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 (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 A quick comparative study between the 43% fraction and Ecotect (generally outdated, but the only software I know that outputs PAR, although I have no idea what calculation is using), gives a constant deviation of some 10% tested on a single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?

Thanks in advance!

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Can you upload a case with the expected results for a location? I’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.

See here:

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

@Mili ,

In my time, I’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 don’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 that’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 you’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 answer… 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 you’ve done for us to play with this incredible tool!

I’ve been looking for information on how to do a precise PAR simulation using Honeybee and Radiance.
Just like you say, it seems there isn’t any standardized method. I would like to look into the complex way though: Taking the PAR wavelength, PAR sensitivity and refraction into consideration.

I’m glad to hear that you’ve seen somthing like this. Is there any chance that you can point me in the right dirrection? At this point I’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 you’ll be able to help. I’ll be using the next 5 months looking into this - so this question will probably be the first of at least a few.

Thanks in advance

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 42°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 µmol 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 everyone’s consideration - hopefully its not too reductive!

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

Ismail Tan, “The effects of urban forms on photosynthetically active radiation and urban greenery in a compact city”, Urban Ecosyst (2015) 18:937-961.

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

Apogee Instruments, “Conversion – PPFD to Lux”,, accessed Nov 15, 2018

Cycloptics, “PPFD Conversion Guide”,, accessed Nov 15, 2018