A New Device to Measure Relative Light Intensity in a Plant Community using Cadmium Sulphide Photoconductive Cells

[摘要] 1. A new device was developed for measuring relative light intensity in plant communitites by means of a pair of CdS photoconductive cells as the light detector. The cells were mounted in metal tubings of 12 mm diameter to avoid disturbing the natural structure of a sward. 2. The principle of the measurement was based on the character of CdS cells that the logarithmic value of the electric resistance vs. that of the light intensity relation is linear under some range of light value. When cells having the same character were placed under two different light intensities the ratio of electric resistance between them corresponds with the ratio of the given two light intensities. 3. If the CdS cells A and B in a bridge circuit (fig. 1-a) were placed under full sunlight and transmitted light respectively, the ratio of the resistance of the cells A : B could be converted to the known ratio of C : D when the bridge was ballanced by the μA meter. The dial of the variable resistor C can be calibrated to the relative light value of B to the full sunlight. The calibration was performed by a series of glass filters having known neutral densities. 4. A slight curvature of the light-resistance relation of the cell was found in the higher range of the light intensity (fig. 2 & 3) and it caused a bias of the measurement under changeable sunlight conditions. The bias was of such a nature that it could be cancelled by a compensating circuit including the third CdS cell D2 placed under full sunlight and attached to the side D as shown in fig. 1-b. The cell D2 reduces the resistance of the side D in response to the increasing light intensity and the compensation is properly regulated automatically so as to accord with the present relative light value by a variable resistor D1 ganged with the measuring dial C. 5. Same results obtained using the device are presented in fig. 6 with respect to the changes in ground level relative light value accompanied by the leaf area indices of various swards. Sampling errors in the case of ladino clover and orchardgrass swards are discussed. It was found that ca. 50 readings are appropriate for sampling and their logarithmic values should be averaged in order to estimate the true mean value within ± 0 06 (ca. 5 % of the length of logarithmic scale 2-0 or 100-1 in percentages) with 95 % confidence.

[学科分类] 农业科学（综合）