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In-situ spectral reflectance measurement of off-axis paraboloidal reflector with a large aperture for radiometric calibration
Optics and Lasers in Engineering 52 (2014) 224–229
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
In-situ spectral reflectance measurement of off-axis paraboloidal reflector with a large aperture for radiometric calibration Ying Zhang, Huijie Zhao n, Pengwei Zhou, Chongchong Li School of Instrumentation Science & Opto-electronics Engineering, Beihang University, No. 37, Xueyuan Road, Haidian District, Beijing 100191, China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 March 2013 Received in revised form 19 May 2013 Accepted 2 June 2013 Available online 23 July 2013
Off-axis paraboloidal reflector with a large aperture is often used as a collimator in radiometric calibration system in the space environment or a simulator for space carrier. Spectral reflectance measurement of the reflector is an important issue that determines the accuracy of radiometric calibration, especially, for imaging spectrometer. Currently, the reflectance of the reflector is generally measured by the sample contrast method. So, only a small sample of the reflector rather than the whole can be measured. In order to overcome this limitation, a method for in-situ measuring spectral reflectance of off-axis paraboloidal reflector, especially, with a large aperture, is proposed in this paper. First of all, the proposed in-situ measurement approach is designed according to in-situ contrast measurement. Then, the proposed measurement is realized and evaluated in experiment for measuring spectral reflectance of an off-axis paraboloidal reflector with an aperture of 110 mm. Finally, the influence of prodigious temperature fluctuation on the measurement is experimentally investigated and the impact of optical devices and environment on the uncertainty is analyzed. The total experimental uncertainty is obtained as 4.2%. The experimental results proved that the spectral reflectance of the reflector will decrease when the experimental temperature falls from 20 1C to −100 1C and the decrease degree is different for different wavelength bands. & 2013 Published by Elsevier Ltd.
Keywords: Spectral reflectance In-situ measurement Off-axis paraboloidal reflector Large aperture Radiometric calibration
1. Introduction The purpose of remote sensing is to derive information about an outlying object or phenomenon under observation using a special technique without physical contact with the object [1–4]. Interest in remote sensing is motivated by several applications such as hydrology [1], environment [2], Geo-science [5–7], climate change [1,8,9], and agriculture [3,4,10,11]. In modern usage, the term generally refers to the use of space and aerial sensing technologies to detect and classify objects on surface of Earth: in the atmosphere and oceans by means of propagated signals, namely, electromagnetic radiation such as light wave emitted from aircraft or satellites. Accurate radiometric calibration for satellite based remote sensing is necessary for converting the detected radiances into physical parameters [12–14]. Many countries have established special equipments for remote sensors calibration in simulated space environment, for example, the Primary Calibration System of NASA [15], the 7 V Chamber and the 10 V Chamber of AEDC [16,17], the Sensor Test Facility of Lockheed Missiles & Space Company [18], the Low-Background Infrared Calibration facility of NIST [19].
n
Corresponding author. Tel./fax: +86 10 82317191. E-mail address: [email protected] (H. Zhao).
0143-8166/$ - see front matter & 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.optlaseng.2013.06.004
However, due to influences of space radiation, prodigious temperature fluctuation and calibration of space carrier, the performances of radiometric calibration are limited. The off-axis paraboloidal reflector with a large aperture is often used as a collimator in radiometric calibration system in the space environment or a simulator for space carrier. The accuracy of radiometric calibration, especially for imaging spectrometer, is seriously affected by the spectral reflectance fluctuation of the reflector. Therefore, it is significant to carry out an in-situ spectral reflectance measurement of off-axis paraboloidal reflector for radiometric calibration. Here, “in-situ measurement” is an immediateness measurement in the condition of environment in space carrier. Currently, the reflectance of reflector is generally measured by the sample contrast method. Keisuke Tamura et al. performed the measurement of reflectivity of gold replicated reflector used in X-ray telescope of Suzaku satellite in 2.2–3.5 keV energy band at synchrotron radiation facility SPring-8 by the sample contrast method and the chamber worked in a constant temperature [20]. Steven R. Meier et al. finished the in-situ far-ultraviolet spectral reflectance of a contaminated sample mirror by the sample contrast method to measure contamination effects on far-ultraviolet optical surfaces and the chamber also worked in a constant temperature [21]. The reflectivity of the reflector for space carrier and simulator is measured by the current sample
Y. Zhang et al. / Optics and Lasers in Engineering 52 (2014) 224–229
