β Scribed by K.J. McCree
No coin nor oath required. For personal study only.
The instrument consists of a solarimeter fitted with a hemisphere of heat-absorbing glass. By this means, a well-tried instrument, already widely used in meteorological stations for the measurement of total incident radiation, is converted into an instrument suitable for measuring photosynthetically active radiation. The good angular response is preserved by using a hemispherical filter. With a commercial electronic microvoltmeter the instrument is sensitive enough for use in phytotrons, and if photosynthetically active radiation is assumed to be bounded by the wavelengths 0.4 and 0.7/~, calculations show that it compares irradiances of common light sources with daylight, in these units, with an error of not more than 20 ~, reducible to 10 ~ with an improved filter glass. Arguments are presented for specifying and measuring light directly in such units, here called "plantwatts/m 2'', rather than using illumination units and a conversion factor for each light source. Daylight measurements with a filtered and an unfiltered solarimeter ove r several months have shown that the number of plantwatts/m 2 is not a constant proportion of the total irradianee but varies (in one locality) from 48 to 65~o. Moreover, the variation is systematic, the highest proportions occurring during the dullest weather, presumably because water vapour is the chief absorber involved. A systematic error would, therefore, be introduced by measuring with an unfiltered solarimeter and assuming that the proportion of photosynthetically active radiation in daylight is constant.
π SIMILAR VOLUMES
Knowledge of the photosynthetically active radiation is necessary in different applications dealing with plant physiology, biomass production and natural illumination in greenhouses. Nevertheless, as a result of the absence of widespread measurements of this radiometric flux, it is often calculated
A robust, relatively low cost (parts $300 Aust., 1976) instrument suitable for field studies was constructed to give rapid estimates of photosynthetically active radiation, both absolute and as percentage transmission, within plant canopies. The instrument consists of 9 selenium photocells mounted i