USE OF A LOW-COST SILICON DIODE-ARRAY SPECTRORADIOMETER TO MEASURE SOLAR UV-B

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1 USE OF A LOW-COST SILICON DIODE-ARRAY SPECTRORADIOMETER TO MEASURE SOLAR UV-B Terence M. Murphy, Section of Plant Biology, University of California, Davis, CA Presented at an NIST Conference on Critical Issues in Air Ultraviolet Metrology, May, 1994 (unpublished) Abstract An Ocean Optics S1000V spectrometer, equipped with a UV-sensitive silicon diode array (glass cover removed) and a UV-transparent optical fiber input together with a MgO diffusive reflectance disk, was used to measure global (whole-sky) UV-B radiation. Reproducibility of the readings of the spectrometer using a standard lamp was about 5%. The calibration function [W m -2 (indicated unit) -1 ] of the system, determined by using a standard quartz-tungsten-halogen lamp, demonstrated interference peaks, the amplitudes of which were ±10% of the mean sensitivity of the sensor. The scattering properties of the reflectance disk provided cosine-corrected irradiance values. The temperature effects on the dark current and the light sensitivity of the sensor were significant, but not correlated, making temperature control important. Readings of solar radiation below 300 nm were overshadowed by dark-current noise and/or stray light. The instrument is inexpensive and easily portable and, with care given to calibration, can be used to measure solar UV-B above 300 nm. Introduction Research on the effects of solar UV-B radiation on biological systems requires an accurate estimation of the effective incident radiation. Because the absorbance of UV-B radiation by many biological molecular (e.g., DNA) varies greatly through this spectral region, in order to predict the relative biological effects of different radiation sources it is necessary either to find a sensor with a spectral response like that of the molecule, tissue, or system in question or to obtain irradiance spectra for the sources (1,2). Spectroradiometers that are sensitive in the UV region have been expensive, generally over $10K. Less expensive instruments that are assembled in the laboratory from a monochromator and photometer are often too clumsy for field students (3, personal experience of the author). Recently, a new instrument has been developed by Ocean Optics, Inc. that is relative inexpensive and easily portable. This paper describes an evaluation of this instrument for biological research on UV effects, emphasizing certain precautions that must be taken when it is used for field studies. Equipment A spectrometer (Model S1000V, Ocean Optics, Inc., Denedin, FL) was equipped with a UV-transparent fiber optic input (50 µm diameter). In this spectrometer, the light from the optical fiber impinges on a concave front-silvered mirror with a holographically inscribed diffraction grating. The diffracted spectrum is directed to a silicon charge-coupled diode array. In the UV-sensitive model, a glass plate is removed from the front surface of the array. The spectrometer was mated to a laptop computer through an analog-digital converted (Model CIODAS16JR, Computer Boards, Inc., Mansfield, MA). The software used for control of the spectrometer and data acquisition was either that provided with the spectrometer by the manufacturer or the more convenient C-SPEC (Ancal, Inc., Las Vegas, NV). The portability of the spectrometer comes from its size (12 x 13 x 4 cm 3 ) and the fact

2 that it draws its power from the computer. List prices (Ocean Optics, Inc.; Ancal, Inc.) as of 1993 were: spectrometer, $1875; optical fiber, $148; analog-digital board, $499; C-CPEC, $580. The software requires an IBM-compatible (or later model) computer. A Pack-in-Tell 386SX laptop computer (Computer Warehouse, Sacramento, CA) was selected because it could accept the analog-digital board directly. To use the spectrometer as a spectroradiometer, the data were calibrated to give an absolute reading of fluence (W m-2 nm-1) using a quartz-tungsten-halogen lamp with a calibrated radiance traceable to an NIST standard (Optronic Laboratories, Inc., Orlando, FL). Confirming values were determined using the output from a monochromator (Jobin Yvon) and a 250 W high-pressure Hg vapor lamp. The monochromator output was measured with an Eppley thermopile radiometer, which itself was calibrated using both a quartz-tungstenhalogen lamp with known energy output (measured electrically) and a laser, the energy of which was measured by a factory-calibrated Newport Corp. Model 835 Si Optical Power Meter. A 100 W quartz-tungsten-halogen lamp (Model L7407, Gilway Technical lamp, Woburn, MA) was used as a secondary standard. Spectral data were calibrated with respect to wavelength by noting the positions of lines from an unfiltered low pressure Hg vapor lamp. For measurement of global (sun plus sky) radiation, a reflectance disk was fabricated from a paste of magnesium oxide in water, which was pressed into a plastic mold, smoothed, and allowed to dry. The optical fiber was positioned normal to the disk. Results As measured with the spectrometer, the median width of Hg vapor lines at one-half maximum height was 2.0 nm (Fig. 1). This compared to the nominal resolutin specified by the manufacturer (1.2 nm). At the highest sensitivity setting (integration time 4096 ms), the dark current was 10-15% of the maximum reading, considerably greater than specified by the manufacturer. A set of eight readings of the secondary standard lamp (unregulated power supply) taken over one week and two cycles from full battery charge to low battery indication gave a coefficient of variation of 4.9%. Over the past 12 months, the wavelength calibration has drifted by less than ±0.8 nm.

