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Photosynthetic photon flux density (PPFD:&thinsp;&micro;mol&thinsp;m^&minus;2&thinsp;s^&minus;1) is indispensable for plant physiology processes in photosynthesis. However, PPFD is seldom measured, so that PPFD has been estimated by using solar radiation (SR: W&thinsp;m^&minus;2) measured in world wide. In method using SR, there are two steps: first to estimate photosynthetically active radiation (PAR: W&thinsp;m^&minus;2) by the fraction of PAR to SR (<i>PF</i>) and second: to convert PAR to PPFD using the ratio of quanta to energy (<i>Q&thinsp;/&thinsp;E</i>: &micro;mol&thinsp;J^&minus;1). PF and <i>Q/E</i> usually have been used as the constant values, however, recent studies point out that <i>PF</i> and <i>Q&thinsp;/&thinsp;E</i> would not be constants under various sky conditions. In this study, we use the numeric data of sky-conditions factors such cloud cover, sun appearance/hiding and relative sky brightness derived from whole-sky image processing and examine the influences of sky-conditions factors on <i>PF</i> and <i>Q&thinsp;/&thinsp;E</i> of global and diffuse PAR. Furthermore, we discuss our results by comparing with the existing methods.