1. Introduction Measurement of sunshine-duration is one of the oldest solar radiation measurements. Sunshine-duration

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1 December 1986 H. Ikeda, T. Aoshima and Y. Miyake 987 Development of a New Sunshine-Duration Meter By Hiroshi Ikeda, Takeshi Aoshima and Yukiharu Miyake EKO Instruments Trading Co., Ltd Hatagaya 1-chome, Shibuya-ku, Tokyo 151, Japan (Manuscript received 24 July 1986, in revised form 16 October 1986) Abstract A new type of sunshine-duration meter,1) providing an optical system that consists of a mirror and a photodetector, was recently developed. The photodetector looks sky through a mirror rotating around an axis parallel to that of the earth's. A uniform sensitivity, independent of the incidence angle of the sun, is performed by an optical mirror with two diffusible surfaces. Effect of diffuse skylight has been remarkably well eliminated by an adoption of a pyroelectric detector. 1. Introduction Measurement of sunshine-duration is one of the oldest solar radiation measurements. Sunshine-duration data are valuable for two main purposes. First is one of the primary parameters for characterizing the climate of a given location. Second is valuable for estimating the total flux of solar radiation at a place where no pyranometeric measurement is available. Because of the simplicity, convenience and relatively low cost, a large number of stations are performing measurements of sunshine-duration. And a number of different types of sunshine-duration meters have been developed over the last 140 years (Coulson,1975). The duration of sunshine is defined as the amount of time when the disk of the sun is not obscured by cloud, but sunshine-duration measured actually is when the intensity of direct solar radiation is large enough to activate the meter. Since the various sunshine recorders have different threshold values for activation, 1) This type of sunshine-duration meter described in this paper was patented by Japanese patent application No , and British patent application No The sunshine-duration meter has been adopted officially as net work instruments by the Japan Meteorological Agency in they do not always give same results. In order to establish an intercomparison method of measuring results between different types of sunshine recorders, the World Meteorological Organization recommended that all data should be referenced to designated standard instruments. In 1962, the Commission for Instruments and Method of Observation of the World Meteorological Organization (CIMO) adopted the Campbell-Stokes sunshine recorder as the standard reference instrument. However, one problem in using the instrument was reported: it is difficult to define a precise lower limit of direct radiant flux. In the extreme condition of a clear, dry atmosphere and a very dry card, the threshold is as low as 70W/m2, while in the opposite extreme conditions it increases to as much as 280W/m2 (Coulson,1975). In 1981 CIMO, considering the problem on the standard reference, adopted a new threshold value for bright sunshine of 120W/m2 with an accuracy of *20%, and recommended a pyrheliometer as a reference instrument for sunshineduration measurement (WMO CIMO-V-III, 1981). This paper described a newly developed sunshine-duration meter which satisfies the recommendation of CIMO 1981.

2 988 Journal of the Meteorological Society of Japan Vol. 64, No Construction The newly developed sunshine-duration meter has two main parts, an optical system and a base (Fig. 1). The optical system consists of a photodetector, a mirror and a stepping motor driving the mirror. They are mounted on the body. A cylindrical glass cover protects optical system from the environment. The metal of chromium is plated on the upper part of the glass surface to prevent rapid temperature increase of the photodetector induced by incoming solar radiation. A spirit level and screws are provided for adjusting the base horizontally and a vertical post is provided for mounting the optical system on the base of the instrument. A circuit board for electric convertor is mounted inside the base in airtight. The sunshine-duration meter is installed on a horizontal surface so that the stepping motor of the optical system faces the south. The mirror rotated with the stepping motor reflects irradiance from a part of the sky to the photodetector, which generates output in proportional to the irradiance. When the sun enters the field of view, a pulse of higher amplitude is generated. The incidence angle of the direct sunlight to the mirror varies in the range of *23.5* depending on the solar declination. In order to provide a uniform electrical output in proportion to the direct solar irradiance, the reflectance of the mirror has been made to be independent of the incidence angles by using two diffusive surfaces (see Fig. 2). The field of view of the sunshine-duration meter can be defined from the optical characteristics of the mirror. Fig. 3 illustrates the field of view on the celestrial sphere. A dense dotted area in Fig. 3 shows the field of view in a given moment, and the dotted area shows the sweepout area. The lengthes of arc BAC (or B'A'C') and arc AA' (or BB', CC') are defined by the characteristics of the mirror. When it is assumed that the field of view is occupied with the area where the relative reflectance of the mirror rises by above 50%, the length of BAC (*90*) and AA' (*8*) can be estimated approximately to 0.20 steradian field of view. On the other hand, the sensitivity of the N.I.P. (Normal Incidence Pyrheliometer) was investigated depending on deviation angles of the sun from the center of the field of view. It was shown that the field of view occupied by the area having the sensitivity above 50% is about ±3* and corresponds to steradian showing only 1/22 of that of the sunshine-duration meter. It is well known that scattering by cloud and haze has a remarkable effect on the skylight Fig. 1. Schematic view of the sunshine-duration meter. 1: stepping motor; 2: mirror; 3: photodetector; 4: glass cover; 5: body; 6: post; 7: base. Fig. 2. The mirror of the sunshine-duration meter.