3 Fig. 1. Line spectrum from an unfiltered low pressure Hg vapor lamp, placed ca 30 cm from the fiber optic input. Integration time was 2048 ms; 10 scans were averaged. Inset: logarithmic plot, showing the symmetry of emission lines and the limits imposed by dark current noise and/or stray light. The sensor of the spectrometer was sensitive to temperature. By placing the spectrometer at different temperatures in incubators in the laboratory, I found that the dark current was the same at -7 o and 12 o C, but was higher at 25 o C and much higher at 30 o C. In contrast, the reading from a standard lamp (less the dark current) was fairly constant from 12 o to 30 o C, but was substantially lower at -7 o C (Fig. 2). Changes in dark current could be compensated by using the values given by the dead pixels (pixels 1-15 and on the diode array), which also varied with temperature, but this procedure did not work for the light response. In practice, it was best to insulate the spectrometer and minimize the differences in temperature between the times that the instrument was calibrated and used.

4 Fig. 2. Effect of temperature on spectroradiometer readings. Open squares, average of a set of 52 light pixels. Closed squares, average of dead pixels Closed triangles, average of dead pixels Each reading was made after the sensor housing had equilibrated to a chamber of a measured temperature; the optical fiber and the quartz-tungsten-halogen lamp remained at a constant room termperature, and the position of the fiber relative to the lamp did not change when the sensor housing was moved from one chamber to another. Integration time was 4096 ms; 10 scans were averaged. In the dark, the readings of the dead pixels were precisely the same as in the light, and the readings of the light pixels were simlar to those of the dead pixels. Used directly, the 50-µm diameter optical fiber had an input sensitivity as a function of angle on incidence that was approximately triangular, dropping to zero for angles of incidence greater than 7.5 o from the fiber axis (Fig. 3). The maximum sensitivity of the instrument was determined by aiming the sensor directly at a standard lamp (Fig. 4). On an energy basis, the sensitivity at 300 nm was 0.52 times that at 450 nm; on a photon fluence basis, the sensitivity ratio was 0.52(450/300) = The calibration function showed a periodic structure that was due to thin-layer interference by the SiO 2 that formed the diode array (M. Morris, Ocean Optics, Inc., personal communication).

5 Fig. 3. Sensitivity of the optical fiber-spectroradiometer system as a function of the angular deviation of the lamp direction from the fiber axis. Fig. 4. Calibration function, showing the absolute sensitivity of the spectroradiometer system with the fiber pointed directly at a point source. The specified irradiances of a standard quartz-tungsten-halogen lamp were divided by the spectrometer readings at the corresponding wavelengths ( ). To approximate a continuous calibration function, a modified black-body

6 spectrum (6) with color temperature chosen to match the spectrum to the specified irradiances of the standard lamp was divided by the spectrometer readings ( ). Finally, a function (B) was derived with a trend taken from a second-order regression formula for the standardized irradiances (A) and an interference component with amplitude and periodicity chosen to match the continuous function. The formula of the calibration function shown is: B = λ + (1.001x10-4 )λ 2 + ( λ) sin (2π[6500/λ] - 0.5) To measure global solar radiation from sun and sky, the fiber was directed downward to a horizontal MgO diffusive reflectance disk. In this mode, the response of the spectroradiometer was proportional to the cosine of the angle of incidence of the light normal to the disk (Fig. 5). The sensitivity of this system was calibrated by directing the output of the secondary standard lamp onto the disk. The reflectance of the disk was % and was considered to be a smooth function from nm (Fig. 6 inset--the sharp peaks from 300 to 350 nm are though to be random variation from the low intensity of the reflected radiation of the standard lamp measured in this mode.) Fig. 5. Sensitivity of the diffusive reflection disk-optical fiberspectroradiometer system as a function of the lamp direction from a line normal to the disk (and along the fiber axis).

