainty than the illuminance values measured with the reference photometer. 2. Method Photometric quantities are obtained from corresponding radiometric quantities by weighting the radiation with the spectral luminous efficiency function V(λ) which represents the relative spectral responsivity of the human eye under daylight illumination levels. Thus, the relation between a photometric quantity Xv and the corresponding radiometric quantity Xe,λ is (1) Xv = Kcd Xe,λ (λ)V(λ) dλ, λ where constant Kcd = 683 lm W−1 is the maximum luminous efficacy for photopic vision [3]. A conventional photometric measurement is carried out by using a filtered detector, a photometer, the spectral responsivity of which closely follows the V(λ) curve. The deviation between the V(λ) and the relative spectral responsivity of the photometer, srel (λ) = s(λ)/s(λ0 ), is taken into account by using the so-called spectral mismatch correction factor e (λ)V(λ) dλ F = , (2) e (λ)srel (λ) dλ where e (λ) is the incident spectral radiant flux. For example, illuminance Ev is then obtained from the measured photocurrent i using the equation [22] Ev = Kcd F i , As(λ0 ) (3) Figure 1. Schematic structure of the PQED and the precision aperture in front of it. Without the aperture the field-of-view (FOV) of the PQED is θ = 21.5◦ . The precision aperture limits the FOV to 4.4◦ . At the distance of 3 m, and with the source diameter of 3 mm, the maximum entrance angle of the incident light is 0.06◦ when the 3 mm aperture is used. where A is the area of the limiting aperture and s(λ0 ) is the absolute responsivity of the photometer at the V(λ) peak wavelength of λ0 = 555 nm. The new method utilizes the PQED together with a precision aperture placed in front of the photodiodes to limit the field of view. The schematic structure of the assembly is shown in figure 1. It should be emphasized that there is no filter or any other optical element between the light source and the detector. Seven reflections of the incident light take place between two 11 mm × 22 mm silicon photodiodes and a small fraction of light is reflected back. Typical shunt resistance and capacitance of the PQED are 3 M and 1 nF. The dust and moisture contamination of the photodiodes is prevented by using a nitrogen flow system [17]. The reflectance loss of the PQED is affected by the spectral distribution and state of polarization of the incident light. The reflectance loss and the photocurrent ratio of the two photodiodes have been modelled for monochromatic light at s and p polarization by using a multilayer model of the photodiodes and ray transfer matrix analysis. As described in [21], these modelled values, together with the measured photocurrent ratio, can be used to determine the reflectance loss of the PQED at individual wavelengths without direct measurement of the reflectance. The method to calculate the responsivity s(λ) of the PQED to be used in equations (2) and (3) with unknown polarization distribution is derived in the appendix. As the V(λ) filter is not used, the new method relies on accurate determination of the relative spectral flux e (λ) in equation (2). The reliability of the measurement of relative spectra can be addressed by measuring the same LED with different calibrated spectroradiometers [23]. In conventional photometry, the relative responsivity of the S277 ¨ T Donsberg et al Metrologia 51 (2014) S276 Figure 2. Schematic of the measurement setup. The distance between the LED and the detector plane is 3 m. The dashed line indicates the locations where white reflecting material was fixed during the stray light studies. photometer resembles the V (λ) function in equation (2), and the uncertainty of the spectral mismatch correction factor due to uncertainty in the incident spectral flux is small. In the absence of V(λ) filter, the relative spectral responsivity srel (λ) is simply that of the PQED—very near to the responsivity of an ideal quantum detector, that is srel (λ) ≈ λ/λ0 . Moreover, the absolute responsivity s(λ0 ) in equation (3) is accurately known. The effect of the measured spectral flux on the spectral mismatch correction factor is the main uncertainty component when measuring photometric quantities with the PQED. In the case of a narrow bandwidth light source, integrating the noise floor of the spectroradiometer over the entire visible wavelength range causes an error in the spectral mismatch correction factor. This effect can be corrected by extrapolating the tails of the peak in the LED spectrum. The most suitable function for extrapolation is an exponential function of photon energy [24–26]. 