Some Characteristics of Radiation Instruments for Measurement of Very Low Intensity Under Artificial Light Source

Abstract

The uses of radiation instruments for measuring artificial light sources are common recently. But the instruments are designed for use in the field so that the performance of the instruments in the laboratory may be different from in the field. Therefore, the purpose of the present investigation is to find the performance of errors of the radiation instruments, Eppley type pyrheliometer and Moll-Gorczynski type solarimeter, under very low light intensity in the growth cabinet. The tests were made: (1) Linearity (2) Temperature coefficient (3) Farred response (4) Transient response.(1) LinearityThe linearities of the outputs of the instruments were checked by comparison with responses of a corrected photometer at various distances under a tungsten lamp (500 watt). In figs 1 and 2, the outputs of Eppley type pyrheliometer and Moll-Gorczynski type solarimeter are plotted against the readings of the photometer. The nonlinearity was not so large over 0.2-10.0mv (0.02-1.0cal/cm2.min) in both instruments. But, according to the calculation using eq. (2), Eppley type had more linear than Moll-Gorczynski type. For spot readings in the laboratory, the linearity is unimportant if the output can be uniquely related to energy through a calibration curve.(2) Temperature coefficientMacDonald and Courvoisier reported that the temperature coefficient for Eppley was -0.041--0.127% per °C, and Anderson for Moll-Gorczynski type was -0.17% per °C. However, these values had been measured under relatively high level of incident radiation, so that the temperature coefficient under low light intensity was checked in the laboratory. Figs. 3 and 4 show data obtained for Eppley type and Moll-Gorczynski type. The responses in figs 3 and 4 are in terms of response at 27.0°C which is assigned the value of 100 percent. It is noted from the figures that: (1) The temperature coefficient is affected by light intensity, (2) Under high intensity (0.15cal/cm2.min) linear relation exists between output and temperature, but under low intensity (0.014cal/cm2.min) nonlinear relation, (3) In Eppley type, the temperature coefficient increases the diversion from 100% with higher temperature, but in Moll-Gorczynski type increases with lower temperature. And it is obviously that the observed values for small I will be seriously affected by temperature.(3) Farred responseThe spectral responses of the radiation instruments are characterized by using a glass dome which transmit about 0.3-4.0μ range. Fluorescent lamps emit the radiation of about 400-700nm range. Therefore, when the energy emitted by fluorescent lamp is measured by a radiation instrument, the energy of about 400-700nm range is able to obtain without the calibration of spectral response of the detector. However, when the output of the instrument and the total radiation (included long-wave radiation) were measured under Day-light fluorescent lamps in combination with farred lamps, the output of the instrument increased with increasing radiation of farred lamps (fig. 5). It is recognized that the output of the instrument is included not only to 400-700nm but to farred energy. It seems the reason that the thermal radiations of glass dome and through glass dome affect the output, as the energy of 400-700nm range is extremely small.(4) Transient responseThe time constant for Eppley type is of the order of 20 seconds and for Moll-Gorczynski type, 10 seconds. But, it is assumed that the response may be delay at low level incident radiation, so that the responses of transition were tested in the case of steady- and unsteady-state between the instrument temperature and ambient temperature. Figs. 6 and 7 show the results for Eppley type and Moll-Gorczynski type, respectively. Transient time under high intensity was more rapid than under low intensity, and in steady-state than in unsteady-state. Especially, in unsteady-state in addition to under