Global Solar Irradiance is usually measured in W/m2, by instruments called Pyranometers, in the waveband from 1100 nm (IR) to 400 nm (UV).

PAR can be also measured in W/m2, but is usually expressed in μmole.m2/s (moles of photons), which is called photosynthetic photon flux density (PPFD), and is not a SI unit. However, PAR and PPFD consider only the waveband from 400 nm to 700 nm.

The conversion is actually not exact and depends on the instantaneous spectrum of the radiation, which means, on the instantaneous atmospheric conditions.

Some researchers use the approximation 1 W/m2 ≈ 4.6 μmole.m2/s.

 Marco Antonio Galdino Thank you very much for clarification. 

The energy associated with this unit depends on the frequency of the radiation considered. Conversion values given by equipment manufacturers are approximations and are not axact. I invite you to consult our publication: Primeras mediciones de la radiación fotosintéticamente activa en San Miguel, Provincia de Buenos Aires. Avances en Energías Renovables y Medio Ambiente, vol. 8 N° 2, pp. 11.13-11.17 versión CD-ROM (2004) and its references.

@Marco Antonio Galdino. would you mind suggesting some reference (paper) which  used that approximation?

First of all, μmole.m2/s is definitely an SI unit. The approximation 1 W/m2 ≈ 4.6 μmole.m2/s comes from the Plant Growth Chamber Handbook (chapter 1, radiation; https://www.controlledenvironments.org/wp-content/uploads/sites/6/2017/06/Ch01.pdf). Note that the value of 4.57 converts Watts/m2 to μmole.m2/s, assuming that the W/m2 is for radiation from 400 to 700 nm. However, I don't know that anyone ever measures solar radiation in W/m2 in the 400 - 700 nm ranhe. Pyranometers measure TOTAL solar radiation. Since only about 45% of the energy of solar radiation is actually in the 400 - 700 nm range. the conversion of TOTAL solar radiation to PAR is ~2.1, rather than 4.57. As mentioned by others, that is an approximation, but a pretty good one.

Hello¡¡¡¡ How could I converted μmole.m2/s to W/m2 in 520-550 wavelenght range in LED light?? I have data in μmole.m2/s but I just measured LED light with this range.

Waiting for your help...

Thanks

You can use this eguation, based on the assumption that the average wavelength of your photons was 535 nm. You'll need to convert wavelength to frequency first: E = hf, where E is the energy of the photon in Joules; h is Planck's constant (6.63 * 10^-34 Joule seconds), and f is the frequency of the light in hertz.

Thank you for the useful information for my masters thesis comparing modeled versus experimental photosynthesis data.

Very nice answer Marc W Van Iersel . I did a pdf file with conversion factors for solar radiation, evapotranspiration, carbon balance, resource use efficiency, and general conversion units for CO2/H2O. If you have some suggestion.....please, let me know. Link:

Data Conversion factors and general equations applied in agricult...

Check next app in Google Play:

"Light Unit Converter app from Apogee Instruments calculates the conversions between different light units for various light sources including T5 Sylvania fluorescent lights, metal halide, high pressure sodium, and sunlight. Units include shortwave radiation (SW), photosynthetic photon flux (PPF, PPFD), Lux (lx) and Foot-candles (fc, lm/ft2, or ft-c)."

Looks very helpful.

For clear-sky situations, you can explore the variation in conversion factor between the two quantities as a function of airmass and atmospheric conditions by using SMARTS:

https://solarconsultingservices.com/smarts.php

Thank a lot for sharing - I spent a bit of time reading these comments and finally I get it now. You need to account both for the difference in the photon energy in the range of the spectrum and the fact that radiation is normally reported as total spectrum and PAR is only a portion of that spectrum. If you apply 2.02 umol/J which already accounts for the 45% fraction of PAR to equator conditions on basis of ~ 400W/m2 you would get a DLI of 70 mol/m2/d which is consistent with most reference sources on internet search. If you apply 4.6 umol/J then you need to apply the 45% factor to 400 W/m2 which is the total spectrum. Using the Planck's -Einstein equation and the ASTM G173-03 Reference Spectra Derived from SMARTS v. 2.9.2 solar spectrum filtered for the range 400-700 nm I get a value of 4.6 umol/J. Here is a link to a google spreadsheet performing the calculation. https://docs.google.com/spreadsheets/d/1qghNKwHNfpbLMD_kIOqtFKdmLvsDIpOTqgAd4dCZuSI/edit?usp=sharing

Taylor Hickem great =) thanks for share this spreadsheet.

@ Mariana Gonçalves dos Reis In the PDF, for conversion of solar irradiance to umol/m2/s it says 1W/m2 =2.022umol/m2/s. How did you get this constant

Sowmya Radhakrishna

The 2.02 conversion factor is described in Mavi & Tupper (2004). But there are many others conversion factors for different region around the world described in Foken (2008).

To Mariana:

Please elaborate on the Foken reference. Is it his book on Micrometeorology? I can't find such info in it...

Hello

This is my question too, I have taken every day observation of PAR by PAR meter. Finally find out intercepted PAR in term of µ mol m-2s-1. how can i convert it to M J m-2day-1

Dear @Payman Hassibi, the energy associated with this unit (micro mol / m2 / s) depends on the frequency of the radiation considered. The conversion values ​​given by the equipment manufacturers are approximations and are not exact. I invite you to consult our publication: First measurements of photosynthetically active radiation in San Miguel, Province of Buenos Aires. Advances in Renewable Energies and the Environment, vol. 8 N ° 2, pp. 11.13-11.17 CD-ROM version (2004) and its references. If you cannot find it on this page, please write to me ([email protected]) and I will send it to you.