3. Measurements and results Figure 3. Normalized spectra of the measured blue and red LEDs and the V(λ) function (dashed line). The illuminance values produced by a blue and a red LED with peak wavelengths of 462 nm and 664 nm, respectively, were measured with the reference photometer and with the PQED as described above. The design of the reference photometer is similar to the filter radiometer described in [27]. The schematic of the measurement setup in a light tight enclosure around the photometric bench is shown in figure 2 [22]. With both methods, a precision aperture with a diameter of 3 mm was used. The detectors were located at a distance of 3 m from the light source. Both LEDs were mounted on a heat sink stabilized to the temperature of 40 ◦ C and they were driven at currents of 700 mA (blue LED) and 1 A (red LED). The relative spectral fluxes e (λ) of the measured LEDs and the V(λ) curve are shown in figure 3. The spectra were measured using a double monochromator scanning spectroradiometer with 1 nm bandwidth. The relative intensity scale of the spectroradiometer was calibrated against the traceable spectral irradiance of an FEL lamp [28], whereas the argon ion laser line at 457.94 nm and the helium–neon laser line at 632.82 nm were used to calibrate the wavelength scale in air. The calculated spectral mismatch correction factors, measured photocurrents and the corresponding illuminance values are given for the PQED and the reference photometer in table 1. As described above, the spectra of the LEDs were extrapolated below the noise floor of the spectroradiometer (see figure 4). LEDs were also studied with the spectroradiometer at a close distance to confirm that their spectra are limited to the single peak. The uncertainty components of the illuminance measurements using the reference photometer and the PQED are given in table 2. The uncertainty due to the spectral mismatch correction factor is dominated by the transmittance of the S278 ¨ T Donsberg et al Metrologia 51 (2014) S276 Table 1. Measured spectral mismatch correction factors, photocurrents and the corresponding illuminance values for the PQED-based method and for the reference photometer. Blue LED Spectral mismatch correction factor F Photocurrent i/nA Illuminance Ev /lx Red LED PQED Photometer PQED Photometer 0.103 86 47.454 1.0378 0.942 13 2.7298 1.0384 0.071 918 135.514 2.0521 1.012 52 4.9954 2.0422 Figure 4. Normalized spectra of the measured blue and red LED and the extrapolated slopes of the peaks (dashed lines). V(λ) filter when the reference photometer is used, and by the measurement of the relative LED spectrum when the PQED is used. These components include the uncertainty due to the wavelength scale, which is 0.08% to 0.10% for the PQED and 0.43% to 0.48% for the reference photometer. Other sources of uncertainty in the spectral mismatch correction factor of the photometer, such as spectral transmittance of the filter and its angular and temperature dependence, were evaluated similarly to [22]. The uncertainties in the LED spectrum due to the non-linearity of the spectroradiometer, temporal drift and the angular dependence were estimated by measuring the spectra of the LEDs at distances of 1.5 m and 3 m. The combined effect of these on the standard uncertainty was concluded to be less than 0.1% for both LEDs when the PQED is used. In addition, the uncertainty of the spectral extrapolation was estimated by changing the wavelength range used for the curve fitting and the wavelength range where the extrapolation was applied. The standard uncertainty due to extrapolation was found to be less than 0.08% for both LEDs. The uncertainty components due to absolute responsivity at λ0 are obtained similarly to [20] and [22]. In addition, the effect of polarization distribution of the source on the responsivity of the PQED was studied. The polarization distribution of the LEDs was measured using a polarizer in front of the reference photometer. The proportions of p polarized components were 49.9% and 50.3% for the blue and the red LED, respectively. On the other hand, the calculation described in the appendix gave corresponding values of (35 ± 16)% and (55 ± 6)%. The quoted standard uncertainty of the calculation is dominated by the uncertainty of the predicted ratio given in [21]. Even without the measurement of the polarization distribution, the standard uncertainty in the responsivity of the PQED due to polarization is less than 0.002% for both LEDs, if the method described in the appendix is used. Two similar precision apertures were used for the reference photometer and the PQED, and their areas had the same uncertainty indicated in table 2. The PQED and the reference photometer were aligned using a laser [29] in such a way that the back-reflections from the wedged trap and the V(λ) filter, respectively, were parallel with the incident beam. The small angle between the aperture normal and the optical axis was then measured and corrected. The transverse movement of the PQED near optical axis was found to have an effect of 0.05% mm−1 in the measured signal. The residual uncertainty due to aperture alignment after the centring and angular correction is 0.02%. The uncertainty due to stray light was estimated to be 0.01% by covering the detector side of the baffle shown in figure 2 with white diffusely reflecting material. On-site calibrations were performed after the photometric measurements for the current-to-voltage converters and multimeters used for photocurrent measurements. The uncertainty due to repeatability of the measurements was estimated on the basis of data collected during one day. Finally, the apertures of the reference photometer and the PQED were at the same entrance plane within a standard uncertainty of 0.2 mm, causing a negligible standard uncertainty of 0.01% in the comparison of illuminance measurements. 