contrast method in [20,21]. However, the size of the sample mirror is only several millimeters, so, the incident light beam arriving at the mirror is very narrow. During the measurement, the switching of the two measurements with different optical passes is realized rotation of the measured sample mirror with a small aperture, which is not very difficult compared with the problem with large aperture. The feasibility of measurements in [20,21] is on the assumption that the performances attenuation of the reflector in the space environment for the whole aperture is uniform. Therefore, this measurement with a small aperture is approximately considered for the full large aperture. However, this paper is aimed at the measurement with full and large aperture. Due to the large aperture of the paraboloidal reflector, the switching of optical passes for the two measurements cannot be realized by the method in [20,21], which is tried to be solved in this paper. Juan Ignacio Larruquert et al. measured the in-situ far ultraviolet overall reflectance of a sample mirror by the sample contrast method to study the degradation of aluminum films exposed to atomic oxygen in-orbit coating application, the chamber temperature is 300 K and no measurement was made of the sample temperature during exposure to the atomic oxygen beam [22]. Niibe et al. established a contamination experimental equipment in the new SUBARU to evaluate the reflectance of Si-capped multilayer sample mirrors by the sample contrast method and the temperature factor was also ignored [23]. Wei et al. studied the change in optical properties of Al film sample mirrors induced by proton irradiation with less than 200 keV in a vacuum environment with a heat sink by the sample contrast method to reveal the deterioration mechanism and the temperature of the vacuum environment did not change during the measurement [24]. Folkman et al. performed in-situ reflectance measurements of 12.5 mm diameter mirror samples at temperatures from 20 K to 373 K while contaminants from representative spacecraft materials were cryoand/or photo-deposited onto the sample surface and the results in different temperatures were different [25]. However, for this current sample contrast method, only a small sample of the reflector rather than the whole can be measured and it is impossible to map the spectral reflectance accurately for the whole reflector with a large aperture. In addition, most of current measurements are not in-situ and only overall reflectance is measured. And the effect induced by the temperature is often ignored. As a result, accuracy of radiometric calibration is still limited due to current measurement of spectral reflectance. In order to try to solve the problem of spectral reflectance measurement for radiometric calibration system in the space environment or a simulator for space carrier, an in-situ spectral reflectance measurement of off-axis paraboloidal reflector with a large aperture is proposed and investigated in this paper. The proposed measurement is realized with advantages including compact configuration and straightforward principle.
225
(1)
Optical Source
Photo Detector
R1
Comparison (2) Sample
Photo Detector
R2
Fig. 1. Schematic diagram of current measurement of overall reflectance.
R1
(1) Spectrometer Optical Source
Comparison (2) Tested Reflector
R2 Spectrometer
Fig. 2. Schematic diagram of proposed in-situ contrast measurement of spectral reflectance.
Our proposed measurement is illustrated in Fig. 2. The improvements are employment of tested reflector instead of sample, and spectrometer instead of photo detector. What is more, implement for first and second measurements is illustrated in Fig. 3. The implement consists of paraboloidal and fold reflectors, heat sink, quartz windows 1 and 2 in a space simulator, converging lens, diaphragm, and solar simulator as a light source, and spectrometer (integrating sphere, grating and photo detector). The measurement can be realized as following. The measured paraboloidal reflector is operated in the space simulator, as shown in Fig. 3(a). The parallel light beams emitted from the solar simulator pass through a converging lens at position a and quartz windows 1. Then, reflected by both fold and paraboloidal reflectors, passing through quartz windows 2 and collected by a spectrometer which consists of an integrating sphere, a grating and a photo detector, out of the space simulator, the intensity is measured by the photo detector. E′1 ðλÞ ¼ τðλÞ R1 ðλÞ R2 ðλÞ E0 ðλÞ T 1 ðλÞ T 2 ðλÞ
ð1Þ
where τ(λ) is the transmissivity of the converging lens, R1(λ) and R2(λ) are the spectral reflectances of the fold and paraboloidal reflectors, E0(λ) is the intensity of light emitted from the solar simulator, respectively, and T1(λ) and T2(λ) are the transmissivities of quartz windows 1 and 2, respectively. Next, the second measurement is implemented. The fold reflector rotate 901 by a computer controlled motor. Then, the converging lens is moved from position a to b in parallel to make the light beams converge at the integrating sphere of the spectrometer. In this case, the intensity can be expressed as E′2 ðλÞ ¼ τðλÞ R1 ðλÞ E0 ðλÞ T 1 ðλÞ T 2 ðλÞ
ð2Þ
2. Principle Current contrast measurement of overall reflectance is to measure the ratio of intensity of light beam reflected by a small sample of the tested reflector to intensity of light beam that is not reflected by the sample, which is illustrated in Fig. 1. Concretely, for a reflector with a large aperture, the overall reflectance can be measured by the following method: at first, the intensity (R1) of light beam emitted by an optical source with very small beam diameter is detected by a photo detector; then, the intensity (R2) of the light beam reflected by a small sample of the tested reflector is detected; finally, the overall reflectance of the sample can be obtained by calculating the ratio of R2 to R1.
So, the spectral reflectance of the measured paraboloidal reflector can be obtained by calculating the ratio of the first and second intensity R2 ðλÞ ¼
E′1 ðλÞ E′2 ðλÞ
ð3Þ
By operating the space simulator in a vacuum and cryogenic environment, the influence of the temperature on the spectral reflectance of the measured paraboloidal reflector can be monitored.
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Y. Zhang et al. / Optics and Lasers in Engineering 52 (2014) 224–229
Fig. 3. Implements for (a) first measurement with paraboloidal reflector and (b) second measurement without paraboloidal reflector.
Inside Space Simulator
Alignment of Source and Lens
Outside Space Simulator
Alignment of Paraboloidal & Fold Reflectors
paraboloidal reflector Height tilt deflection
reflector
Alignment of Spectrometer
Light beams can converge at the integrating sphere Fig. 4. Procedure of optical alignment.
rotating platform
3. Experimental results and discussion
Fig. 5. Experimental setup for measurement of spectral reflectance inside space simulator.