3 December 1986 H. Ikeda, T. Aoshima and Y. Miyake 989 Fig. 3. The field of view in the celestrial sphere. intensity and that the effect is most pronounced in the region of the solar aureole. Therefore, it is supposed that the measurement of direct solar radiation with the sunshine-duration meter having such wide field of view can be considerably influenced by the diffuse skylight. However, the problem of the field of view has been considerably improved by adopting a pyroelectric detector. As shown in Fig. 4, the time response curve of the detector is a pattern of polarization. Its output is in proportion to the time rate of change of irradiance and is useful in eliminating the background of diffuse skylight. The output of the pyroelectric detector is transmitted to the electric convertor producing two types of output: one is a sunshine switch and the other is an analog output. As shown in Fig. 5, the output of the pyroelectric detector is converted to a positive pulses by means of peak hold. Time constant of the peak hold is adjusted to match that of analogue recorder to be connected. The analogue output is provided for the sensitivity adjustment of the sunshineduration meter. On the other hand, this pulses is also transmitted to a comparator, and when the peak goes beyond some threshold value, the comparator triggers a binary counter. When the counter counts the pulses some given times successively, one shot of pulse operates the relay, and a switch of the relay works as a sunshine switch. The sensitivity of the sunshineduration meter is adjusted with GAIN ADJUST to 7 volts per kwm-2 in comparison with a Fig. 4. Time response of pyroelectric detector. pyrheliometer, and a reference voltage for the comparator is adjusted with THRESHOLD ADJUST to 0.84 volts corresponding to 120 W/m2 designated by CIMO The two revolution speeds of the mirror, 100 and 120 r.p.h. are alternatively available depending on a data logger. The actuation of the sun switch specified as 30 times per hour is realized by 120 r.p.h. and 2 binary counters, and 100 times actuation per hour is realized by 100 r.p.h. In this case, the binary counters are removed. 3. Characteristics The threshold value is affected by reflectance of the mirror, nonlinearity of the analog output, thermal dependency of the sensitivity and conditions of the diffuse skylight. Ideally, the reflectance of the mirror must be independent of the incidence angle of sunlight. In an early investigation, some kinds of ideal specular mirror having a curvature was tried, however it was found difficulty to produce a mirror with permissible accuracy. Furthermore it was found that the reflectance of a specular mirror is considerably affected by a small crack, small uneveness and small contamination on its surface. As a result, it was concluded that the mirror having a diffusive surface (make of aluminum) is most suitable from. the view point of optical stability and also easy to process the

4 990 Journal of the Meteorological Society of Japan Vol. 64, No. 6 Fig. 5. Circuit diagram of the convertor, production. As shown in Fig. 2, the mirror adopted in this paper consists of two surfaces making angles of 30* and 60* to the axis; and each surface is given some diffusivity by grinding with emery paper to a direction of traverse to the axis. Thus the reflectance of the mirror is given by a synthetic result of the reflectances of the two surfaces. Two convenient methods for the mirror examination have been provided: 1) a sun method in which the optical system of the sunshineduration meter is installed on the post so that incidence angle of irradiance to the mirror can be controlled in a range of the -25 and +25* sun declinations and 2) a lamp method in which the optical system is placed on the post on an optical bench with a lamp of Halogen 250 watt. The distance between the mirror and the lamp is about 50cm. As shown in Fig. 6, significant discrepancies exist between the two methods which may be caused by a complicated interaction between different type of light sources and by response time of the detector itself. It can be pointed out that the response time of the detector is on the same order as the time that the image of the light source passes through the detector. The reason for the discrepancy is due to the different shape of image projected on the detector and by the speed of that image as it pass through the detector. However it can be concluded that these systematic discrepancies can be conserved as far as the specifications of the lamp and the detector, the distance between the mirror and the lamp, and the revolutions of the mirror are maintained without alteration. These discrepancies depending on incidence angles are plotted in Fig. 7. The remarkable scatter Fig. 6. Examples of the mirror reflectance. solid line: by the sun method; broken line: by the lamp method; point: measured point.