4. Conclusions A new method has been developed for the basic realization of photometric units based on the PQED operated at room temperature. The PQED is used without the V(λ) filter, and the photometric weighting is taken into account in the spectral mismatch correction factor F . As a result, the new method overcomes the problems associated with V(λ) filters, such as low values at the blue and red slopes as well as temporal and temperature drifts. The method is applicable to light sources whose spectral irradiance is limited to silicon region, i.e. the region where PQED has predictable responsivity. In both the PQED-based method and the traditional photometry, the accuracy of the wavelength scale produces the largest contribution to the uncertainty when narrowband sources in the blue and red region are measured. However, the wavelength scale of the spectroradiometer can be conveniently calibrated using lasers as wavelength S279 ¨ T Donsberg et al Metrologia 51 (2014) S276 Table 2. Uncertainty components of illuminance measurements using the reference photometer and the PQED. If the uncertainties for the blue and red LED deviate, the value for the red LED is given in parenthesis. 100 × Relative standard uncertainty Source of uncertainty Photometer PQED Spectral mismatch correction factor, F LED spectrum relative spectral responsivity of the photometer/detector Absolute responsivity of the detector, s(λ0 ) Aperture area, A Aperture alignment Stray light Photocurrent measurement, i Repeatibility of the measurement 0.006 (0.004) 0.44 (0.49) 0.10 0.07 0.02 0.01 0.006 0.03 0.15 (0.16) 0.002 0.007 0.07 0.02 0.01 0.003 0.03 Combined standard uncertainty 0.46 (0.51) 0.17 (0.18) Expanded uncertainty (k = 2) 0.92 (1.01) 0.34 (0.36) standards, and the method is more accurate than the wavelength scale calibration of spectrophotometers using wavelength transmission standards. Illuminance values of narrow-band blue and red LED were measured with the PQED-based method and with the reference photometer. The congruent measurement results demonstrate that the new method is suitable for characterization of narrowband LED sources, such as separately measured colours of RGB lamps. In addition, the PQED-based method has significantly lower uncertainty than the reference photometer in the illuminance measurements of both LEDs. As the spectral weighting is not dependent on a physical device, i.e. the filter, the new method is not limited to photopic weighting. Thus, any weighting in the visible spectrum can be used, such as mesopic, scotopic, or spectrally flat for determination of broadband radiometric quantities. The PQED-based method was here demonstrated to be applicable to illuminance measurements of narrow-band LEDs. The method can also be extended to the realization of other photometric units as with traditional photometers. Furthermore, the new method is in principle applicable to broadband solid state light sources based on phosphor LEDs. In conclusion, realizing the illuminance and luminous intensity units without a V(λ) filter using the PQED may give lower uncertainty and a simpler method than with V(λ) weighted photometers when narrow-band or white LEDs are measured. Acknowledgments Minna Santaholma is acknowledged for the design of precision mechanics. The research leading to these results has received funding from the European Metrology Research Programme (EMRP) project SIB57 ‘New Primary Standards and Traceability for Radiometry’. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. Appendix light source with uncontrolled state of polarization is used. The method requires the measurement of the relative spectral distribution of the light source and the photocurrent ratio of the PQED photodiodes. As the photocurrents of the two photodiodes can be measured separately, the photocurrent ratio is easily obtained. The photocurrent ratio, denoted here as α, is defined as α= i1 , i2 (A1) where i1 and i2 are the photocurrents measured by the first and second photodiode, respectively, and the total photocurrent i is the sum (A2) i = i1 + i2 . From equations (A1) and (A2) we can obtain i1 = αi 1+α and i2 = i . 1+α (A3) The photocurrent ratio can be predicted for monochromatic light at s and p polarization [21], denoted here as αs (λ) and αp (λ). In turn, the photocurrents can be divided into components due to incident light at s and p polarization: i1 = i1,s + i1,p and i2 = i2,s + i2,p . By applying equation (A3), the photocurrent ratio α can be written as i1,s + i1,p α= i2,s + i2,p α (λ) αs (λ) dλ + e,p (λ)Rp (λ) 1+αp p (λ) dλ e,s (λ)Rs (λ) 1+α s (λ) = , e,s (λ)Rs (λ) 1+α1s (λ) dλ + e,p (λ)Rs (λ) 1+α1p (λ) dλ (A4) where e,s (λ) and e,p (λ) are the incident spectral fluxes and Rs (λ) and Rp (λ) are the responsivities of the PQED at s and p polarization, respectively. If the polarization distribution of the light source is assumed to be wavelength independent—a good approximation for narrow-band light source—we can define a wavelength independent polarization ratio This appendix describes the method to determine the reflectance losses of the PQED when a non-monochromatic S280 b= e,p (λ) . e,s (λ) (A5) ¨ T Donsberg et al Metrologia 51 (2014) S276 From (A4) and (A5) we can then obtain s (λ) e (λ)Rs (λ) α−α dλ 1+αs (λ) . b = − α−αp (λ) e (λ)Rp (λ) 1+αp (λ) dλ (A6) Further, this can be used to calculate the responsivity of the PQED at given polarization ratio b Rb (λ) = Rs (λ) + bRp (λ) , 1+b (A7) which can be used as the absolute responsivity s(λ) for equations (2) and (3). 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