Leaf density affects R/FR and diffuse light proportion can affect it too. If the instrument allows you may try to build a small disk that takes off sun image on sensor  under clear sky. Moving it inside the canopy you may take diffuse light value and the corresponding RFR. Without the disk you obtain RFR value of direct and diffuse radiation. If you need  to integrate daily records under different LAI, soil  and plants you should borrow some micromet experience and set-up

I would just like to mention that the R/FR ratio, measured in the environment, should not be subsequently used to determine the phyochrome photoequilibrium. The R/ FR ratio within the leaf, or a green stem, is very differnt from that incident on the plant - due to massive chlorophyll absorption.

  Ciao, Guido.

Hi robert, hope you in good shape, I guess that in he field the main difference is about transmitted  and reflected light from leaves and soil, and  penumbra, the portion of incident light that travels near a partial occlusion. With small leaves near to sun spot size we got a lot of penumbra. Eventually the RFR at a molecular target is a remote value, but the shade quality  can affect understory 

To answer Robert, I want to characterize R:FR within the canopy in order to have an idea of "natural conditions" in a particular plant stand. I'm not looking for the phytochrome photoequilibrium.

To answer Guido, I can log global and diffuse incident light, let's say, at 1Hz frequency in order to access to beam fraction and use timestamp to link spectrum data that I will automatically or manually scan.

Concerning R:FR within the canopy, I try to have an idea of what plants are locally perceiving. Is it pertinent to consider that plant organs have an isotropic 180° 'field of view', and therefore are comparable to a cosine corrected sensor? In this case, I will try to avoid making measurements under clear sky or setting the sensor active surface in sunflecks, or as you advise me, take off the sun image with small shading.

Is that clearly exlained ? and is it OK ?

Diffuse light can be frequent and under diffuse there would be more FR than in residual from direct light absorption. Leaf angle distribution and leaf size play a strong role, but I miss your last sentence about avoiding clear sky...You probably need some biological trait variation that is thought to depend on RFR ratio and relate it to RFR taken with a cosine corrected sensor. You can shade it with a small bridge and collect both direct and diffuse. Sun raise and set are diffusers as well fog clouds or planofile trasmitted light. There would be a variable FRF but when and where to characterize it is depending on the proxy you look for. Far red blocking on small plants in greenhouse can help.

Greenhouses under our latitude (Paris) are only transmitting about 50% of PAR. It has an impact on photosynthesis and therefore, we're not able to discriminate signal effect from trophic effect. I have already tried...