3.1. Optical alignment
3.2. Experimental measurement of spectral reflectance
The optical alignment is executed before the measurement. The procedure of optical alignment is shown in Fig. 4. The height, tilt and deflection of the optical devices inside and outside the space simulator including light source, reflectors, and spectrometer are well adjusted to make sure that light beams can converge at the integrating sphere for 1st and 2nd measurements.
Experimental setup for measurement of spectral reflectance inside space simulator is shown in Fig. 5. The spectral reflectance of the paraboloidal reflector with an aperture of 110 mm is measured in the cases of normal and cryogenic temperatures in vacuum. The procedure of experiment is illustrated in detail in Table 1. The temperature is measured by a temperature sensor
Y. Zhang et al. / Optics and Lasers in Engineering 52 (2014) 224–229
227
Table 1 Procedure of experiment. No.
Temperature (1C)
Atmosphere
Times of measurement
1 2 3
20 Dropped to −100 Rose to 20
Vacuum Vacuum Vacuum
3 3 3
attached to the paraboloidal reflector in our experiment and it will drop slowly while sending liquid nitrogen to the heat sink. The vacuum is realized by starting the vacuum system of the space carrier. Experimental results of measured spectral reflectance in case of normal temperature (20 1C) in vacuum are shown in Fig. 6 and the average values are used as the calibration values of the spectral reflectance of the reflector. Compared to the average values, the relative errors of the measurement results is smaller than 1% for λ ¼0.35–1 μm and smaller than 2.5% for λ¼1–1.8 μm, respectively. However, for λ¼ 1.8–2.5 μm, the average relative error is so high as 21.8% that the measurement data is almost invalid. This is because the light energy of the solar simulator begins to decline when λ is larger than 1 μm and it will decline more quickly for λ ¼1.8–2.5 μm, so that the signal to noise ratio (SNR) of the photo detector for this wavelength range is too low to work availably. Experimental results in case of cryogenic temperature (−100 1C) in vacuum are shown in Fig. 7 and the average values are used as the final measured values of the spectral reflectance of the reflector. Compared to the calibrated values, it is found that the experimentally measured average spectral reflectance in the case of −100 1C in vacuum is reduced by 2.32% for λ¼0.3–1 μm and 4% for λ¼1– 1.8 μm, respectively. Then the experiment temperature rose to 20 1C and the measured results in this case are shown in Fig. 8. Compared to the calibrated values, it is found that the measured average spectral reflectance is reduced by 0.41% for λ¼0.3–1 μm and 0.61% for λ¼ 1–1.8 μm, respectively. The experimental results proved that the spectral reflectance of the reflector will decrease when the experimental temperature falls from 20 1C to −100 1C and the decrease degree is different for different wavelength bands. Compared with the calibration data in the initial temperature 20 1C, the spectral reflectance of the reflector will decrease to a certain extent when the experimental temperature rises back to 20 1C, which proves that the properties of the film on the reflector will decline after the reflector works in case of cryogenic temperature for some time.
4. Uncertainty in measurement Then, the errors in our experiment are analyzed and described by the uncertainty. The uncertainty induced by the devices and environment is expounded as follows: 4.1. Uncertainty induced by the non-stability of light source s1 In our method, the certainty degree of measuring spectral reflectance is determined by the stability of light source. So, the uncertainly induced by the non-stability of light source can be obtained as s1 o 1% for our used solar simulator. 4.2. Uncertainty induced by the photo detector s2 In fact, the uncertainty of photo detector can be eliminated due to the contrast measurement. However, it is found from (3) that the certainty degree of spectral reflectance is also determined by
Fig. 6. Experimental results of measured spectral reflectance in cases of No. 1 as in Table 1.
Fig. 7. Experimental results of measured spectral reflectance in cases of No. 2 as in Table 1.
Fig. 8. Experimental results of measured spectral reflectance in cases of No. 3 as in Table 1.
the repeatability of the photo detector, which is s2 ¼2% in our experiments. 4.3. Uncertainty induced by the transmissivity fluctuation of quartz windows s3 Another factor inducing uncertainty is the transmissivity fluctuation of quartz windows. The transmissivity of quartz windows can be expressed as τðλÞ ¼ ½1−ρðλÞ2 τi ðλÞ
ð4Þ
where ρ(λ) is the spectral reflectance of the interface of the window, which increases with the increase of convergence angle of the input light beam, τi(λ) is the transmissivity of the internal window, and can be considered as a constant.
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5. Conclusion
Table 2 The maximum convergence angles calculated by ZEMAX software.
