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1. Introduction Measurement of Sunshine-Duration Is One 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. 21-8 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 sky- light has been remarkably well eliminated by an adoption of a pyroelectric detector. 1. Introduction they do not always give same results. In order Measurement of sunshine-duration is one of to establish an intercomparison method of meas- the oldest solar radiation measurements. Sun- uring results between different types of sun- shine-duration data are valuable for two main shine recorders, the World Meteorological Organi- purposes. First is one of the primary parameters zation recommended that all data should be for characterizing the climate of a given location. referenced to designated standard instruments. Second is valuable for estimating the total flux In 1962, the Commission for Instruments and of solar radiation at a place where no pyrano- Method of Observation of the World Mete- meteric measurement is available. Because of orological Organization (CIMO) adopted the the simplicity, convenience and relatively low Campbell-Stokes sunshine recorder as the stand- cost, a large number of stations are performing ard reference instrument. However, one problem measurements of sunshine-duration. And a in using the instrument was reported: it is dif- number of different types of sunshine-duration ficult to define a precise lower limit of direct meters have been developed over the last 140 radiant flux. In the extreme condition of a years (Coulson,1975). clear, dry atmosphere and a very dry card, the The duration of sunshine is defined as the threshold is as low as 70W/m2, while in the amount of time when the disk of the sun is not opposite extreme conditions it increases to as obscured by cloud, but sunshine-duration meas- much as 280W/m2 (Coulson,1975). ured actually is when the intensity of direct In 1981 CIMO, considering the problem on solar radiation is large enough to activate the the standard reference, adopted a new threshold meter. Since the various sunshine recorders value for bright sunshine of 120W/m2 with an have different threshold values for activation, accuracy of *20%, and recommended a pyrhelio- meter as a reference instrument for sunshine- duration measurement (WMO CIMO-V-III, 1) This type of sunshine-duration meter described in 1981). this paper was patented by Japanese patent application No. 1111917, and British patent application No. This paper described a newly developed sun- 8324831. The sunshine-duration meter has been shine-duration meter which satisfies the recom- adopted officially as net work instruments by the mendation of CIMO 1981. Japan Meteorological Agency in 1986. 988 Journal of the Meteorological Society of Japan Vol. 64, No. 6 2. Construction ing on the solar declination. In order to provide The newly developed sunshine-duration meter a uniform electrical output in proportion to the has two main parts, an optical system and a base direct solar irradiance, the reflectance of the mirror has been made to be independent of the (Fig. 1). The optical system consists of a photo- detector, a mirror and a stepping motor driving incidence angles by using two diffusive surfaces the mirror. They are mounted on the body. A (see Fig. 2). cylindrical glass cover protects optical system The field of view of the sunshine-duration from the environment. The metal of chromium meter can be defined from the optical character- istics of the mirror. Fig. 3 illustrates the field is plated on the upper part of the glass sur- face to prevent rapid temperature increase of of view on the celestrial sphere. A dense dotted the photodetector induced by incoming solar area in Fig. 3 shows the field of view in a given radiation. A spirit level and screws are provided moment, and the dotted area shows the sweep- out area. The lengthes of arc BAC (or B'A'C') for adjusting the base horizontally and a vertical and arc AA' (or BB', CC') are defined by the post is provided for mounting the optical system characteristics of the mirror. When it is assumed on the base of the instrument. A circuit board for electric convertor is mounted inside the base that the field of view is occupied with the area in airtight. where the relative reflectance of the mirror rises The sunshine-duration meter is installed on a by above 50%, the length of BAC (*90*) and horizontal surface so that the stepping motor AA' (*8*) can be estimated approximately to of the optical system faces the south. The mirror 0.20 steradian field of view. rotated with the stepping motor reflects irradi- On the other hand, the sensitivity of the ance from a part of the sky to the photodetector, N.I.P. (Normal Incidence Pyrheliometer) was which generates output in proportional to the investigated depending on deviation angles of irradiance. When the sun enters the field of view, the sun from the center of the field of view. It a pulse of higher amplitude is generated. was shown that the field of view occupied by the area having the sensitivity above 50% is about The incidence angle of the direct sunlight to ±3* and corresponds to 0.009 steradian showing the mirror varies in the range of *23.5* depend- 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. 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 Fig. 4. Time response of pyroelectric detector. radiation with the sunshine-duration meter having such wide field of view can be consider- ably influenced by the diffuse skylight. pyrheliometer, and a reference voltage for the However, the problem of the field of view comparator is adjusted with THRESHOLD has been considerably improved by adopting a ADJUST to 0.84 volts corresponding to 120 W/m2 designated by CIMO 1981. The two pyroelectric detector. As shown in Fig. 4, the revolution speeds of the mirror, 100 and 120 time response curve of the detector is a pattern of polarization. Its output is in proportion to r.p.h. are alternatively available depending on a data logger. The actuation of the sun switch the time rate of change of irradiance and is use- specified as 30 times per hour is realized by ful in eliminating the background of diffuse 120 r.p.h. and 2 binary counters, and 100 times skylight. actuation per hour is realized by 100 r.p.h. In The output of the pyroelectric detector is this case, the binary counters are removed. transmitted to the electric convertor producing two types of output: one is a sunshine switch 3. Characteristics and the other is an analog output. As shown The threshold value is affected by reflectance in Fig. 5, the output of the pyroelectric detector of the mirror, nonlinearity of the analog output, is converted to a positive pulses by means of thermal dependency of the sensitivity and condi- peak hold. Time constant of the peak hold is tions of the diffuse skylight. adjusted to match that of analogue recorder to Ideally, the reflectance of the mirror must be be connected. The analogue output is provided for the sensitivity adjustment of the sunshine- independent of the incidence angle of sunlight. duration meter. On the other hand, this pulses In an early investigation, some kinds of ideal is also transmitted to a comparator, and when specular mirror having a curvature was tried, however it was found difficulty to produce a the peak goes beyond some threshold value, the mirror with permissible accuracy. Furthermore comparator triggers a binary counter. When the it was found that the reflectance of a specular counter counts the pulses some given times mirror is considerably affected by a small crack, successively, one shot of pulse operates the small uneveness and small contamination on relay, and a switch of the relay works as a sun- its surface. As a result, it was concluded that shine switch. The sensitivity of the sunshine- the mirror having a diffusive surface (make of duration meter is adjusted with GAIN ADJUST aluminum) is most suitable from. the view point to 7 volts per kWm-2 in comparison with a of optical stability and also easy to process the 990 Journal of the Meteorological Society of Japan Vol. 64, No. 6 Fig. 5. Circuit diagram of the convertor, production. the lamp and the detector, the distance between As shown in Fig. 2, the mirror adopted in the mirror and the lamp, and the revolutions of this paper consists of two surfaces making angles the mirror are maintained without alteration. of 30* and 60* to the axis; and each surface is These discrepancies depending on incidence an- given some diffusivity by grinding with emery gles are plotted in Fig.
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