# ext\_co

This coefficient is used to calculate the amount of intercepted photosynthetically active radiation.

Differences in canopy structure for a species are described by the number of leaves present (leaf area index) and the leaf orientation. Leaf orientation has a significant impact on light interception and consequently on radiation-use efficiency. More erect leaf types spread the incoming light over a greater leaf area, decreasing the average light intensity intercepted by individual leaves (see figure below). A reduction in light intensity interception by an individual leaf favors a more complete conversion of total canopy-intercepted light energy into biomass.

![](https://1348478613-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MTFpIHUB1K63DLhLJaf%2Fuploads%2FRKA4NWnmsdBbwUMbojMb%2Fimage.png?alt=media\&token=f25efd8a-2f66-4e22-8c19-f618f4de1203)

Using the light extinction coefficient value (kℓ) in the Beer-Lambert formula to quantify efficiency of light interception per unit leaf area index, more erect leaf types have a smaller kℓ.

To calculate the light extinction coefficient, the amount of photosynthetically active radiation (PAR) intercepted and the mass of aboveground biomass (LAI) is measured several times throughout a plant’s growing season using the methodology described in the previous sections. The light extinction coefficient is then calculated using the Beer-Lambert equation:

$$\frac{TPAR}{PAR}=(1-exp⁡(-k\_l⋅LAI))$$

or

$$k\_l=-ln⁡(\frac{TPAR}{PAR}) \* \frac{1}{LAI}$$

where *TPAR* is the transmitted photosynthetically active radiation, and *PAR* is the incoming photosynthetically active radiation.