Fig. 3(a) Fig. 3(b)
On the left of normal line (deg)
On the right of normal line (deg)
1.74 4.51
7.64 4.57
Actually, the difference between the two convergence angles of the input light beams firstly and secondly arriving at the window for the first and the second measurements shown in Fig. 3(a) and (b), respectively, may lead to uncertainty. The convergence angles are numerically calculated by means of ZEMAX software and the results are illustrated in Table 2. Then, the uncertainty is induced by the different transmissivity of the internal window τi(λ) due to the difference between the convergence angles. The transmissivity of the internal window τi(λ) is measured for the first and second implements as shown in Fig. 3(a) and (b), and the results are 85% and 83%, respectively. Therefore, the uncertainty induced by the transmissivity fluctuation of quartz windows can be obtained as s3 ¼2% in our experiment. 4.4. Uncertainty induced by the alignment and mechanical process s4 The uncertainty is also induced by the alignment and mechanical processes such as optical alignment of height, tilt and deflection of the optical devices as well as the rotation of fold reflector. The alignment and mechanical processes induced uncertainty is measured as s4 ¼ 2% by repeating the measurement without quartz windows. 4.5. Uncertainty induced by the stray light s5 In our experiment, it is hard to eliminate the influences of stray light. Furthermore, the influences of stray light may not be the same for the first and second measurements, as a result, the uncertainty induced by the stray light is limited to be s5 o2%. 4.6. Uncertainty induced by the absorption of air s6 The absorption of air may induce uncertainty because the optical path is different for the first and second measurements. However, the space simulator is operated in the case of vacuum with a vacuum degree o 10−4 Pa. So, the absorption of air in the space simulator can be ignored. Then, the uncertainty induced by the absorption of air out of space simulator can be measured as s6 o 1%. 4.7. Total uncertainty stotal In conclusion, the total uncertainty stotal can be obtained with the assumption that uncertainties due to different factors described above are independent on each other: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi stotal ¼ s21 þ s22 þ s23 þ s24 þ s25 þ s26
≈
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 12 þ 22 þ 22 þ 22 þ 22 þ 12 ≈4:24%
ð5Þ
A method for in-situ measuring spectral reflectance of off-axis paraboloidal reflector with a large aperture is proposed. The proposed in-situ measurement approach is improved by utilizing a spectrometer instead of only a photo detector. The feasibility is evaluated in experiment for measuring spectral reflectance of an off-axis paraboloidal reflector with an aperture of 110 mm in different temperatures (−110 to 20 1C) in vacuum. The experimental results proved that the spectral reflectance of the reflector will decrease when the experimental temperature falls from 20 1C to −100 1C and the decrease degree is different for different wavelength bands. It is also proved that the properties of the film on the reflector will decline after the reflector works in case of cryogenic temperature for some time. It is found that the proposed spectral reflectance measurement can be used for in-situ measuring spectral reflectance of wide spectrum range. Then, the uncertainties due to light source, photo detector, quartz windows, alignment and mechanical processes, stray light and absorption of air are analyzed and the total uncertainty is 4.24%.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 61107013) and the National Natural Science Foundation of China (No. 61227806). The authors are grateful for all of the valuable suggestions received during the course of this research. References [1] Juergen Dietza Andreas, Kuenzera Claudia, Gessnera Ursula, Decha Stefan. Remote sensing of snow—a review of available methods. Int J Remote Sensing 2012;33(13):4094–134. [2] Garbulsky Martín F, Peñuelas Josep, Gamon John, Inoue Yoshio, Filella Iolanda. The photochemical reflectance index (PRI) and the remote sensing of leaf, canopy and ecosystem radiation use efficiencies: a review and meta-analysis. Remote Sensing Environ 2011;115(2):281–97. [3] Panda Sudhanshu Sekhar, Hoogenboom Gerrit, Paz Joel O. Remote sensing and geospatial technological applications for site-specific management of fruit and nut crops: a review. Remote Sensing 2010;2(8):1973–97. [4] Meroni M, Rossini M, Guanter L, Alonso L, Rascher U, Colombo R, et al. Remote sensing of solar-induced chlorophyll fluorescence: review of methods and applications. Remote Sensing Environ 2009;113(10):2037–51. [5] David Doelling, Constantine Lukashin, Patrick Minnis, Benjamin Scarino, Daniel Morstad. Spectral reflectance corrections for satellite inter calibrations using SCIAMACHY data. IEEE Geosci Remote Sensing Lett 2012;9(1):119–23. [6] Marjorie Schmeltz, Jean-Marie Froidefond, Frédéric Jourdin, Peter Gege. Remote sensing reflectance reconstruction to obtain water optical properties from MERIS multi-spectral satellite images, In: Proceedings of the SPIE, remote sensing of the coastal ocean, land, and atmosphere environment, 78580C, 7858; 2010. [7] Shahraiyni Taheri, Shouraki Bagheri, Schaale Fell, Tavakoli Fischer, Tajrishy Preusker, Vatandoust, et al. Application of the active learning method to the retrieval of pigment from spectral remote sensing reflectance data. Int J Remote Sensing 2009;30(4):1045–65. [8] C. W. Chang, S. V. Salinas, S. C. Liew and L. K. Kwoh, Spectral reflectance of clouds in multiple-resolution satellite remote sensing images, in: Proceedings of the 27th Asian conference on remote sensing; 2006. p. 149–153. [9] Vitaly Marchuk, Felix Yanovsky. Use of spectral differential reflectivity at remote sensing of precipitation. In: Proceedings of the microwaves, radar and remote sensing symposium, MRRS; 2008. p. 182-185. [10] Carol Jones, Paul Weckler, Niel Maness, Roshani Jayasekara, Marvin Stone, Donna Chrz. Remote sensing to estimate chlorophyll concentration in spinach using multi-spectral plant reflectance. Trans ASABE 2007;50(6):2267–73. [11] Craig Daughtry, Guy Serbin, James Reeves, Paul Doraiswamy, Earle Raymond Jr. Hunt. Spectral reflectance of wheat residue during decomposition and remotely sensed estimates of residue cover. Remote Sensing 2010;2(2):416–31. [12] Teillet Fedosejevs, Thome, Barker. Impacts of spectral band difference effects on radiometric cross-calibration between satellite sensors in the solarreflective spectral domain. Remote Sensing Environ 2007;110(3):393–409. [13] Seng William, Bruce Rafert J, Otten LJ, Meigs A. Multiple layer reflectance and spectroscopic calibration for remote sensing spectral imagers. Proc SPIE 1998;3491:1004–9.