7 Fig. 6. Calibration function for use with the MgO diffusive reflection disk, calculated as the product of the calibration functin for direct radiation (B, Fig. 4) with the reciprocal of reflectance (C, inset and formula below). The reflectance was measured by placing the sensor the standard distance from the MgO disk (8 cm) while illuminating the disk with the secondary standard lamp at a measured distance (0.25 m) and angle (9.2 o ). Data for reciprocal of reflectance were obtained by the following calculation, Reciprocal of reflectance = [S(λ)/J(λ)][cos(9.2 o )π(0.5) 2 /π(0.25) 2 ], where S(λ) was the reading of the standard lamp measured by pointing the fiber directly at the source from 0.5 m and J(λ) was the reading of the reflected radiation. The right-hand expression corrects for the distance and angle of incidence of the standard lamp radiation on the disk. The function C represents a third-order regression fit to the reciprocal of the data for reflectance and follows the equation, C = λ λ 2 + (2.05x10-5 )λ 3 Fig. 7 shows the spectrum of global solar UV (and blue) radiation in April in Davis, CA; the structure of the spectrum in the UV range is similar to that produced by more elaborate instruments. The upper inset shows how the measured irradiance compared to that predicted by a semi-empirical analytical function (4,5). The lower inset shows a logarithmic ploy, demonstrating the limits imposed below 300 nm by dark current noise and/or stray light. A comparison of spectra taken at Lake Tahoe, CA and Davis, CA (Fig. 8) shows a ratio, greater than predicted by the program of Björn and Murphy (4) on the basis of altitude alone, that may reflect the presence of ozone or other pollutants in the Central Valley.

8 Fig. 7. Global solar UV and blue spectral intensities, measured at Davis on 26 April 1994, 1200 PST; data corrected for spectral sensitivity and reflectance of MgO disk (Fig. 6). Upper inset: UV-B portion of the spectrum, compared to values predicted by the DAYLIGHT program of Björn and Murphy (4), using appropriate parameters and three values, 3,4, and 5, for aerosol. Note that an aerosol value below 4 overestimates the amount of UV-B reaching the Earth s surface. Visibility at that time was over 80 km in all directions. Lower inset: logarithmic plot.

9 Conclusions Fig. 8. A comparison of representative solar spectra in Davis, Ca (15 July 1993, 1215 PST, 10 m altitude) and Lake Tahoe (25 July 1993, 1145 PST am, 1920 m altitude). Six Davis spectra and four Tahoe spectra were compared by t test: the difference at 340 nm was significant at the 5% level of confidence. Inset: ratio of the Tahoe spectral values to the Davis spectral values. Although visibility was excellent in both locations when the spectra were taken, the higher ratio in the UV-B band suggests that the difference is due to an accumulation of pollutants, including ozone, in the more populated Central Valley around Davis. The S1000V spectrometer provides the basis for an inexpensive, easy-to-operate, fieldportable spectroradiometer. Although subject to uncertainties generated by the limited spectral resolution and stray-light rejection characteristic of a single-grating monochromator and by temperature effects on both light and dark currents, the sensitivity [signal:(noise+stray light)] of the instrument above 300 nm for measurements of global UV-B radiation reflected from a MgO diffusive reflectance disk, and the disk gives a good cosine response. In using this instrument, the large uncertainty at wavelengths less than 300 nm must be acknowledged. Is an instrument of this type useful for studies of the biological effects of UV-B? The answer depends on the biological weighting function of the phenomenon being studied. This instrument cannot measure solar radiation at those wavelength most affected by ozone; however, a substantial proportion of solar damage to DNA and photosynthesis occurs from wavelengths above 300 nm. Also, the spectroradiometer has some advantages relative to a broad-band meter, since its measurements can easily be adapted to different light sources and to living systems with different biological weighting functions. The low cost, simplicity, and portability of this instrument make it inevitable that it or similar instruments by other manufacturers will be used increasingly by physiologists and ecologists.

10 The complexity of the calibration curve (Fig. 4) emphasizes the importance of having a standard lamp readily available. The high cost of absolute spectral standards for the UV wavelengths presents a danger of data appearing in the literature from instruments that have not bee calibrated or have been calibrated too infrequently. There is a need for less expensive standard lamps of reasonable, if lower, accuracy (5%). Acknowledgement--I am grateful to Jana Steiger, Laboratory of Chemical Biodynamics, University of California, Berkeley, for providing the standard lamp. References 1. R.D. Rundel, Action spectra and estimation of biologically effective UV radiation, Physiol. Plant. 58: , Y. Furusawa, K. Suzuki, and M. Sasaki, Biological and physical dosimeters for monitoring solar UV-B light, J. Radiat. Res. 31: , W.F. Kaufmann and K.M. Hartmann, Low cost digital spectroradiometer, Photochem. Photobiol. 49: , L.O. Björn and T.M. Murphy, Computer calculation of solar ultraviolet radiation at ground level, Physiolgie Vegetale 23: , A.E.S. Green, K.R. Cross, and L.A. Smith, Improved analytic characterization of ultraviolet skylight, Photochem. Photobiol. 31:59-65, J.C. DeVos, A new determination of the emissivity of tungsten ribbon, Physica 20: , 1954.