I am reading a bit about solar energy, and for my own curiosity, I would really like to know the insolation on my balcony. That could tell me how much a solar panel could produce.
Now, I don't have any equipment, but I do have a smartphone, and an app called Light Meter, which tells me the luminious flux per area in the unit lux.
Can I in some way calculate W/m2 from lux? E.g. the current value of 6000lux.
Answer
I'm afraid that it is not easily possible to take the luminous flux and obtain the insolation (as radiant flux). Here's why:
The luminous flux $F$ is calculated from the radiant spectral power distribution $J(\lambda)$ by weighting each wavelength with a luminosity function $y(\lambda)$ as per
$$ F = c \int J(\lambda)y(\lambda)\mathrm{d}\lambda$$
where $c$ is some unit conversion constant between lumen and watts. The total radiant flux $\Phi$ would be
$$ \Phi = \int J(\lambda)\mathrm{d}\lambda $$
The problem is that the calculation of the luminous flux is not invertible - portions of $J(\lambda)$ lying outside the visible range are cut off by the luminosity function being zero there, and it is perfectly possible for two $J(\lambda)$ of different radiant flux $\Phi$ to have a similar luminous flux $F$.
However, in the case of solar radiation, there might be a way - we know the spectral composition of sunlight, and so we know the form of $J(\lambda)$ already quite well - you could try to run an algorithm that fits the scaling of the known spectrum $J(\lambda)$ to yield the value of $F$ you measure and then calculate $\Phi$ from that spectral function. I'm not sure how good this idea is though.
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