In midsummer, near noon on a clear day, full sunlight approaches light intensity of 1000 Wm-2. Since about 45% of sunlight is in the 400nm to 700 nm range used for photosynthesis, plants can only use about 450 Wm-2 or about 2000 micromol m-2 s-1. Similar or higher intensities can be achieved using artificial light sources like HPS,MH, LED grow light, but this requires considerable costs for both power and lamps.
If we consider a plant, for individual leaves oriented at right angles to incoming radiation, it needs roughly 1/3 of full sunlight (about 150 Wm-2) for photosynthesis. These values would be typical for sun-adapted species such as greenhouse vegetable crops. In shade-adapted species, including many foliage plants need lower intensities of receiving LED grow light. This might lead to a misconception that maximum crop growth can be achieved at only 1/3 full sunlight. For many several reasons, however this is not the case. These are:
• Whole plants are not composed of single leaves exposed to direct sunlight coming in at right angles to their surfaces. In a real canopy, leaves are often positioned oblique to the incoming beam lowering the amount of available light.
• In a shoot canopy the leaves overlap, and the incoming light is absorbed before it reaches the lower leaves. (PS. LED grow light emit enough intensity for overlap leaves)
• Differences in light absorbing pigment contents in different tissues cause variation in absorbing light.
For these reasons, well developed leaf canopies require higher intensities to reach the light compensation point than do single leaves. These considerations help us to understand a relationship accepted by many greenhouse vegetable producers, the "1% Rule": for every 1% increase in light intensity reaching the crop there is a 1% increase in productivity.