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[14] Alexander Trishchenko, Josef Cihlar, Zhanqing Li, Byongjun Hwang. Long-term monitoring of surface reflectance, NDVI and clouds from space: what contribution we can expect due to effect of instrument spectral response variations? Proc SPIE 2002;4815:108–19. [15] Williamson William R, Otte Allen A. Attitude-referenced radiometer study: Part II—primary calibration system. NASA Contractor Report, NASA CR-1902. 2–38. [16] Simpson WR. The AEDC aerospace chamber 7 V—an advanced test capability for infrared surveillance and seeker sensors. Proc SPIE 1995;2470:369–79. [17] Lowry Simpson, Nicholson. Cryogenic optical system development for AEDC’s 10 V chamber. Proc SPIE 2002;4822:82–93. [18] La Croix LV, Smietanowska BS. The lockheed sensor test facility—a 1990 update. Proc SPIE 1990;1311:360–81. [19] Jung Timothy M, Carter Adriaan C, Lorentz Steven R, Datla Raju U. NIST-BMDO transfer radiometer (BXR). Proc SPIE 2000;4028:404–10. [20] Tamuraa K, Ogasakaa Y, Naitous M, Maeda Y, Naitoua Masataka, Ebara M, et al. Measurements of reflectivity of X-ray mirror for Suzaku satellite. Proc SPIE 2006;6266:62663D.
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[21] Meiera Steven R, Tveekrem June L, Keski-Kuha Ritva AM. A far-ultraviolet contamination-irradiation facility for in situ reflectance measurements. Rev Sci Instrum 1998;69(10):3642–4. [22] Larruquert Juan Ignacio, Méndez José Antonio, Aznárez José Antonio. Degradation of far ultraviolet reflectance of aluminum films exposed to atomic oxygen in-orbit coating application. Opt Commun 1996;124:208–15. [23] Niibe M, Kakutani Y, Terashima S, Takase H, Gomei Y, Matsunari S, et al. New contamination experimental equipment in the new SUBARU and evaluation of Si-capped multilayer mirrors using it. Proc SPIE 2006;6151:615134. [24] Wei Q, Liu H, Wang D, Liu SX. Degradation in optical reflectance of Al film mirror induced by proton irradiation. Thin Solid Films 2011;519(15):5046–9. [25] Folkman MA, Darnton LA, Silott SG, Frink ME. Optical scatter and contamination effects facility. Proc SPIE 1990;1329:221–32.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
In-situ spectral reflectance measurement of off-axis paraboloidal reflector with a large aperture for radiometric calibration Ying Zhang, Huijie Zhao n, Pengwei Zhou, Chongchong Li School of Instrumentation Science & Opto-electronics Engineering, Beihang University, No. 37, Xueyuan Road, Haidian District, Beijing 100191, China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 March 2013 Received in revised form 19 May 2013 Accepted 2 June 2013 Available online 23 July 2013
Off-axis paraboloidal reflector with a large aperture is often used as a collimator in radiometric calibration system in the space environment or a simulator for space carrier. Spectral reflectance measurement of the reflector is an important issue that determines the accuracy of radiometric calibration, especially, for imaging spectrometer. Currently, the reflectance of the reflector is generally measured by the sample contrast method. So, only a small sample of the reflector rather than the whole can be measured. In order to overcome this limitation, a method for in-situ measuring spectral reflectance of off-axis paraboloidal reflector, especially, with a large aperture, is proposed in this paper. First of all, the proposed in-situ measurement approach is designed according to in-situ contrast measurement. Then, the proposed measurement is realized and evaluated in experiment for measuring spectral reflectance of an off-axis paraboloidal reflector with an aperture of 110 mm. Finally, the influence of prodigious temperature fluctuation on the measurement is experimentally investigated and the impact of optical devices and environment on the uncertainty is analyzed. The total experimental uncertainty is obtained as 4.2%. The experimental results proved that the spectral reflectance of the reflector will decrease when the experimental temperature falls from 20 1C to −100 1C and the decrease degree is different for different wavelength bands. & 2013 Published by Elsevier Ltd.