You can also consult a very specific work by María José Denegri, PHOTOSINTHETICALLY ACTIVE RADIATION IN LUJAN, BUENOS AIRES. ESTIMATION AS A FUNCTION OF GLOBAL SOLAR IRRADIATION. You can find it on this site.

The first known conversion study was by McCree (1972):

Article Test of Current Definitions of Photosynthetically Active Rad...

He obtained a quanta-to-energy ratio conversion value of 4.57 for global radiation (sun+sky).

A more recent study (2004):

Article Spectral composition and quanta-to-energy ratio of diffuse p...

Things would be different if you only consider the diffuse light, however.

Dear Payman Hassibi,

Please find the following articles cited below to discover all answers to your questions.

K. J. McCree

Test of Current Definitions of Photosynthetically Active Radiation Against Leaf Photosynthesis Data

December 1972, Agricultural Meteorology 10(13):443-453

DOI: 10.1016/0002-1571(72)90045-3

Dennis G. Dye

"Spectral composition and quanta-to-energy ratio of diffuse photosynthetically

active radiation under diverse cloud conditions". 2004

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, D10203,

doi:10.1029/2003JD004251, 2004

MATTI MOTTUS, MADIS SULEV, , FREDERIC BARET, RAOUL LOPEZ-LOZANO

, ANU REINART

Chapter of book: "Photosynthetically Active Radiation: Measurement and Modeling", 2011

Matti Mõttus, Madis Sulev,, Baret Frederic, Anu Reinart Noorma

"Photosynthetically Active Radiation: Measurement and Modeling"

January 2011; In book: Encyclopedia of Sustainability Science and Technology Publisher: Springer, Editors: Robert Meyers (Editor-in-Chief)

Inmaculada Alados, I. Foyo‐Moreno , Lucas Alados Arboledas

"Photosynthetically active radiation: Measurements and modelling"

January 1996Agricultural and Forest Meteorology 78(1-2):121-131

DOI: 10.1016/0168-1923(95)02245-7

A. Aculinin

"Latitudinal variability of surface solar radiation in Moldova" -

Moldavian Journal of the Physical Sciences, Vol.7, N2, 2008, p.247-253.(pdf)

(pls. download respective file "aculinin_photosynthetically.pdf" from the

ARG IAP web-site : https://arg.ifa.md/publ.html )

The following article will give all answers related to solar irradiance

DOI: 10.1016/0168-1923(95)02245-7

The following article will give all answers related to solar irradiance

DOI: 10.1016/0168-1923(95)02245-7

Dear Masura Shammi,

Please find the following articles cited below to discover all answers to your questions.

Abbriviation PAR is the "Photosynthetically Active Radiation".

K. J. McCree

Test of Current Definitions of Photosynthetically Active Radiation Against Leaf Photosynthesis Data

December 1972, Agricultural Meteorology 10(13):443-453

DOI: 10.1016/0002-1571(72)90045-3

Dennis G. Dye

"Spectral composition and quanta-to-energy ratio of diffuse photosynthetically

active radiation under diverse cloud conditions". 2004

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, D10203,

doi:10.1029/2003JD004251, 2004

MATTI MOTTUS, MADIS SULEV, , FREDERIC BARET, RAOUL LOPEZ-LOZANO

, ANU REINART

Chapter of book: "Photosynthetically Active Radiation: Measurement and Modeling", 2011

Matti Mõttus, Madis Sulev,, Baret Frederic, Anu Reinart Noorma

"Photosynthetically Active Radiation: Measurement and Modeling"

January 2011; In book: Encyclopedia of Sustainability Science and Technology Publisher: Springer, Editors: Robert Meyers (Editor-in-Chief)

Inmaculada Alados, I. Foyo‐Moreno , Lucas Alados Arboledas

"Photosynthetically active radiation: Measurements and modelling"

January 1996Agricultural and Forest Meteorology 78(1-2):121-131

DOI: 10.1016/0168-1923(95)02245-7

A. Aculinin

"Latitudinal variability of surface solar radiation in Moldova" -

Moldavian Journal of the Physical Sciences, Vol.7, N2, 2008, p.247-253.(pdf)

(pls. download respective file "aculinin_photosynthetically.pdf" from the

ARG IAP web-site : https://arg.ifa.md/publ.html )

Dear Sir Marc W Van Iersel,

Thank you very much for sharing a good reference.

However when I looked at table 2, page 3, I think there is a concern about ratio between W/m2, photons (micromol/m2/s), and lux. According to table 2:

a, 1 W/m2 ~ 4.6 photons (micromol/m2/s)

b, 1 W/m2 ~ 0.25 lux

c, 1 lux ~ 0.019 micromol/m2/s

from b and c: 1 W/m2 ~ 0.25*0.019 = 0.00475 micromol/m2/s

The calculation from b and c is different from a.

What do you think about it? Do you have field data to test the exchange rates?

Thanks very much,

Thuong H. Pham

The way to convert is based on the light source. The conversion coefficient will be different for each light source.

http://www.egc.com/useful_info_lighting.php

This paper may be helpful (Conversion factors and general equations applied in agricultural and forest meteorology). I have found it very helpful.

Article Conversion factors and general equations applied in agricult...