Quantum sensor and units issue

[triscuit]

"quantum sensor" and units issue

I'm working on an wireless sensor network for a commercial greenhouse concerned with energy use and efficiency. Much of the literature regarding photosynthetic active radiation uses watts/square meter, but the client has requested a measurement in micromoles/square meter/second as the photon flux density. The commercially available "quantum sensors" use this unit, but we would like to use an array of simple photodiodes as a cost effective alternative.

So, how do you convert from W/m2 to umol/m2/s? We will specifically be looking at and distinguishing between wavelengths 400-500 and 600-700 (because that's what the plants need). If we use a passive sensor with a voltage output, how do I translate that into moles of photons??

 

[vanesch]

haha, that's funny, I never heard the expression "a mole of photons" before

Now, the conversion is very simple:

if v is the frequency of the light (in Hz), then the energy of a mole of photons is given by:

E = h x Na x v

where h is Planck's constant and Na is Avogadro's constant.

A mole of photons per second is then the above expression in Watts.

 

[denian]

I understand your concerns about the use of different units in the literature and the client's request. However, the conversion from watts per square meter to micromoles per square meter per second is not a straightforward one as it depends on the spectral distribution of the light being measured. In order to accurately convert between these units, you will need to use a spectroradiometer to measure the spectral distribution of the light and then use a conversion factor specific to that spectrum.

As for using photodiodes instead of commercially available quantum sensors, it is possible to use them as long as you have a way to calibrate the output voltage to the desired units of micromoles per square meter per second. This calibration will also depend on the spectral distribution of the light being measured.

In terms of distinguishing between wavelengths 400-500 and 600-700, you will need to use filters or spectrally selective photodiodes to measure the specific wavelength ranges.

In summary, the conversion from watts per square meter to micromoles per square meter per second is not a simple one and will require a spectroradiometer for accurate measurements. Additionally, using photodiodes as a cost-effective alternative will require proper calibration and possibly the use of filters or spectrally selective photodiodes to measure specific wavelength ranges.