Keywords: Spectral reflectance In-situ measurement Off-axis paraboloidal reflector Large aperture Radiometric calibration
1. Introduction The purpose of remote sensing is to derive information about an outlying object or phenomenon under observation using a special technique without physical contact with the object [1–4]. Interest in remote sensing is motivated by several applications such as hydrology [1], environment [2], Geo-science [5–7], climate change [1,8,9], and agriculture [3,4,10,11]. In modern usage, the term generally refers to the use of space and aerial sensing technologies to detect and classify objects on surface of Earth: in the atmosphere and oceans by means of propagated signals, namely, electromagnetic radiation such as light wave emitted from aircraft or satellites. Accurate radiometric calibration for satellite based remote sensing is necessary for converting the detected radiances into physical parameters [12–14]. Many countries have established special equipments for remote sensors calibration in simulated space environment, for example, the Primary Calibration System of NASA [15], the 7 V Chamber and the 10 V Chamber of AEDC [16,17], the Sensor Test Facility of Lockheed Missiles & Space Company [18], the Low-Background Infrared Calibration facility of NIST [19].
n
Corresponding author. Tel./fax: +86 10 82317191. E-mail address: [email protected] (H. Zhao).
0143-8166/$ - see front matter & 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.optlaseng.2013.06.004
However, due to influences of space radiation, prodigious temperature fluctuation and calibration of space carrier, the performances of radiometric calibration are limited. The off-axis paraboloidal reflector with a large aperture is often used as a collimator in radiometric calibration system in the space environment or a simulator for space carrier. The accuracy of radiometric calibration, especially for imaging spectrometer, is seriously affected by the spectral reflectance fluctuation of the reflector. Therefore, it is significant to carry out an in-situ spectral reflectance measurement of off-axis paraboloidal reflector for radiometric calibration. Here, “in-situ measurement” is an immediateness measurement in the condition of environment in space carrier. Currently, the reflectance of reflector is generally measured by the sample contrast method. Keisuke Tamura et al. performed the measurement of reflectivity of gold replicated reflector used in X-ray telescope of Suzaku satellite in 2.2–3.5 keV energy band at synchrotron radiation facility SPring-8 by the sample contrast method and the chamber worked in a constant temperature [20]. Steven R. Meier et al. finished the in-situ far-ultraviolet spectral reflectance of a contaminated sample mirror by the sample contrast method to measure contamination effects on far-ultraviolet optical surfaces and the chamber also worked in a constant temperature [21]. The reflectivity of the reflector for space carrier and simulator is measured by the current sample
Y. Zhang et al. / Optics and Lasers in Engineering 52 (2014) 224–229
contrast method in [20,21]. However, the size of the sample mirror is only several millimeters, so, the incident light beam arriving at the mirror is very narrow. During the measurement, the switching of the two measurements with different optical passes is realized rotation of the measured sample mirror with a small aperture, which is not very difficult compared with the problem with large aperture. The feasibility of measurements in [20,21] is on the assumption that the performances attenuation of the reflector in the space environment for the whole aperture is uniform. Therefore, this measurement with a small aperture is approximately considered for the full large aperture. However, this paper is aimed at the measurement with full and large aperture. Due to the large aperture of the paraboloidal reflector, the switching of optical passes for the two measurements cannot be realized by the method in [20,21], which is tried to be solved in this paper. Juan Ignacio Larruquert et al. measured the in-situ far ultraviolet overall reflectance of a sample mirror by the sample contrast method to study the degradation of aluminum films exposed to atomic oxygen in-orbit coating application, the chamber temperature is 300 K and no measurement was made of the sample temperature during exposure to the atomic oxygen beam [22]. Niibe et al. established a contamination experimental equipment in the new SUBARU to evaluate the reflectance of Si-capped multilayer sample mirrors by the sample contrast method and the temperature factor was also ignored [23]. Wei et al. studied the change in optical properties of Al film sample mirrors induced by proton irradiation with less than 200 keV in a vacuum environment with a heat sink by the sample contrast method to reveal the deterioration mechanism and the temperature of the vacuum environment did not change during the measurement [24]. Folkman et al. performed in-situ reflectance measurements of 12.5 mm diameter mirror samples at temperatures from 20 K to 373 K while contaminants from representative spacecraft materials were cryoand/or photo-deposited onto the sample surface and the results in different temperatures were different [25]. However, for this current sample contrast method, only a small sample of the reflector rather than the whole can be measured and it is impossible to map the spectral reflectance accurately for the whole reflector with a large aperture. In addition, most of current measurements are not in-situ and only overall reflectance is measured. And the effect induced by the temperature is often ignored. As a result, accuracy of radiometric calibration is still limited due to current measurement of spectral reflectance. In order to try to solve the problem of spectral reflectance measurement for radiometric calibration system in the space environment or a simulator for space carrier, an in-situ spectral reflectance measurement of off-axis paraboloidal reflector with a large aperture is proposed and investigated in this paper. The proposed measurement is realized with advantages including compact configuration and straightforward principle.
225
(1)
Optical Source
Photo Detector
R1
Comparison (2) Sample
Photo Detector
R2
Fig. 1. Schematic diagram of current measurement of overall reflectance.
R1
(1) Spectrometer Optical Source
Comparison (2) Tested Reflector
R2 Spectrometer
Fig. 2. Schematic diagram of proposed in-situ contrast measurement of spectral reflectance.