5 December 1986 H. Ikeda, T. Aoshima and Y. Miyake 991 Fig. 8. Frequency distribution of temperature coefficient of the pyroelectric detector sensitivity. Fig. 7. Discrepancies of mirror reflectances between the sun and the lamp methods. may be caused by an irregularity belonging to each mirror. The averaged curve showing systematic discrepancies is useful for performing the mirror production or the mirror examination with the lamp method. As shown in Fig. 6, the accuracy of the mirror can be easily improved to be less than *2 or *3% over a range of incidence angles *23.5*. In actually the mirrors are supplied under a specified accuracy *3% typically cally and *5% in maximum. The pyroelectric detector having two windows, a differential type (P2LD-F Horiba Japan) is.used. The pyroelectric detector without window material has uniform sensitivity in a wide range of wavelength. In this study, quartz glass was selected as the window material, because the measurement of the solar radiation must be accommodate the 0.3-3*m of wavelength. Moreover, one of the window is evaporated by a metal of low emissivity to intercept irradiance, so that the characteristics of detector have been improved not only in the stability but in the temperature dependency. The sensitivity of the detector decreases almost linearlly with increasing temperature in a wide range. Frequency distribution of the temperature coefficients of the detectors measured in a temperature range -20*40* is shown in Fig. 8. It was found that 80% of the sampling detectors belongs to the decrease of the sensitivity by less than 0.l%/ Fig. 9. Comparison between output of sunshineduration meter with that of pyrheliometer in the clear sky. * and 98% of those belongs to the same by 0.18%/*. In other words, the sensitivity of the sampling detectors changes by *3% or less in typically and by *5% in maximum within temperature range of *30*. Fig. 9 shows the output of the sunshine duration meter as plotted in comparison with that of a pyrheliometer for the clear sky condition. It is observed that the sensitivity of the sunshine duration meter is decreasing so gradually as increasing of the direct solar irradiance. This seems

6 992, Journal of the Meteorological Society of Japan Vol. 64, No. 6 to be unavoidable because the effect of the diffuse skylight contributes to the measurement of direct solar irradiance in each different manner. The sensitivity of the sunshine duration meter is determined in comparison with pyrheliometer. Therefore, when the determination is performed in a higher intensity such as 1kWm-2, an exceeding of the sensitivity by aobut 8% will be given at lower level of irradiance; this corresponds to a lower value of threshold. For the purpose of accurate determination of the threshold value, it is desired that it must be performed as close as possible to the threshold value. Fig. 10 shows the comparison between the sunshine-duration meter and the pyrheliometer performed in a cloudy sky prevailing with thin clouds (cirrus and cirro-stratus). It was observed that the almost data of the sunshine-duration meter appeared in higher value than that in the clear sky. The virtual increasing of the sunshineduration meter can be also considered as the result of the influence of the abounding diffuse skylight in cloudy sky. However, when it is considered that the field of view of the sunshineduration meter is very wide in comparison with that of pyrheliometer, the amount of virtual increasing of the sensitivity that is approximately 10% in extreme condition can be said to be fairly small. When the accuracy of the sunshine-duration meter is assumed as the accuracy for the threshold value determination, it is also equivalent to the accuracy for the measurement of the direct solar irradiance. An excess of the sensitivity will give too low value of the threshold in the same amount of per cent. The accuracy of the threshold value determination due to various factors is listed in Table 1. This suggests that the accumulated error of -18*l0% under the extreme conditions might exceed the permitted error of *20% defined by WMO (1981). Among of the listed errors, the systematic errors caused by the nonlinearlity and the diffuse skylight are significant. As described above, the systematic error caused by diffuse skylight is unavoidable. Howeever, when the determination of the sensitivity is performed at the irradiance close to the specified threshold, the error of -8% arising from nonlinearlity can be eliminated. As a result, the accumulation of errors under the extreme conditions is %, that is, it can satisfy the specified accuracy. * 4. Conclusion According to WMO's recommendation defining the threshold value which is based on the direct solar irradiance and its accuracy for sunshine-duration measurement, a new sunshine duration meter has been developed. To provide the electric output proportional to the direct Table 1. The accuracies for threshold value determination depending on various factors. Fig. 10. Comparison between output of sunshineduration meter with that of pyrheliometer in the cloudy sky. The area enclosed by broken lines shows the permitted limit for the threshold determination.

7 December 1986 H. Ikeda, T. Aoshima and Y. Miyake 993 solar irradiance and independent of incidence angle varied with solar declination, the mirror having two diffusive surfaces was introduced, and it rotated with constant speed around earth axis. Thus automatic operation was realized through year without troublesome maintenance. Furthermore the pyroelectric detector whose output is proportional to the time rate change of irradiance is used as a sensor to eliminate undesirable effect of the diffuse skylight. This adoption led also to eliminate error due to spectral variation of direct solar irradiance. As a result when using as sunshine-duration meter, error range as the threshold value is limited within -10*10% under extreme condition. Acknowledgements The authors wish to thank Mr. S. Koinuma and Mr. M. Anzai of section manager of Japan Meteorological Agency for their help in developing this instrument and we also acknowledge valuable discussion and suggestion of Mr. T. Yamauchi, Mr. K. Matubara, Mr. H. Shimura and other staff of the same sections. References Coulson, K.L., 1975: Solar and Terrestrial Radiation. Academic Press, WMO CIMO-VIII, 1981: Radiation Measurement: The definition of sunshine occurrence in terms of a threshold of solar irradiance.