You could get charged a lot of money for this but you could use the sun to calibrate it on a nice clear day.  You will need the time of solar midday at your location and also latitude and longitude.  I also have a file that will calculate the Equation of Time but it is not easy to use.  You can file websites that will give you solar midday for your location that are easier to use.

I had to resort to this with my spectroradiometer.  A student had lost the calibrate certificate and the files.

This solar calculator of mine might help.

Dear Melina,

you may calibrate your instrument against one which is already calibrated. So for PAR most of us use the LiCor Sensors. But in the UV range you need a calibrated spectroradiometer, as most of the sensors working with light sensitive diodes and filters (measuring not very accurate wavebands) but do not fit well to the defined wavebands, or show an erythem weighted irradiances.  If you calibarte it I would recommend high noon with clear sky and a comparision to early morning or late afternoon, to see if the cosinus factor (or maybe 2 pi, depending on the entrance of your instrument) fits. The same I would do on a cloudy day with scattered light.

The factors between the instrumentscan be used for calculation.

Convertion between Wm2 and µmol is easy done by the wavelength using Einsteins formular.  For sun light and blue sky ( so the respective  wavelength distribution) it is about 1 Wm-2 = 4,2 µmol m-2s-1 (PAR)

There a companies which also can do the calibration, and they generally use a calibrated standard lamp and a turning table which change the direction of the instrument to proof the dependence of the entrance angle (i.e.sun position). In Germany we have a special state office which calibrate such lamps (Eichamt).

Goodmorning once more!

: )

thank you both for the answers. And mr. Kazantzis for his contribution.

You both agree that I could use an already calibrated instrument or a spectroradiometer than measures at least up to 700nm (spectral response of PAR is needed for the second option).

Unfortunately we don't have these options... Our spectroradiometer measures up to 365,5 nm and we don't have any other instrument measuring PAR.

The other option is to use a model... I'm not so excited though since using a model for calibration isn't the appropriate way of calibrating.

But I guess that there is no other choice. Fortunately as Dr. Stelios Kazantzidis proposed, we have a CM measuring total irradiance, so I can tune the model to the total measurment for some szas, aods and water vapor and then get the calibration factor for PAR. Fingers crossed! :)

Dear Hanelt,

I wish we could send it for calibration but we can't afford to send the NILU radiometer there every year. And we got 10 of them!!! We have also noticed that there could be changes in data (e.g. shifts) after re - placing the instrument to its base, since our NILUs don't have an incorporated bubble.

It's a mess...

I'm wondering also about converting to μE/m2/sec. Since I'm going to use a model (no other option is available) I will have the spectrum, so I can create a table with aod, water vapor, ozone and the conversion factor to μΕ/m2/s. Have you already done something like this? Are  there any significant changes? Or I can use a mean value? I have read somewhere that 1 Wm-2 = 4,6 µmol m-2s-1 should cover all the atmospheric situations.

Thank you once more!!!

Melina.

Dear Melina,

I have looked into my files, for 552nm  1Wm-2 are 4.612 µmol m-2s-1. As it is in  the middle of PAR the conversion can be done with the factor of 4.6, it follwos a linearity.

You do not need to send the instruments each year. One possibilty to have a constant lamp which is calibrated and you use it only for calibration purposes, or you have one accurat calibrated instrument and you compare the data of the several daily used instrument once a year with this instrument. I have done simply  this with a calibrated LED and it works nicely. For the lamp you will need a stabilised accurate adjustable power supply.

To convert mole photons into watts you have to calculate it for each wavelength from the Top of Atmosphere (TOA) solar spectrum.  Unfortunately the terrestrial solar spectrum also changes according to how many atmospheric thicknesses the light has to pass through.  So the solar spectrum does vary with solar angle. This is worse in the near-UV.

By the way uE is not an official SI unit.  The correct figure is umol photon m-2 s-1.  Show off your superior knowledge to your friends.

Here is my solar calculator that is in Watts and it is based on calculations at each nm.

You are right.  You will go broke paying for calibration certificates, especially when students manage to lose them.

Be careful with LiCor PAR sensors.  They are designed to give a reasonable estimate for 400-700 nm white light.  They are much less trustworthy if you use them to measure coloured light from filters or LEDs.  They cannot be trusted for monochromatic light 380-430 nm or about 680-710 nm red light.  In other words the blue and red absorption peaks for Chl a!  Makes using LiCor light data for photosynthetic action spectra rather untrustworthy.

thank you both!

I will let you know for my progress on this.

: )