Our proposed measurement is illustrated in Fig. 2. The improvements are employment of tested reflector instead of sample, and spectrometer instead of photo detector. What is more, implement for first and second measurements is illustrated in Fig. 3. The implement consists of paraboloidal and fold reflectors, heat sink, quartz windows 1 and 2 in a space simulator, converging lens, diaphragm, and solar simulator as a light source, and spectrometer (integrating sphere, grating and photo detector). The measurement can be realized as following. The measured paraboloidal reflector is operated in the space simulator, as shown in Fig. 3(a). The parallel light beams emitted from the solar simulator pass through a converging lens at position a and quartz windows 1. Then, reflected by both fold and paraboloidal reflectors, passing through quartz windows 2 and collected by a spectrometer which consists of an integrating sphere, a grating and a photo detector, out of the space simulator, the intensity is measured by the photo detector. E′1 ðλÞ ¼ τðλÞ R1 ðλÞ R2 ðλÞ E0 ðλÞ T 1 ðλÞ T 2 ðλÞ
ð1Þ
where τ(λ) is the transmissivity of the converging lens, R1(λ) and R2(λ) are the spectral reflectances of the fold and paraboloidal reflectors, E0(λ) is the intensity of light emitted from the solar simulator, respectively, and T1(λ) and T2(λ) are the transmissivities of quartz windows 1 and 2, respectively. Next, the second measurement is implemented. The fold reflector rotate 901 by a computer controlled motor. Then, the converging lens is moved from position a to b in parallel to make the light beams converge at the integrating sphere of the spectrometer. In this case, the intensity can be expressed as E′2 ðλÞ ¼ τðλÞ R1 ðλÞ E0 ðλÞ T 1 ðλÞ T 2 ðλÞ
ð2Þ
2. Principle Current contrast measurement of overall reflectance is to measure the ratio of intensity of light beam reflected by a small sample of the tested reflector to intensity of light beam that is not reflected by the sample, which is illustrated in Fig. 1. Concretely, for a reflector with a large aperture, the overall reflectance can be measured by the following method: at first, the intensity (R1) of light beam emitted by an optical source with very small beam diameter is detected by a photo detector; then, the intensity (R2) of the light beam reflected by a small sample of the tested reflector is detected; finally, the overall reflectance of the sample can be obtained by calculating the ratio of R2 to R1.
So, the spectral reflectance of the measured paraboloidal reflector can be obtained by calculating the ratio of the first and second intensity R2 ðλÞ ¼
E′1 ðλÞ E′2 ðλÞ
ð3Þ
By operating the space simulator in a vacuum and cryogenic environment, the influence of the temperature on the spectral reflectance of the measured paraboloidal reflector can be monitored.
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Fig. 3. Implements for (a) first measurement with paraboloidal reflector and (b) second measurement without paraboloidal reflector.
Inside Space Simulator
Alignment of Source and Lens
Outside Space Simulator
Alignment of Paraboloidal & Fold Reflectors
paraboloidal reflector Height tilt deflection
reflector
Alignment of Spectrometer
Light beams can converge at the integrating sphere Fig. 4. Procedure of optical alignment.
rotating platform
3. Experimental results and discussion
Fig. 5. Experimental setup for measurement of spectral reflectance inside space simulator.
3.1. Optical alignment
3.2. Experimental measurement of spectral reflectance
The optical alignment is executed before the measurement. The procedure of optical alignment is shown in Fig. 4. The height, tilt and deflection of the optical devices inside and outside the space simulator including light source, reflectors, and spectrometer are well adjusted to make sure that light beams can converge at the integrating sphere for 1st and 2nd measurements.
Experimental setup for measurement of spectral reflectance inside space simulator is shown in Fig. 5. The spectral reflectance of the paraboloidal reflector with an aperture of 110 mm is measured in the cases of normal and cryogenic temperatures in vacuum. The procedure of experiment is illustrated in detail in Table 1. The temperature is measured by a temperature sensor
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Table 1 Procedure of experiment. No.
Temperature (1C)
Atmosphere
Times of measurement
1 2 3
20 Dropped to −100 Rose to 20
Vacuum Vacuum Vacuum
3 3 3
attached to the paraboloidal reflector in our experiment and it will drop slowly while sending liquid nitrogen to the heat sink. The vacuum is realized by starting the vacuum system of the space carrier. Experimental results of measured spectral reflectance in case of normal temperature (20 1C) in vacuum are shown in Fig. 6 and the average values are used as the calibration values of the spectral reflectance of the reflector. Compared to the average values, the relative errors of the measurement results is smaller than 1% for λ ¼0.35–1 μm and smaller than 2.5% for λ¼1–1.8 μm, respectively. However, for λ¼ 1.8–2.5 μm, the average relative error is so high as 21.8% that the measurement data is almost invalid. This is because the light energy of the solar simulator begins to decline when λ is larger than 1 μm and it will decline more quickly for λ ¼1.8–2.5 μm, so that the signal to noise ratio (SNR) of the photo detector for this wavelength range is too low to work availably. Experimental results in case of cryogenic temperature (−100 1C) in vacuum are shown in Fig. 7 and the average values are used as the final measured values of the spectral reflectance of the reflector. Compared to the calibrated values, it is found that the experimentally measured average spectral reflectance in the case of −100 1C in vacuum is reduced by 2.32% for λ¼0.3–1 μm and 4% for λ¼1– 1.8 μm, respectively. Then the experiment temperature rose to 20 1C and the measured results in this case are shown in Fig. 8. Compared to the calibrated values, it is found that the measured average spectral reflectance is reduced by 0.41% for λ¼0.3–1 μm and 0.61% for λ¼ 1–1.8 μm, respectively. The experimental results proved that the spectral reflectance of the reflector will decrease when the experimental temperature falls from 20 1C to −100 1C and the decrease degree is different for different wavelength bands. Compared with the calibration data in the initial temperature 20 1C, the spectral reflectance of the reflector will decrease to a certain extent when the experimental temperature rises back to 20 1C, which proves that the properties of the film on the reflector will decline after the reflector works in case of cryogenic temperature for some time.
