J.A. Stoffers
In a greenhouse climate it is necessary to know something about the radiation absorption of the canopy in two respects.
  1. As a source of energy for heat and vapour transfer; this means the absorption of long-wave radiation and the radiation of sun and sky in the region 250 – 30000 nm and
  2. The radiation in the visible region of 400 – 700 nm, necessary for photosynthesis.

The plant physiologist looks at a single leaf or a single plant; in a greenhouse there are canopy rows. A simplified model is drawn in figure 1. The rows have have a height H, a width B and a centre-to-centre distance L. There is an infinite number of infinitely long rows. For these rows, it is better not to speak about a leaf area index, but to define a density K. The density is the ratio of the leaf area in a volume-element and the volume of this element.

The direction of single leaf-elements has a distribution; for example, a spherical leaf distribution when all possible directions have the same probability.

The direction of the sun is usually defined by the height of the sun and the orientation of the rows, but for computations, it is easier to define the direction in a cylindrical co-ordinate system with the two angles theta and beta. The x direction is the direction of the rows.

Part of the radiation falling on a leaf is reflected and part is transmitted by the leaf. The amount of scattered light depends on a scattering coefficient henceforth called 2C.

The bottom can reflect light, or at least should reflect light. The reflection of the bottom is called R.

Stoffers, J.A. (1975). RADIATION ABSORPTION OF CANOPY ROWS. Acta Hortic. 46, 91-96
DOI: 10.17660/ActaHortic.1975.46.8