PAR (Photosynthetically Active Radiation)

I wonder if anyone has experience with PAR (Photosynthetically Active Radiation)? Is it simply a fraction of Solar Irradiance? If yes, how much is this fraction? Based on this source https://core.ac.uk/download/pdf/29218727.pdf (Appendix A), it is 43%. A quick comparative study between the 43% fraction and Ecotect (generally outdated, but the only software I know that outputs PAR, although I have no idea what calculation is using), gives a constant deviation of some 10% tested on a single fully exposed surface in various locations.

Any idea if this approach is correct? Any intention of including PAR as a default ladybug output?

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Can you upload a case with the expected results for a location? Iâ€™m not sure if you can compare the results of Ecotect against Radiance-based studies like Ladybug/Honeybee. Ecotect uses the Equal-Angle subdivision and Radiance/Honeybee is using Prez sky model which is 145 Equal-Area.

It could be something similar to a radiation map on a grid. Yes, I understand the comparison with Ecotect wasnâ€™t fruitful. I believe there is a multiplication factor that would do the trick, however, itâ€™s probably not that straightforward to define it. I wonder if this can be done without using Radiance and just weight the solar incidence.

In my time, Iâ€™ve seen implementations of PAR calculations that range from incredibly simple (basically just a radiation study with no bounces) to complex ones that input reflectances of materials only in the photosynthetic wavelengths and translate the units to the molecular level (mol/m2-s).

For this reason, I donâ€™t think there is really a standardized method for calculating PAR and I would suggest customizing the level of detail of your analysis to the species of plants you are growing and the conditions under which you are trying to grow them. A baseball field thatâ€™s exposed to the sky might be fine with an assumption of no bounces and factors like the hours of direct sun might be the most relevant. If youâ€™re trying to grow lower-light plants in the shadow of a building, a more detailed study with bounces would help.

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Thank you Chris for the response. I figured that there is no straightforward answerâ€¦ I have in the past used a simplified calculation that takes into account reflectances, but without using the photosynthetic wavelength, that could be an interesting one. Not being a botanist myself and not having any related experience, I would have liked a simplified approach that can nevertheless be able to use the recommended PAR levels for each plant for comparison and evaluation purposes. Thank you both again for your responses!

Dear @chris

Thank you for the work youâ€™ve done for us to play with this incredible tool!

Iâ€™ve been looking for information on how to do a precise PAR simulation using Honeybee and Radiance.
Just like you say, it seems there isnâ€™t any standardized method. I would like to look into the complex way though: Taking the PAR wavelength, PAR sensitivity and refraction into consideration.

Iâ€™m glad to hear that youâ€™ve seen somthing like this. Is there any chance that you can point me in the right dirrection? At this point Iâ€™m trying to collect information on how to change the inputs in Radiance but also which parameters that needs to be taken into consideration in order to get a precise estimation.

I hope youâ€™ll be able to help. Iâ€™ll be using the next 5 months looking into this - so this question will probably be the first of at least a few.

/Lasse

This is a very interesting subject.
As I work on permaculture farm design, I alleady had a look at some agro-engineering paperwork.
What I have in mind is :

• level of photosynthesis is fonction of humidity ratio, diffuse sunlight, temperature, and stage of plant growth (for annual crops).
• beyond a specific level of temperature (around 42Â°C) photosynthesis stops.
• below a specific soil wetness index, photosynthesis stops
• below a certain sunlight access, phosynthsesis stops
Anyway, it is not a single variable function.

I imagine that reflexion can be interesting for urban agriculture. In other cases, it might be less relevant.

I am very interested in this subject, and looking forward to learn more. in France, we have a lot of old walls that have been built to modify microclimate and grow warmer climate species (peachs or grapes near Paris), and I would like to investigate this kind of landscape architecture.

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It sounds like this discussion has surpassed my knowledge on the topic but I would like to share some resources that provide more insight into how to convert illuminance to PAR for indoor applications. These sources demonstrate that 54 lux / 1 Âµmol m-2 s-1 as an appropriate conversion factor for sunlight and that PAR should be calculated daily. It seems like the individuals on this thread are beyond the simple conversion but I thought I would share for everyoneâ€™s consideration - hopefully its not too reductive!

Chun Liang Tan, Nyuk Hien Wong, Puay Yok Tan, Mirza Ismail, Ling Yan Wee, â€śGrowth light provision for indoor greenery: A case study.â€ť Energy and Buildings (2017) 144:207-217.

Ismail Tan, â€śThe effects of urban forms on photosynthetically active radiation and urban greenery in a compact cityâ€ť, Urban Ecosyst (2015) 18:937-961.

Brian Chabot, Thomas Jurik, Jean Chabot, â€śInfluence of instantaneous and integrated light-flux density on leaf anatomy and photosynthesis,â€ť American Journal of Botany (1979) 66:940-945.

Apogee Instruments, â€śConversion â€“ PPFD to Luxâ€ť, https://www.apogeeinstruments.com/conversion-ppfd-to-lux/, accessed Nov 15, 2018

Cycloptics, â€śPPFD Conversion Guideâ€ť, https://www.cycloptics.com/sites/default/files/Greenbeams%20PPFD%20Conversion%20Tables_0.pdf, accessed Nov 15, 2018

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