4. Uncertainty in measurement Then, the errors in our experiment are analyzed and described by the uncertainty. The uncertainty induced by the devices and environment is expounded as follows: 4.1. Uncertainty induced by the non-stability of light source s1 In our method, the certainty degree of measuring spectral reflectance is determined by the stability of light source. So, the uncertainly induced by the non-stability of light source can be obtained as s1 o 1% for our used solar simulator. 4.2. Uncertainty induced by the photo detector s2 In fact, the uncertainty of photo detector can be eliminated due to the contrast measurement. However, it is found from (3) that the certainty degree of spectral reflectance is also determined by
Fig. 6. Experimental results of measured spectral reflectance in cases of No. 1 as in Table 1.
Fig. 7. Experimental results of measured spectral reflectance in cases of No. 2 as in Table 1.
Fig. 8. Experimental results of measured spectral reflectance in cases of No. 3 as in Table 1.
the repeatability of the photo detector, which is s2 ¼2% in our experiments. 4.3. Uncertainty induced by the transmissivity fluctuation of quartz windows s3 Another factor inducing uncertainty is the transmissivity fluctuation of quartz windows. The transmissivity of quartz windows can be expressed as τðλÞ ¼ ½1−ρðλÞ2 τi ðλÞ
ð4Þ
where ρ(λ) is the spectral reflectance of the interface of the window, which increases with the increase of convergence angle of the input light beam, τi(λ) is the transmissivity of the internal window, and can be considered as a constant.
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5. Conclusion
Table 2 The maximum convergence angles calculated by ZEMAX software.
Fig. 3(a) Fig. 3(b)
On the left of normal line (deg)
On the right of normal line (deg)
1.74 4.51
7.64 4.57
Actually, the difference between the two convergence angles of the input light beams firstly and secondly arriving at the window for the first and the second measurements shown in Fig. 3(a) and (b), respectively, may lead to uncertainty. The convergence angles are numerically calculated by means of ZEMAX software and the results are illustrated in Table 2. Then, the uncertainty is induced by the different transmissivity of the internal window τi(λ) due to the difference between the convergence angles. The transmissivity of the internal window τi(λ) is measured for the first and second implements as shown in Fig. 3(a) and (b), and the results are 85% and 83%, respectively. Therefore, the uncertainty induced by the transmissivity fluctuation of quartz windows can be obtained as s3 ¼2% in our experiment. 4.4. Uncertainty induced by the alignment and mechanical process s4 The uncertainty is also induced by the alignment and mechanical processes such as optical alignment of height, tilt and deflection of the optical devices as well as the rotation of fold reflector. The alignment and mechanical processes induced uncertainty is measured as s4 ¼ 2% by repeating the measurement without quartz windows. 4.5. Uncertainty induced by the stray light s5 In our experiment, it is hard to eliminate the influences of stray light. Furthermore, the influences of stray light may not be the same for the first and second measurements, as a result, the uncertainty induced by the stray light is limited to be s5 o2%. 4.6. Uncertainty induced by the absorption of air s6 The absorption of air may induce uncertainty because the optical path is different for the first and second measurements. However, the space simulator is operated in the case of vacuum with a vacuum degree o 10−4 Pa. So, the absorption of air in the space simulator can be ignored. Then, the uncertainty induced by the absorption of air out of space simulator can be measured as s6 o 1%. 4.7. Total uncertainty stotal In conclusion, the total uncertainty stotal can be obtained with the assumption that uncertainties due to different factors described above are independent on each other: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi stotal ¼ s21 þ s22 þ s23 þ s24 þ s25 þ s26
≈
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 12 þ 22 þ 22 þ 22 þ 22 þ 12 ≈4:24%
ð5Þ
A method for in-situ measuring spectral reflectance of off-axis paraboloidal reflector with a large aperture is proposed. The proposed in-situ measurement approach is improved by utilizing a spectrometer instead of only a photo detector. The feasibility is evaluated in experiment for measuring spectral reflectance of an off-axis paraboloidal reflector with an aperture of 110 mm in different temperatures (−110 to 20 1C) in vacuum. The experimental results proved that the spectral reflectance of the reflector will decrease when the experimental temperature falls from 20 1C to −100 1C and the decrease degree is different for different wavelength bands. It is also proved that the properties of the film on the reflector will decline after the reflector works in case of cryogenic temperature for some time. It is found that the proposed spectral reflectance measurement can be used for in-situ measuring spectral reflectance of wide spectrum range. Then, the uncertainties due to light source, photo detector, quartz windows, alignment and mechanical processes, stray light and absorption of air are analyzed and the total uncertainty is 4.24%.
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