I measured LAI (leaf area index) by hand, using the method of Carmassi 2007. My first attempts were not very accurate/reproducible, but I was eventually able to get reproducible results by carefully measuring out from both sides of the leaf stem separately. My leaflets often grew off at angles, but I measured along the center line of each. I will have to validate this method destructively at the end of the experiment by pressing the leaves flat and scanning them for area, but it seems doable this way even if I have to re-calibrate the model for this cultivar. The amount of reaching around and pushing other leaves out of the way makes me very pessimistic about the hope of any 3D scanning process working. It takes about 15-20 min per plant to measure LAI using this method, which is not extreme and probably less than it would take for a good 3d scan, so this is the way forward.
I promised new calibrations of the ray tracing last week and I have failed. Well, sort of. It took me a long time to figure out that my new spot light source is not completely stable! Light output is +/- 2% over a 30 min interval, seemingly at random. I think I just have to accept that level of inaccuracy because I’ve spent far too long trying to calibrate as is. I measured the transmission of the glazing that I’m currently using on the greenhouse, and got 93.39% transmission of diffuse light, which is pretty high. Of course, that could be partly the light’s fault.
The analytical calculation from Fresnel equations gives, for index of refraction of 1.5, 0.84% transmission. The data sheet for this film claims 90% transmission, but that’s usually direct normal light, since that’s the best possible situation and marketing people like to report high transmission values. My simulation reported 86.7% std=0.5%. So what’s happening?
First, it’s tempting to think of doing two experiments with and without the greenhouse film as measuring the transmission, but this is not quite true. The side shade on my sensor means I’m not measuring exactly diffuse light, but a subspace of directions that is weighted toward the normal direction. So measuring a transmission higher than pure fresnel equations for diffuse light is to be expected, but not higher than the fresnel equations for direct light.
The other problem that I think is present is that when a ray is emitted from my diffuser, which is simply baker’s parchment paper, it may hit the film and reflect back to the diffuser. But the diffuser is white with high albedo so that ray is more likely than not re-reflected back towards the greenhouse film at a different angle. This albedo of diffuser effect is not taken into account by my simulation.
So I have two choices, measure the albedo of parchment paper and include that in the simulation, or revise my light source to avoid this problem. I haven’t decided yet.
While I was waiting on the sensors I also went back to compare my modelica greenhouse simulation with that of the thesis Vanthoor 2011. I actually wrote my simulation without looking at his because I wanted it to be completely independent and… Just kidding. I just didn’t see that he had all the details in an appendix until I had finished mine. So that was kind of a waste of my time, but I did do something better I think and left out some complications that he has and I don’t need. However, now going back I was able to fix some stuff I wrote and most of all use his calibration values for crop transpiration. I was hoping to be able to simulate my growth chamber, but I’m not sure that’s possible this far in. I don’t have records plant height, the LAI (leaf area index), nor any measurement for K in the canopy absorption. I could just use vanthoor’s estimate for K, but not having the LAI and height of the plants makes it hard. Perhaps it can still be done though and it’s probably good preparation for the full model validation to try.
Sun Aquasystems 