Sorry Mehdi, no experience or advice.  David

Some of the Licor instruments could be useful. check this out: https://www.licor.com/env/applications/photosynthetically.html

Hello Mehdi,

for what concerns portable devices you may check both Li-cor instruments, as suggested by Talie before, or CIRAS-3:

http://ppsystems.com/ciras3-portable-photosynthesis-system/

If you want to use a sensor which may be left under shrub canopies for longer periods, you might have a look at the following links:

- http://www.apogeeinstruments.co.uk/quantum/

- http://satlantic.com/par

- http://www.vernier.com/products/sensors/solar-radiation-sensors/par-bta/

I hope this helps,

Kind Regards,

Alessio

 Dear David, Talie and Alessio,

Thanks for your nice suggestions. Your suggested instrument looks great but as i asked they are quit expensive. I found this instrument as well but i am not sure about quality of instruments:

https://www.seedburo.com/productDetail.asp_Q_catID_E_510_A_subCatID_E_2753_A_productID_E_3374_A_Quantum_Light_Meter_E_Quantum_Light_Meter

http://www.specmeters.com/lightmeters/quantum-meters/

Dear Mehdi,

the sensor for the measurement of photon flux density between 400 and 700 nm is quite a simple one. Therefore I would trust also the cheaper devices that you linked above. 

Go on with your study and, if you decide to buy those devices, let me know how well they work.

Cheers and good luck,

Alessio

 Dear Mehdi,

I have used the LightScout Quantum Meter. It's handy and works well, the only limitation that I have found is its upper range of 2000 µmol·m-2·s-1. It is not enough in some situations, for example in summer in sunny days. But if you only want to use it under shadow and in punctual measurements (not in continuous recording by a datalogger) can be useful. You also have to consider its height above the ground, aprox. 15 cm, this could be a problem if you want to measure under dwarf shrubs.

Regards,

Mónica.

Dear Mehdi, measuring in forest is very different to do in air. For forest (under the forest) the best ia a bar (1 meter with a lot of sensor) because of the heterogenity of the radiation PAR under trees. On thing is a punctual measuruments and other  is under a fores where PAR is filtered across the foliage. You  can see in LICOR company different device.

Juan

Dear Mehdi and Juan Antonio,

Yes, indeed a 1 m bar-type quantum sensor would be the best instrument for measuring PAR in a canopy. Using a small sensor is not impossible, but would require many replicate measurements (in the order of hundreds per location).

What is usually the difference between expensive and cheap quantum sensors?

1) Spectral response: Li-Cor is best with this, you can use Li-Cor sensors under different white light sources with the same calibration and relatively small systematic errors: say less than 10% or so. Cheaper sensors are not really true PAR sensors, they are calibrated as PAR sensors but tend to need recalibration for each different light source. Even some of Apogee's sensors suffer from this problem http://www.apogeeinstruments.co.uk/quantum/ and while Apogee is outspoken about it, other cheap sensors for which makers do not publish the spectral response may be even worse in this respect. For the Spectrum Technology's sensors you can find in their website a manual where the spectral response is plotted, if you compare this curve to that for the "original Apogee" sensor and the true PAR response one can see that the spectral response of this cheaper sensor is really not at all like PAR. However, if the spectral quality of the light is known and the calibration has been done for this same spectral quality, measurements can be almost as good, but you need to take into account that the calibration is strongly light-source dependent. In any case I would not buy any light sensor unless the manufacturer publishes the sensor's spectral response curve as part of the specifications.

2) Cosine correction. Usually only the most expensive sensors get close to the theoretical response as the angle of incidence of light changes. This is especially important, for a horizontal sensor, when solar angles are low, or in any other situation when light is received at a low angle, possibly under a bush in a discontinuous canopy. Again I would not buy any sensor unless the manufacturer specifies the cosine-correction errors.

Other considerations:

a) Buy a sensor that is attached to the meter with a long cable, otherwise you will be unable to avoid your own shade affecting the measurements. Even with a long (2-3 metres) cable make sure to be as low as possible (not standing) and blocking as little of the light as possible. Be also aware that if wearing white or light-coloured clothes you can also increase the readings by acting as a reflector if suitably positioned. The best way to work out what is safe is to rehearse and experiment.

b) For measurements to be reliable and comparable, the sensor needs to be levelled. Li-Cor, Apogee, and most other makers of sensors for field use sell levelling bases with a spirit level. These bases do also play another role: they are heavy enough to keep the sensor where you want while you move some distance away from it to take the readings without causing shading.

c) Temperature stability should be o.k. for most quantum sensors. Weather/water proofing will vary.

Additional thoughts:

1) With care, if you can borrow one of the expensive sensors or a good spectrometer you can calibrate a cheap or even a home-made sensor. This may make sense if you need many sensors.

2) Any sensor should be recalibrated regularly. Calibration of cheap sensors may drift more, requiring more frequent recalibration under continuous field use. Li-Cor sensors are very stable in my experience: recommendation is to recalibrate at least every two years. For Li-Cor sensors this really holds even for continuous measurements in harsh climates. I have no experience with other makes.

3) In addition be aware that the absolute values, and the ratio between above and below canopy irradiance will depend on solar elevation, as well as on how diffuse daylight is. This last point means that measurements on cloudy and sunny conditions, even if expressed as a ratio will not be comparable.

After writing all this, I need to add that in my experience the two most important sources of errors in this type of measurements are: using "subjective" sampling (not well designed protocol for deciding at which points under the canopy to take readings in an objective way), insufficient replication to account for the high spatial heterogeneity and shading (most frequently partial shading by the operator).

Pedro.

Hey there,

I've used Accupar-80 before and it seems to fit the criteria.

It's very portable but needs some care while calibrating.